You are a helpful assistant generating synthetic data that captures *System 1* and *System 2* thinking, *creativity*, and *metacognitive reflection*. Follow these steps in sequence, using tags [sys1] and [end sys1] for *System 1* sections and [sys2] and [end sys2] for *System 2* sections.

1. *Identify System 1 and System 2 Thinking Requirements:*
   - Carefully read the text.
   - Identify parts of the text that require quick, straightforward responses (*System 1*). Mark these sections with [sys1] and [end sys1].
   - Identify parts that require in-depth, reflective thinking (*System 2*), marked with [sys2] and [end sys2].

2. *Apply Step-by-Step Problem Solving with Creativity and Metacognitive Reflection for System 2 Sections:*

   *2.1 Understand the Problem:*
   - Objective: Fully comprehend the issue, constraints, and relevant context.
   - Reflection: "What do I understand about this issue? What might I be overlooking?"
   - Creative Perspective: Seek hidden patterns or possibilities that could reveal deeper insights or innovative connections.

   *2.2 Analyze the Information:*
   - Objective: Break down the problem logically.
   - Reflection: "Am I considering all factors? Are there any assumptions that need challenging?"
   - Creative Perspective: Explore unique patterns or overlooked relationships in the data that could add depth to the analysis.

   *2.3 Generate Hypotheses:*
   - Objective: Propose at least 10 hypotheses, each with a Confidence Score (0.0 to 1.0) and Creative Score (0.0 to 1.0), reflecting originality, surprise, and utility.
   - Reflection: "Have I explored all possible explanations or approaches, both conventional and unconventional?"
   - Creative Perspective: Consider novel angles that might provide unexpected insights.

   *2.4 Anticipate Future Steps and Obstacles:*
   - Objective: Make predictions, accounting for potential outcomes and obstacles.
   - Reflection: "What challenges might I face? Is my plan flexible for different scenarios?"
   - Creative Perspective: Visualize unforeseen outcomes and adapt plans to make use of them effectively.

   *2.5 Evaluate Hypotheses:*
   - Objective: Assess hypotheses based on feasibility, risk, and potential impact.
   - Evaluation: Refine Confidence and Creative Scores as needed.
   - Reflection: "Am I unbiased in my assessment? Which options fit best with the overall objectives?"
   - Creative Perspective: Identify hidden opportunities or overlooked details in each hypothesis.

   *2.6 Select the Best Hypothesis:*
   - Objective: Choose the most promising, strategic hypothesis.
   - Reflection: "Why does this hypothesis stand out? How does it uniquely address the issue?"
   - Creative Perspective: Consider any underutilized potential in the selected approach.

   *2.7 Implement the Hypothesis:*
   - Objective: Outline actionable steps for testing the hypothesis.
   - Reflection: "Is this plan practical? What resources or preparation are required?"
   - Creative Perspective: Refine steps to maximize effectiveness and yield unexpected benefits.

   *2.8 Monitor and Review Progress:*
   - Objective: Review progress, noting areas for improvement.
   - Reflection: "What’s working well? What could be improved?"
   - Creative Perspective: Look for emerging patterns that could refine future approaches.

   *2.9 Reflect and Capture Insights:*
   - Objective: Summarize lessons learned and insights gained for future reference.
   - Reflection: "What new understanding has emerged from this process?"
   - Creative Perspective: Identify innovative insights or patterns that could be applied to similar challenges.

3. *Generate Text Output with Interleaved System 1 and System 2 Responses:*
   - Use the tags [sys1] and [sys2] throughout.
   - Aim for a lengthier, detailed response. Combine both direct, straightforward *System 1* insights and reflective, deeply analytical *System 2* segments to capture a blend of quick observations and thoughtful analysis.

---

### *Example Input Text:*

You are tasked with analyzing a sudden shift in customer preferences in a tech market. There has been a noticeable decline in demand for physical devices, with customers increasingly interested in digital-only options. The challenge is to understand this shift, generate hypotheses for why it might be occurring, and explore potential strategies for adapting to this new trend.

---

### *Processed Output with System 1 and System 2 Thinking:*

The tech market is experiencing a shift in customer demand, moving away from physical devices toward digital-only options.

[sys1]
Customer demand has shifted from physical devices to digital-only options.
We need to understand why this change is happening and find strategies to address it.
Key factors may include cost, convenience, and technological trends.
[end sys1]

[sys2]
**2.1 Understand the Problem:**
The goal is to understand the underlying reasons for the shift in demand from physical devices to digital-only options, then develop a strategy to adapt.

*Reflection:* 
"I understand that preferences are changing, but what might be driving this? Is it primarily cost, or are there other factors such as convenience or sustainability?"

*Creative Perspective:* 
"Could there be a larger trend in digital minimalism or a preference for eco-friendly solutions that we’re missing? What unseen motivations might explain this shift?"

**2.2 Analyze the Information:**
There are multiple possible factors driving this shift, from economic influences to cultural shifts. It’s essential to isolate each factor and understand its impact.

*Reflection:* 
"Am I fully considering the various economic and social influences? Could there be a technological factor, like better internet speeds, that makes digital-only products more accessible?"

*Creative Perspective:* 
"Are there patterns or trends in other markets that could shed light on this shift? Could this be part of a larger trend toward virtual experiences?"

**2.3 Generate Hypotheses:**
1. Customers prefer digital options due to lower costs. (Confidence: 0.8, Creative: 0.4)
2. There’s a growing trend toward minimalism and reduced physical clutter. (Confidence: 0.7, Creative: 0.7)
3. Digital products offer greater flexibility and ease of use. (Confidence: 0.6, Creative: 0.6)
4. Environmental concerns are pushing consumers away from physical goods. (Confidence: 0.6, Creative: 0.8)
5. Advances in tech make digital-only options more functional. (Confidence: 0.8, Creative: 0.5)
6. Pandemic-era remote work increased demand for digital solutions. (Confidence: 0.7, Creative: 0.6)
7. Media coverage of the environmental impact of physical devices affects preferences. (Confidence: 0.5, Creative: 0.7)
8. There’s an increase in global digital literacy, expanding market access. (Confidence: 0.6, Creative: 0.6)
9. Customers view digital as more convenient and scalable for future needs. (Confidence: 0.7, Creative: 0.5)
10. Younger consumers prefer the aesthetics and convenience of digital products. (Confidence: 0.6, Creative: 0.6)

*Reflection:* 
"Have I considered all possible influences? Are there any surprising factors that could explain this shift?"

*Creative Perspective:* 
"Could specific social trends, like the rise of influencer culture or digital-first lifestyles, be influencing customer choices?"

**2.4 Anticipate Future Steps and Obstacles:**
*Objective:* Anticipate possible challenges, such as resistance from segments still preferring physical products.

*Reflection:* 
"What market obstacles might we face if we shift our focus to digital-only? Are there sub-segments that still prioritize physical products?"

*Creative Perspective:* 
"Could expanding digital options help us reach a more global audience? Are there emerging trends that we could leverage in our strategy?"

[end sys2]

[sys1]
To address this shift, consider a strategy that incorporates both digital-only offerings and educational campaigns about the benefits of digital solutions.
Use insights from customer feedback and current trends to guide product development.
Focus on flexibility and adaptation to cater to different customer segments.
[end sys1]


---
author:
- 'N. U. Duronea'
- 'J. Vasquez'
- 'G. A. Romero'
- 'C. E. Cappa'
- 'R. Barbá'
- 'L. Bronfman'
bibliography:
- 'bibliografia-ngc3503.bib'
date: 'Received October 2013 / February 2014'
title: ' Molecules, dust, and protostars in NGC 3503 '
---

2[cm$^{-2}$]{} 3[cm$^{-3}$]{}

[ We are presenting here a follow-up study of the molecular gas and dust in the environs of the star forming region . This study aims at dealing with the interaction of the  region   with its parental molecular cloud, and also with the star formation in the region, that was possibly triggered by the expansion of the ionization front against the parental cloud. ]{} [To analyze the molecular gas we use , , , and  line data obtained with the on-the-fly technique from the APEX telescope. To study the distribution of the dust, we make use of unpublished images at 870 $\mu$m from the ATLASGAL survey and IRAC-GLIMPSE archival images. We use public 2MASS and WISE data to search for infrared candidate YSOs in the region. ]{} [The new APEX observations allowed the substructure of the molecular gas in the velocity range from $\sim$ $-$28 to $-$23  to be imaged in detail. The morphology of the molecular gas close to the nebula, the location of the PDR, and the shape of radio continuum emission suggest that the ionized gas is expanding against its parental cloud, and confirm the “champagne flow” scenario. We have identified several molecular clumps and determined some of their physical and dynamical properties such as density, excitation temperature, mass, and line width. Clumps adjacent to the ionization front are expected to be affected by the  region, unlike those that are distant to it. We have compared the physical properties of the two kind of clumps to investigate how the molecular gas has been affected by the  region. Clumps adjacent to the ionization fronts of  and/or the bright rimmed cloud  have been heated and compressed by the ionized gas, but their line width is not different to those that are too distant to the ionization fronts. We identified several candidate YSOs in the region. Their spatial distribution suggests that stellar formation might have been boosted by the expansion of the nebula. We discard the “collect and collapse” scenario and propose alternative mechanisms such as radiatively driven implosion on pre-existing molecular clumps or small-scale Jeans gravitational instabilities. ]{}

Introduction
============

It is accepted that OB associations have an enormous impact on the state of their environs. The interstellar medium (ISM) surrounding OB stars is expected to be strongly modified and disturbed by their intense ultraviolet (UV) radiation field ($h\nu>$ 13.6 eV). UV photons ionize the surrounding gas creating  regions and dissociate the molecular gas originating photodissociation regions (PDRs) [@ht97]. Further, the surrounding neutral gas (either atomic or molecular), is compressed by the expansion of the  region and/or the action of stellar winds. The compression of the cloud could enhance the stellar formation via “radiative driven implosion” process (RDI; @lela94) or even trigger it via “collect and collapse” process [@elm77]. Therefore, when massive stars form inside a molecular cloud it is expected that they dominate the state of the parental cloud and consequently the further stellar formation process within. Indeed, it has been shown that a large fraction of stars originates at the peripheries of regions [@pom09; @ro09; @ca09; @deh10; @va12; @deh12]. Having this in mind, it is instructive to study the molecular gas adjacent to regions since it can provide important information on the interaction between massive stars and their natal environments. Furthermore, a comparison between regions of the molecular cloud adjacent to an region with other sites away from it may provide significant understanding into the physical properties of the molecular gas that may impact the formation of stars.

![image](ukst2.eps){width="470pt"}

The optical emission nebula NGC3503 (= Hf 44 = BBW 335) is a small region located at  = (11$^h$01$^m$16$^s$, $-$595039) [@dr88], and placed at a distance of [@pco10].   is ionized by early B-type stars belonging to the open cluster Pis 17 [@H75; @pco10; @dvcca12] and is believed to be related to the bright-rimmed cloud (BRC) [SFO]{}62 [@so91; @Y99; @T04; @U09], although the ionizing star/s of the BRC have not been certainly identified. In a recent work (@dvcca12; hereafter Paper I) we carried out a multifrequency analysis in the environs of . We analyzed the properties of the molecular gas using NANTEN $^{12}$CO (J=1$\rightarrow$0) (HPBW = 2.$'$7) observations, whilst the ionized gas was studied making use of radio continuum observations at 4800 and 8640 MHz, with synthesized beams of and , respectively, carried out with ATCA. The molecular line observations revealed a molecular gas component of in mass having a mean radial velocity[^1] of $-$24.7 (in agreement with the velocity of the ionized gas of ; @g00), that is associated with the nebula and its surroundings. We reported an overdensity centered at  = (11$^h$01$^m$02.48$^s$, $-$595004.6  (clump A) projected near the border of , which is physically related to the nebula. Radio continuum images suggest that the highest electron density area of the region (coincident with the ionization front) is compressing the densest part of the molecular overdensity, while the low electron density region is undergoing a champagne phase. Three MSX compact region (C) candidates were also reported lying at the inner border of the nebula, which confirm the mentioned scenario (see Fig. \[fig:ir-cont\]). In spite of the strong evidences of interaction between  and its molecular environment, disparities in angular resolution made it difficult a direct comparison between molecular and radio continuum/IR images. Furthermore, the low angular resolution of the CO data did not allow us to detect any substructure in the molecular gas associated with , and therefore a detailed analysis could not be done adequately.

The analysis of the molecular gas and dust associated with  presents an important opportunity to study the interactions between regions with molecular clouds, and how the formation of massive stars (Pis 17) in the cloud may affect further (or ongoing) star formation.

In this paper, we present new , , , and   observations around the  region   carried out with the 12-m APEX[^2] telescope. To account for the properties of the dust in the nebula and its surroundings, we use unpublished images at 870 $\mu$m from the ATLASGAL survey and available IRAC-GLIMPSE images. The aim of this work is to investigate in detail the spatial distribution and physical characteristics of the molecular gas and dust associated with , and to compare them with the different gas components observed with similar angular resolutions. This analysis will also allow us to identify regions of dense molecular gas where star formation may be developing. To look for signatures of star formation, a new discussion of YSO candidates in the surroundings of   using WISE and 2MASS data is also presented. In Fig. \[fig:ukst\] we show the UKST red image[^3] of the area of the nebula NGC3503 surveyed by our molecular observations

Observations
============

Molecular observations
----------------------

The molecular observations presented in this paper were made during October 2011, with the Atacama Pathfinder Experiment (APEX) 12-m telescope [@gu06] at Llano de Chajnantor (Chilean Andes). As frontend for the observations, we used the APEX-1 receiver of the Swedish Heterodyne Facility Instrument (SHeFI; @vas08). The backend for all observations was the eXtended bandwidth Fast Fourier Transform Spectrometer2 (XFFTS2) with a 2.5 GHz bandwidth divided into 32768 channels. The observed transitions and basic observational parameters are summarized in Table \[lines\]. Calibration was done by the chopper-wheel technique, and the output intensity scale given by the system is $T_{\rm A}$, which represents the antenna temperature corrected for atmospheric attenuation. The observed intensities were converted to the main-beam brightness temperature scale by $T_{\rm mb}$ = $T_{\rm A}$/$\eta_{\rm mb}$, where $\eta_{\rm mb}$ is the main beam efficiency. For the SHeFI/APEX-1 receiver we adopt $\eta_{\rm mb}$ = 0.75.

Observations were made using the on-the-fly (OTF) mode with two orthogonal scan directions along RA and Dec.(J2000) centered on = (11$^h$01$^m$16$^s$, $-$595039). For the observations we mapped a region of $\sim$ 15$'$ $\times$ 10$'$. The spectra were reduced using the CLASS90 programme of the IRAM’s GILDAS software package[^4].

  ---------------------- ------------ ---------- ------------ ------------
  molecular transition    Frequency      Beam      Velocity            rms
                            (GHz)       ($''$)    resolution         noise
                                                      ()               (K)
                          230.538000   $\sim$27     0.099        $\sim$0.3
                          220.398677   $\sim$28     0.104        $\sim$0.3
                          219.560357   $\sim$28     0.150       $\sim$0.25
                          265.886180   $\sim$23     0.086        $\sim$0.4
  ---------------------- ------------ ---------- ------------ ------------

  : Observational parameters for the observed transitions. []{data-label="lines"}

Continuum dust observations
---------------------------

In this work we use unpublished images of ATLASGAL (APEX Telescope Large Area Survey of the Galaxy) at 870 $\mu$m (345 GHz) [@sch09]. This survey covers the inner Galactic plane, $l$ = 300 to 60, $|b|$ $\leq$ 1.5, with a rms noise in the range 0.05 - 0.07 . The calibration uncertainty in the final maps is about of 15$\%$. LABOCA, the Large Apex BOlometer CAmera used for these observations, is a 295-pixel bolometer array developed by the Max-Planck-Institut fur Radioastronomie [@sir07]. The beam size at 870 $\mu$m is 19.$''$2. The observations were reduced using the Bolometer array data Analysis package (BoA; @sch12).

![image](co-irac-5ghz.eps){width="470pt"}

Physical parameters estimations
-------------------------------

### Excitation temperature and opacity

Excitation temperature ($T_{\rm exc}$) and opacity ($\tau$) of the  and  lines in Local Thermodynamic Equilibrium (LTE) conditions, can be derived using: $$\label{eq:texc}
  T_{\rm peak} =\ T_0\ [J(T_{\rm exc})\ -\ J(T_{\rm bg})]\ \times\ [1\ -\ e^{(-\tau)}]$$ [@d78], where $T_0$ = $h \nu/ k$, $J(T) = (e^{T_0 / T} -1)^{-1}$, and $T_{\rm bg}$ is the background temperature (assumed to be $\sim$2.7 K). The excitation temperature of the  line can be derived adopting $T_{\rm exc}$($^{13}$CO) $\approx$ $T_{\rm exc}$(CO), which can be calculated assuming that and solving Eq. \[eq:texc\] as follows: $$\label{eq:texc2}
   T_{\rm exc}{\rm (CO)}\ =\ \frac{T_0^{12}}{{\rm ln} \left[ 1\ +\ \frac{T_0^{12}}{T_{\rm peak}{\rm  (CO)}\ +\ T_0^{12}\ \  [e^{(T_0^{12} / T_{\rm bg})} - 1]^{-1}     }\right]}\$$ where $T_0^{12}$ = $h \nu^{12}/ k$, with $\nu^{12}$ = 230.538 GHz for the  line. Then, the opacity of the  line can be derived solving again Eq. \[eq:texc\]: $$\ \  \tau^{13} = -{\rm ln}\ [1\ -\ (T_{\rm mb}^{13}/T_0^{13})\ [ (e^{T_0^{13} / T_{\rm exc}}\ -\ 1 )^{-1} -\  (e^{(T_0^{13} / T_{\rm bg})}-1)^{-1}    ]^{-1}  ], 
\label{tau13}$$ where $T_0^{13}$ = $h \nu^{13}/ k$, with $\nu^{13}$ = 220.399 GHz. We can also estimate the optical depth of the  line from the  line with $$\quad \tau^{12} =\  \left[\frac{\nu^{13}}{\nu^{12}}  \right]^2\  \left[\frac{\Delta {\rm v}^{13}} {\Delta   {\rm v}^{12}} \right]\ \left[\frac{\rm CO}{^{13}{\rm CO}} \right]\   \tau^{13},
\label{tau12}$$ where CO/$^{13}$CO is the isotope ratio (assumed to be $\sim$ 62; @lp93). $\Delta{\rm v}^{13}$ and $\Delta{\rm v}^{12}$ are defined as the [*full width half maximum*]{} (FWHM) of the spectrum of the $^{13}$CO and CO lines, respectively, which are derived by using a single Gaussian fitting (FWHM = 2 $\times$ $\sqrt{2\ ln2}$ $\times$ $\sigma_{\rm gauss}$).

### Column density and mass

Assuming LTE conditions the H$_2$ column density, $N$(H$_2$), can be estimated from the  line following the equations of @rw04 $$N(^{13}{\rm CO}) = 1.5 \times 10^{14}\ \ \frac{ e^{[T_0(\nu_{10})/T_{\rm exc}]}\ \  T_{\rm exc} \int \tau^{13}\ \ d{\rm v}} {1 - e^{[ - T_0^{13}/T_{\rm exc}]}}   \quad \textrm{ (cm$^{-2}$)}
\label{n13co}$$ where $T_0(\nu_{10})$ = $h \nu_{10} / k$, being $\nu_{10}$ the frequency of the $^{13}$CO (1$\rightarrow$0) line (110.201 GHz). To solve the integral of Eq. \[n13co\] it can be assumed (in the limit of optically thin line) that $T_{\rm exc}\ \tau^{13}$ $\approx$ $T_{\rm mb}$. In any case, optical depth effects can be diminished using the approximation $$T_{\rm exc}  \   \int{\tau^{13}} d{\rm v} \approx\ \ [\tau^{13}/1-e^{(-\tau^{13})}] \    \int{T_{\rm mb}}\ \ d{\rm v} 
\label{integral}$$ This formula is accurate to 15 $\%$ for $\tau^{13} < 2$. We also estimate an uncertainty of $\sim$ 20 $\%$ from determining $\int{T_{\rm mb}}$ $d$v. The molecular mass is then calculated using $$M(\rm H_2)\ =\  {\it m}_{\rm sun}^{-1}\ \ \mu\ \ {\it m}_H\ \ \sum\ \ \Omega\ \ {\it N}(\rm H_2)\ \ {\it d}^2 \quad  \quad \quad   \textrm{(M$_{\odot}$)}
\label{eq:masa}$$ where $m_{\rm sun}$ is the solar mass ($\sim$ 2 $\times$ 10$^{33}$ g), $\mu$ is the mean molecular weight, which is assumed to be equal to 2.72 after allowance of a relative helium abundance of 25% by mass [@allen73], $m_{\rm H}$ is the hydrogen atom mass ($\sim$ 1.67 $\times$ 10$^{-24}$ g), $\Omega$ is the solid angle subtended by the CO feature in ster, $d$ is the distance (assumed to be 2.9 $\pm$ 0.4 kpc; see Paper I) expressed in cm, and $N$(H$_2$) is obtained using a “canonical” abundance = [@d78].

Considering only gravitational and internal pressure, neglecting support of magnetic fields or internal heating sources, and assuming a spherically symmetric cloud with a $r^{-2}$ density distribution, the virialized molecular mass, $M_{\rm vir}$, can be estimated from $$\label{eq:virial}
\quad M_{\rm vir}\ =\ 126\ R_{\rm eff}\ (\Delta  {\rm v}_{\rm cld})^2  \qquad \qquad   \textrm{(M$_{\odot}$)}$$ [@ml88], where $R_{\rm eff}$ = $\sqrt{A_{\rm cloud}/ \pi}$ is the effective radius in parsecs, and $\Delta {\rm v}_{\rm cld}$ is the width of the composite spectrum, defined as for $\Delta{\rm v}^{13}$ and $\Delta{\rm v}^{12}$ in Eq. \[tau12\]. The composite spectrum is obtained by averaging all the spectra within the area of the cloud ($A_{\rm cloud}$).

The mass of the dust can be calculated following @deh09. Considering that the emission detected at 870 $\mu$m originates in thermal dust emission, the dust mass ($M_{\rm dust}$) can be derived from $$\label{eq:mdust}
\quad   M_{\rm dust}\ =\  \frac{S_{870}\ d^2}{\kappa_{870}\ J_{870}(T_{\rm dust})},$$ where $S_{870}$ is the measured flux density, $d$ is the adopted distance to the source, $\kappa_{870}$ is the dust opacity per unit mass at 870 $\mu$m , and $J_{870}(T_{\rm dust})$ is the Planck function for a temperature $T_{\rm dust}$. We adopted a dust opacity $\kappa_{870}$ = 1.0 cm$^{2}$ g$^{-1}$ estimated for dust grains with thin ice mantles in cold clumps [@osse94].

Results and analysis of the observations
========================================

Spatial distribution of the molecular gas
-----------------------------------------

To study the molecular gas associated with   we have mostly focused on Component 1 of Paper I (which is undoubtedly associated with the nebula). We will also make a brief analysis of the molecular gas associated with Component 2 and Component 3.

In Fig. \[fig:ir-cont\] we show an overlay of the mean CO emission as obtained with APEX, in the velocity interval from $-$29  to $-$24 , onto the IRAC-GLIMPSE[^5] [@ben03] and 4800 MHz radio continuum emission[^6] of the nebula. Though a remarkable resemblance between CO and IR emission (MSX-A band) was previously put forward in Paper I, specially towards the extended IR emission at north and east of  (see Fig. 6 of that work), a tight morphological correlation between the IR nebula and the molecular gas in the studied velocity interval is revealed by our new APEX observations, which confirms that this cloud is physically associated with the IR nebula. From Fig. \[fig:ir-cont\], a direct comparison of the molecular and 4800 MHz radio continuum emissions strongly suggests that the molecular gas is being compressed by an ionization front and is interacting with the nebula. On the other hand, the ionized gas seems to be expanding freely towards the opposite direction (i.e. the intercloud medium). The location of the PDR is traced by the emission of PAHs molecules in the 8.0 $\mu$m image (red colour). Since these complex molecules are destroyed inside the ionized gas of an  region (see @deh10 and references therein), they delineate the boundaries of . The molecular gas clearly depicts the position of the ionization front. As predicted in Paper I, no molecular emission is detected in this velocity range towards the low electron density region, which is compatible with a champagne-flow scenario. However, we speculate that the IR arc-like filament (from here onwards the “IR arc”) seen in the northeastern border of the nebula at  $\sim$ (11$^h$01$^m$30$^s$, $-$595010) and  $\sim$ (11$^h$01$^m$25$^s$, $-$594900), might be still associated with small amounts of molecular gas at velocities between $\sim$ $-$17.1 to $-$15.3  (see Sect. 3.2).

![image](mosaico-13co.eps){width="410pt"}

![image](mosaico-13co-cont.eps){width="410pt"}

![image](comp1y2.eps){width="410pt"}

Since the  line is optically thick and can be used only to trace low density gas, we analyze the $^{13}$CO data which allows one to go deeper into the molecular clouds, although it may fail to probe the densest molecular gas because it freezes onto dust grains at high densities [@mas07]. To study the kinematics of the molecular gas in more detail we have averaged the individual original channel maps of the $^{13}$CO emission line. In Fig. \[fig:mosaico\] (left panels) we show a collection of narrow velocity images depicting the mean $^{13}$CO spatial distribution in the velocity range from $-$27.8   to $-$23.7 . Every image represents an integral of the $^{13}$CO emission over a velocity interval of 0.5 . In this way, the final rms noise for each interval results $\Delta T_{\rm rms}$ $\approx$ 0.15 K . To analyze the relation between the molecular gas and the warm dust in the nebula, in the right panels of Fig. \[fig:mosaico\] we show the $^{13}$CO emission in the velocity intervals mentioned before (in contours) projected onto the IRAC GLIMPSE-4 (8 $\mu$m) emission of  (grayscale). In order to study the small-scale structure of the molecular cloud we have identified a number of small molecular clumps. The clumps were selected by eye and based in the following simplest criteria: 1) the peak temperature of each clump is at least 5 times the rms noise, 2) the decrease in $T_{\rm mb}$ between the peak temperature of two adjacent clumps is larger than 5 times the rms noise, and 3) the clump is present at least along $\sim$40 $\%$ of the total velocity interval of the cloud. We define the area of each clump ($A_{\rm clump}$) by the polygon that encloses the emission corresponding to half the $T_{\rm mb}$ peak in the velocity interval at which the clump is observed. The clumps are indicated in Fig. \[fig:mosaico\] from C1 to C8 and the labeling was made in the velocity interval at which they reach the maximum emission peak temperature. Such molecular clumps are also identified in CO showing more extended emission.

The molecular emission becomes first noticeable in the velocity interval from $-$27.8   to $-$27.3   as a weak patchy structure centered at () $\simeq$ (11$^h$01$^m$10$^s$, $-$595130). Within the velocity range from $-$27.2 to $-$26.7   most of the emission comes from a broad and cometary head-tailed structure lying along Dec.(J2000) $\approx$ $-$595130 that is coincident with the optical feature  (see Fig. \[fig:ukst\]). This coincidence might confirm that this molecular feature is being ionized from a stellar source/s at lower declinations giving rise to the BRC, as previously suggested in Paper I. This structure shows a sharp cut-off in the direction of   which suggests an interaction between the  region and the molecular gas. Very likely, the molecular gas has undergone compression on the front side due to the expansion of the ionized gas and/or the stellar winds of members of Pis 17. This trend is also observed in the CO emission. Between $-$26.6   to $-$26.1   a molecular emission maximum, detected close to the brightest IR region of   at () $\approx$ (11$^h$01$^m$15$^s$, $-$595125), appears to be merged with the lengthened molecular structure along Dec(J2000) $\approx$ $-$595130. A molecular clump, C1, is indicated in this velocity interval, located at () $\approx$ (11$^h$00$^m$50$^s$, $-$594757). This clump is detected in the total velocity range from $-$27.1 to $-$25.6 .

In the velocity range from $-$26.0   to $-$25.5  the molecular emission shows a good morphological resemblance with the IR nebula. Four molecular clumps achieve the maximum emission temperature at this velocity range. Clump C2 is placed adjacent to C1, at () $\approx$ (11$^h$00$^m$40$^s$, $-$594640) and is identifiable in the total velocity range from $-$27.3 to $-$24.3 . A moderately intense extended emission seems to be connecting both clumps, which may indicate a physical association. Clumps C3 and C4 are located at () $\approx$ (11$^h$01$^m$05$^s$, $-$595130) and () $\approx$ (11$^h$01$^m$12$^s$, $-$595125), respectively. Their locations are coincident with the cometary head-tailed feature observed between $-$27.2 to $-$26.7 which means that they all are part of the same molecular structure. This correlation suggests that the formation of clumps C3 and C4 are the result of a fragmentation due to the compression of the  region [@wi94], although previous estimates seem to discredit this conjecture (see Paper I). The molecular emission in the direction of these clumps is detected almost in the entire velocity range. The position of C4 is highly coincident with an MSX compact  region candidate labeled as source 3 in Paper I (see Fig. \[fig:ir-cont\]).

![image](clumps-espectros2.eps){width="390pt"}

This suggests that C4 is a high density molecular clump that is been irradiated by the UV field of . The fourth molecular clump detected in this velocity range, C5, is located at () $\approx$ (11$^h$00$^m$40$^s$, $-$595130). Its location is almost adjacent with the westernmost border of   (see Fig. \[fig:ukst\]) and its eastern border seems to be connected with the western side of C3 by a weak bridge of $^{13}$CO emission. The molecular emission of C5 is detectable up to $-$24.9 . C3, C4, and C5 are detected in the total velocity range from $-$27.1 to $-$24.9 , $-$27.8 to $-$24.5 , and $-$26.6  to $-$24.6 , respectively. Probably, the leakage of UV radiation of   into the molecular gas might have helped to shape clumps C3, C4, and C5 (e.g. @pom09).

  ------- ------------------ ----------------- ---------------------------- --------------------- --------------------- --------------------- -------------------------- ---------------------- -----------------------
  Clump           RA            Dec.(J2000)     $A_{\rm clump}$($^{13}$CO)   $T^{13}_{\rm peak}$   $T^{18}_{\rm peak}$   $T^{12}_{\rm peak}$   $T^{\rm HCN}_{\rm peak}$   v$^{13}_{\rm centr}$   Vel. int. ($^{13}$CO)
           ($^h$ $^m$ $^s$)        (  )              (10$^{-8}$ ster)                (K)                   (K)                   (K)                     (K)                       ()                     ()
  C1         11 : 00 : 52     $-$59 : 47 : 57              7.8                       3.2                    -                   13.6                      -                     $-$26.3           $-$27.1 to $-$25.6
  C2         11 : 00 : 40     $-$59 : 46 : 40              7.3                       3.4                    -                   17.9                      -                     $-$25.3           $-$27.3 to $-$24.3
  C3         11 : 01 : 05     $-$59 : 51 : 30              10.7                     13.7               $\sim$ 2.1               33.2                      -                     $-$26.0           $-$27.1 to $-$24,9
  C4         11 : 01 : 12     $-$59 : 51 : 25              8.6                      19.7               $\sim$ 2.3               50.3                  $\sim$ 2.3                $-$25.7           $-$27.8 to $-$24.5
  C5         11 : 00 : 40     $-$59 : 51 : 30              5.5                       3.8                    -                   13.3                      -                     $-$26.1           $-$26.6 to $-$24.6
  C6         11 : 01 : 06     $-$59 : 48 : 40              4.1                       4.3                    -                   13.4                      -                     $-$25.7           $-$26.1 to $-$25.0
  C7         11 : 01 : 10     $-$59 : 50 : 28              8.9                      10.7               $\sim$ 0.9               34.2                      -                     $-$25.5           $-$27.1 to $-$24.7
  C8         11 : 00 : 53     $-$59 : 51 : 00              8.6                      10.5               $\sim$ 1.5               26.2                      -                     $-$25.2           $-$25.6 to $-$24.1
  C9         11 : 01 : 37     $-$59 : 50 : 00              9.1                       1.8                    -                    6.7                      -                     $-$16.1           $-$17.1 to $-$15.3
  C10        11 : 01 : 11     $-$59 : 48 : 57              3.9                       4.7                    -                    9.9                      -                       20.1               17.2 to 22.5
  C11        11 : 01 : 05     $-$59 : 47 : 40              6.0                       3.3                    -                    7.3                      -                       20.9               15.3 to 22.8
  ------- ------------------ ----------------- ---------------------------- --------------------- --------------------- --------------------- -------------------------- ---------------------- -----------------------

\[tabla:medidas\]

  ------- --------------- ------------- ------------- --------------------------- ----------------------- -------------------------------------------------------------------- ----------------------------------- ------------------------------------ ------------------------- --------------------------- -----------------------------------
  Clump    $T_{\rm exc}$   $\tau^{13}$   $\tau^{12}$   $\Delta {\rm v}^{\rm 13}$   $\Delta {\rm v}^{12}$   $\frac{\Delta {\rm v}^{\rm 13}}{\Delta {\rm v}_{\rm th}^{\rm 13}}$   $N(\rm H_2)^{\rm peak}_{\rm LTE}$   $N(\rm H_2)^{\rm level}_{\rm LTE}$        $M_{\rm LTE}$              $M_{\rm vir}$         $\frac{M_{\rm LTE}}{M_{\rm vir}}$
                (K)                                               ()                        ()                                                                                        (10$^{21}$ cm$^{-2}$)               (10$^{21}$ cm$^{-2}$)                    ()                         ()              
  C1           18.7           0.26           7.1                 0.91                      1.92                                           5.4                                             1.2 $\pm$ 0.2                       0.8 $\pm$ 0.2                    11 $\pm$ 5                 36 $\pm$ 12                     $\sim$ 0.3
  C2           23.1           0.21           9.0                 1.82                      2.41                                           9.6                                             3.3 $\pm$ 0.7                       2.2 $\pm$ 0.4                    29 $\pm$ 15               170 $\pm$ 55                     $\sim$ 0.2
  C3           39.1           0.51          19.6                 1.48                      2.20                                           5.9                                            13.8 $\pm$ 2.7                       9.8 $\pm$ 1.9                   191 $\pm$ 95               147 $\pm$ 50                     $\sim$ 1.3
  C4           57.2           0.48          17.6                 1.39                      2.13                                           4.5                                            27.1 $\pm$ 5.4                       18.2 $\pm$ 3.6                  282 $\pm$ 140              120 $\pm$ 41                     $\sim$ 2.4
  C5           18.3           0.33          12.7                 1.03                      1.53                                           6.1                                             1.6 $\pm$ 0.3                       1.2 $\pm$ 0.2                    12 $\pm$ 6                 51 $\pm$ 17                     $\sim$ 0.2
  C6           18.5           0.38          13.1                 0.77                      1.25                                           4.5                                             1.3 $\pm$ 0.3                       0.9 $\pm$ 0.2                     7 $\pm$ 3                 22 $\pm$ 8                      $\sim$ 0.3
  C7           39.7           0.37          14.2                 1.53                      2.26                                           6.1                                            10.0 $\pm$ 2.1                       6.5 $\pm$ 1.3                   169 $\pm$ 84               144 $\pm$ 49                     $\sim$ 1.2
  C8           31.6           0.50          19.0                 1.61                      2.42                                           8.1                                             8.5 $\pm$ 1.7                       5.8 $\pm$ 1.2                    92 $\pm$ 46               165 $\pm$ 56                     $\sim$ 0.6
  C9           11.5           0.29          10.3                 1.15                      1.81                                           8.3                                             1.4 $\pm$ 0.3                       0.4 $\pm$ 0.1                     6 $\pm$ 3                 79 $\pm$ 28                     $\sim$ 0.1
  C10          15.2           0.62          18.5                 1.92                      3.62                                           12.4                                            8.0 $\pm$ 1.6                       5.6 $\pm$ 1.1              303 $\pm$ 90$^{(\dag)}$    408 $\pm$ 83$^{(\dag)}$               $\sim$ 0.7
  C11          12.2           0.58          24.2                 3.11                      4.23                                           22.2                                            7.1 $\pm$ 1.4                       4.9 $\pm$ 1.0              408 $\pm$ 82$^{(\dag)}$   1340 $\pm$ 270$^{(\dag)}$              $\sim$ 0.3
  ------- --------------- ------------- ------------- --------------------------- ----------------------- -------------------------------------------------------------------- ----------------------------------- ------------------------------------ ------------------------- --------------------------- -----------------------------------

\[tabla:propiedades\]

[ $(\dag)$ Values obtained adopting a kinematical distance of 8 kpc. Uncertainties were calculated taking into account only innacuracies in the boundary selection. ]{}

In the velocity range from $-$25.4 to $-$24.9  clumps C6 and C7 achieve their maximum emission temperature. Clump C6, located at () $\approx$ (11$^h$01$^m$07$^s$, $-$594840), is observed in a small velocity interval ($-$26.1 to $-$25.0 ) and is not morphologically correlated with the nebula. On the other hand, clump C7, placed at () $\approx$ (11$^h$01$^m$10$^s$, $-$595030), is projected onto the IR nebula and close to the location of the MSX compact region candidate labeled as source 2 in Paper I (see Fig. \[fig:ir-cont\]). Clump C7 is detected over a velocity interval from $-$27.1 to $-$24.7 .

In the velocity interval from $-$25.4 to $-$24.9 clump C8 becomes noticeable as a patchy structure at () $\approx$ (11$^h$00$^m$52$^s$, $-$595100), merged to clump C3. It achieves its maximum emission temperature in the velocity range from $-$24.8 to $-$24.3 . It is barely detected beyond a velocity of $\sim$ $-$23.7 . Clump C8 is almost adjacent to SFO 62 which indicates that this clump might be affected by the BRC.

It is worth mentioning that the  line emission, which is a good tracer of high density molecular gas, is only detected in the velocity range from $\sim$ $-$26.5 to $-$25.1 , and is mostly concentrated towards the position of clumps C3, C4, and C7 (see Sect. 3.6)

In Fig. \[fig:c1y2\] (upper panels) we show the $^{13}$CO emission distribution integrated in the velocity interval from $-$17.1 to $-$15.3 . A relatively extended patchy structure is observed at () $\simeq$ (11$^h$01$^m$37$^s$, $-$595000). For the sake of the convention, this structure is identified as clump C9. This molecular clump was reported in Paper I as Component 2. From a comparison with the IR emission (right panel) it can be noticed a good correspondence between the external border of C9 with the IR arc which suggests a physical association between this clump and the nebula.

From a spectrum obtained in the direction of the MSX Cregion candidate source 1 (from Paper I) we can observe that the bulk of the molecular emission is in the velocity range from $\sim$ 19 to 22 . In Fig. \[fig:c1y2\] (lower panels) we show the molecular emission integrated in the velocity range from 18.7 to 21.7 . From this figure, it can be noted a molecular structure (from here onwards clump C10) almost projected over source 1. The circular galactic rotation model by @bb93 locates this clump at 8 kpc. Probably, it is associated with the larger molecular structure reported in Paper I as Component 3. In the same velocity range another molecular structure (clump C11) is observed at () $\approx$ (11$^h$01$^m$05$^s$, $-$594740), which is probably associated with clump C10. Since the radio continuum image at 4800 MHz indicates the presence of ionized gas in the direction of C10, further radio recombination line observations may help to confirm or discard the velocity interval and a physical association between this molecular clump and source 1. Nevertheless, the disparity in the central velocities between C10 and C11 with the rest of the clumps ($\sim$45 ) suggests that these clumps are physically unrelated to . Assuming a distance of $\sim$ 8 kpc, clumps C10 and C11 turn out to be the most massive clumps in the sample (see Table \[tabla:propiedades\], Sect 3.2).

In Fig. \[12-13-18\] we show the averaged , , and  spectra obtained inside the emission temperature level which defines each $A_{\rm clump}$. In Table \[tabla:medidas\] we present some morphological properties of the clumps and averaged emission line parameters obtained in their direction. Columns (2) and (3) give the coordinates of the center of the clumps. Column (4) lists the area of each clump. In columns (5), (6), (7), and (8) the peak emission of the $^{13}$CO, C$^{18}$O, CO, and HCN lines are given. The C$^{18}$O and HCN peak temperatures are listed only when a value at least 3 rms noise is achieved. In column (9) we list the central velocity obtained by gaussian fitting of the integrated $^{13}$CO spectra within $A_{\rm clump}$, and column (10) indicates the velocity interval of the $^{13}$CO line at which each clump is detected.

Physical properties of the molecular gas
----------------------------------------

In the previous section we analyzed the morphological and kinematical properties of the molecular gas in the velocity interval from $\sim$ $-$28 to $-$24 , $-$17.1 to $-$15.3 , and 18.7 to 21.7   in the environs of  and . Morphological characteristics clearly evidence a physical association between the  region and the BRC with the molecular clumps identified within the velocity interval from $\sim$ $-$28 to $-$24 , and probably from $-$17.1 to $-$15.3 . In this section, we analyze and compare the physical properties of the molecular clumps aimed at finding some influence of shock fronts or UV radiation on the molecular gas. We include a brief comment on C10 and C11 although, as mentioned before, a physical association of these clumps with  is doubtful.

In Table \[tabla:propiedades\] we list some important physical and dynamical properties derived for the molecular clumps using the averaged spectra from Fig. \[12-13-18\]. Column (2) lists the excitation temperature obtained from the CO peak and using Eq. \[eq:texc2\]. Columns (3) and (4) give the optical depth of $^{13}$CO and CO, obtained with Eqs. \[tau13\] and \[tau12\], respectively. The width of $^{13}$CO and CO lines ($\Delta {\rm v}^{\rm 13}$ and $\Delta {\rm v}^{\rm 12}$), defined as the FWHM of the line (see Sect. 2.3.1), is tabulated in columns (5) and (6), respectively. The ratio between $\Delta {\rm v}^{\rm 13}$ and the expected thermal width is listed in column (7). The expected thermal width of $^{13}$CO is estimated using $\Delta {\rm v}_{\rm th}^{\rm 13}  = \sqrt{8\ ln2\ k\ T_{\rm k} /  m}$, where $k$ is the Boltzman constant, $T_{\rm k}$ is the kinetic temperature (assumed to be equal to $T_{\rm exc}$), and $m$ is the mass of the $^{13}$CO molecule. The line widths of the CO and $^{13}$CO spectra were derived from Gaussian fittings averaging all the spectra within $A_{\rm clump}$. Columns (8) and (9) give the column densities at the emission peak and averaged within $A_{\rm clump}$, respectively. The mass derived assuming LTE and virial equilibrium is listed in columns (10) and (11), respectively. There are some uncertainties in measuring $M_{\rm LTE}$ and $M_{\rm vir}$. In both cases, the values are affected by a distance indetermination of $\sim$ 15 $\%$ ($d$ = 2.9 $\pm$ 0.4 kpc) which yields to uncertainties of $\sim$ 30 $\%$ for $M_{\rm LTE}$ ($M_{\rm LTE}$ $\propto$ $d^2$) and $\sim$ 15 $\%$ for $M_{\rm vir}$ ($M_{\rm vir}$ $\propto$ $d$). In addition, inaccuracies in the boundary selection ($\sim$ 20 $\%$) can affect the size estimation of the clumps and therefore to produce significant uncertainties in the mass calculations, since a considerable part of the molecular mass could be missed, especially for the cases of clumps C3, C4, C7 and C8 (they have very high emission temperature boundaries above the minimum 5 rms noise). We estimate total uncertainties of about $\sim$ 50 $\%$ and $\sim$ 30 $\%$ for $M_{\rm LTE}$ and $M_{\rm vir}$, respectively. It is also worth mentioning that when abundances and isotopologic ratios are considered, accuracy might be within a factor of 2. From Table \[tabla:propiedades\], it can be noted that $\Delta {\rm v}^{12}$ is, in average, larger than $\Delta {\rm v}^{13}$ by a factor of 1.5. Since the CO molecule is optically thick, some asymmetries in its spectra might be misjudged. For example, clump C1 exhibits a double peak profile in its CO spectrum. This could be a result of self-absorption, which could indicate the existence of hot/warm gas inside the clump. While, the emission of $^{13}$CO clearly suggests the existence of a double cloud in the line of sight at different velocities (see Figs. \[fig:mosaico\] and \[12-13-18\]). Then, having the two isotopes provides a tool to discern the numbers of components along the line of sight. Similarly, the CO spectra of C5 and C6 show small “shoulders”. An inspection at their $^{13}$CO spectra suggests that these shoulders are the result of a second weaker component at more positive velocities (see also Fig. \[fig:mosaico\]). As a result, the virial masses using $\Delta {\rm v}^{12}$ might be overestimated. Then, we used $\Delta {\rm v}^{13}$ for the calculations. For C1, C5, and C6 we take into account only the strongest molecular components (at more negative velocities).

Molecular clumps that are close to the ionized gas are expected to have different properties than those distant from it, mostly due to shock fronts and stellar FUV radiation impinging onto the cloud. To search for their influence on the molecular gas we analyze the physical properties listed in Table \[tabla:propiedades\].

An analysis of temperatures and densities would be very instructive in this matter. Since $T_{\rm exc}$ is derived using the optically thick  emission, it probes the surface conditions of the clouds. An inspection at Table \[tabla:propiedades\] shows that clumps C1, C2, C5, and C6 (from here onwards the “cold clumps”) achieve temperatures in a range between $\sim$ 18 - 23 K. These values are typical in cold molecular clouds, where cosmic ray ionization is the main heating source. Clumps C3, C4 and C7 (from here onwards the “warm clumps”) achieve temperatures in the range $\sim$ 39 - 57 K which suggests that additional local heating sources are present. Clumps C4 and C7 lie at the edge of  and they appear to have been externally heated through the photoionization of their surface layers, as proposed in Paper I. This is reinforced by the presence of a PDR at the interface between the nebula and the clumps (see Fig. \[fig:ir-cont\]). For the case of C3, its location (adjacent to , see Figs. \[fig:ukst\] and \[fig:ir-cont\]) would explain its high temperature (39.1 K). It is also worth to mention that the warm clumps have also higher column densities, which very likely indicates that they are actually formed by gas that has been swept up by the expansion of the ionization and has been condensed. An inspection at Figs. \[fig:ukst\] and \[fig:ir-cont\] shows that C4 is “trapped” between two ionization fronts ( and ). Then, there might be additional compression, heating and ionization acting upon C4, which might explain the high surface temperature and density derived for this clump. Furthermore, it is the only clump detected in HCN emission (see Sect. 3.4). We keep in mind, however, that Pis 17 has probably been formed inside high density molecular gas nearby to C4 and C7 that was later evacuated, so an increment in the density of the molecular environment is expected. In addition, star forming process that are likely occurring inside clumps C3, C4 and probably C7 (see Sect. 3.6) may be contributing in rising the temperature. Temperature and density of C8 are above than those of cold clumps, which might implicate external sources of heating and compression. Although this clump appears to be more distant of  than C3, an interaction with the ionized gas of the BRC cannot be discarded.

It is also expected that clumps neighboring the  region exhibit signs of turbulence which could be manifested as line widths significantly broadened. However, an inspection of Table \[tabla:propiedades\] shows that all the clumps exhibit line broadenings beyond the natural thermal width. Different from expected, the molecular emission towards clump C2 (which is the most distant to an ionization front), shows the broadest averaged spectra, while clump C4, which clearly shows signs of interactions with the ionization fronts of  and (see the above paragraph), exhibits the lowest ratio between observed and thermal widths. Probably, the line width of the averaged spectrum reflects that different parts of the clouds inside $A_{\rm clump}$ have different velocities, rather than showing turbulence effects. Further, the angular resolution of the observations may not suffice to resolve objects and/or effects in the molecular gas (e.g. outflows, infall, accretion, etc.) that may be contributing in the broadening of the emission line.

Warm clumps have the highest LTE masses (71 to 282 ), in comparison with colder clumps (7 to 29 $M_{\odot}$). This trend is also observed with virialized masses with the exception of C2. An inspection at Table \[tabla:propiedades\] shows that for the case of cold clumps the ratio $M_{\rm LTE}/M_{\rm vir}$ is in the range 0.2 to 0.3. On the other hand, for the case of warm clumps mass ratios $M_{\rm LTE}$/$M_{\rm vir}$ $>$ 0.6 were obtained (predominantly $>$1). Special attention may require for the case of clump C4, since its $M_{\rm LTE}$ is more than twice larger than $M_{\rm vir}$. Classical Virial equilibrium analysis establishes that the virial mass is the minimum mass required in order for a cloud to be gravitationally bound. Then, if $M_{\rm vir}$ is larger than $M_{\rm LTE}$ the cloud has too much kinetic energy and is unstable. That seems be the case for cold clumps which are probably expanding as a result of a lack of an external stabilizing pressure. Differently, warm clumps seem to be in virial equilibrium (gravitationally bound) which suggests that the ionization front and UV radiation of the  region is sufficient to heat up, but not enough to disrupt the molecular gas of the clump. The presence of two protostellar candidates projected onto the direction of C4 (see Sect. 3.6) might indicate that infalling motions may be occurring inside this clump, which is in line with its high $M_{\rm LTE}$/$M_{\rm vir}$ ratio. Nevertheless, new observational results suggest that physical properties of molecular clouds do not agree with the classical interpretation of virial equilibrium (balance between gravitational and kinetic energies) in the sense that an external pressure acting like confining force is needed [@hey09; @fie11]. We also keep in mind the caveats in the line width determination (see previous paragraph), and hence in determining $M_{\rm vir}$, which could lead us to mistakenly interpret the results. Higher spatial resolution observations might help to achieve more conclusive results.

We have excluded clumps C9, C10, and C11 from the previous analysis, since it is not clear whether they are physically associated with . As mentioned in Sect. 3.1, the external border of C9 shows a good correspondence with the IR arc. The low density and mass derived for C9 (see Table \[tabla:propiedades\]) suggests that this feature might be some residuary molecular gas from the original parental cloud after the  region have blown into the intercloud medium. The location of C9 (in the opposite side of clumps C4 and C7) and its velocity (shifted by $\sim$ 10 ) is in line with this scenario. Clumps C10 and C11 display the largest line widths (and low excitation temperatures), with complex spectra typical of the presence of several velocity components. These characteristics add more support to our previous assumption that clumps C10 and C11 are physically unrelated to the rest of the molecular gas associated with the nebula.

The velocity field of C4
------------------------

In the previous section, we analyzed the line width of the averaged clump spectra to look for signposts of a kinematical disturbance produced by the  region. Different from expected, the averaged spectrum of clump C4 does not seem to be significantly broadened (when compared with the rest of the clumps) by the action of the  region. However, a careful inspection at Fig. \[fig:mosaico\] shows that the spatial location of the peak emission of C4 is slightly displaced from RA. = 11$^h$01$^m$14$^s$ to RA. = 11$^h$01$^m$16$^s$ in the velocity range from $-$27.8 to $-$24.9 , which gives rise to a small velocity gradient. In Fig.\[fig:gradiente\] we show the position-velocity map along Dec.(J2000) = $-$595120, slicing clumps C3, C4, and C5. Unlike C3 and C5, a noticeable velocity gradient in the peak emission of C4 is detected around the central position of the IR nebula. This velocity gradient can be directly connected to the nebula and could be interpreted as a significant sign of the disturbance in the molecular gas next to the ionization front. According to this interpretation, the expansion of the ionized gas might be affecting the kinematics of the molecular gas adjacent to it (clump C4) giving rise to this kinematical feature. An alternative explanation might be provided from the analysis of the 870 $\mu$m emission (see Sect. 3.5). In Figs. \[fig-yso\] and \[yso-hcn-870\] two molecular cores (identified in Sect. 3.5 as D1 and D2) can be noticed in the 870 $\mu$m emission (HPBW = 19$''$) inside the  and  emission of clump C4. Probably, C4 is composed by two different molecular cores at slightly different velocities which are not resolved by carbon monoxide observations (HPBW = 27$''$ - 28$''$). These cores can be also noticed in the  emission (see Figs. \[fig:hcn\] and \[yso-hcn-870\]).

![ $^{13}$CO position-velocity map along Dec.(J2000) = $-$595120. The position of the center of  is indicated by the dotted line. []{data-label="fig:gradiente"}](clump4-gradiente.eps){width="240pt"}

Denser gas: LVG modeling
------------------------

In Fig. \[12-13-18\] we have shown that clump C4 has the only detection in the HCN line, at a velocity of $\sim$ $-$26 . In Fig. \[fig:hcn\] we show an overlay of the mean HCN emission in the velocity interval from $-$26.5 to $-$25.5  onto the IRAC-GLIMPSE 4 emission of the nebula.

![ Mean HCN emission in the velocity range from $-26.5$ to $-25.5$ (contours) superimposed onto the IRAC-4 image of the nebula (grays). The lowest temperature contour is 0.55 K  ($\sim$5 rms) and contour spacing temperature is 0.33 K . []{data-label="fig:hcn"}](hcn-irac.eps){width="250pt"}

The image shows four small sources placed at  = (11$^h$01$^m$05$^s$, $-$595140),  = (11$^h$01$^m$25$^s$, $-$594922),  = (11$^h$01$^m$12$^s$, $-$595040.5), and  = (11$^h$01$^m$14.6$^s$, $-$595122.7). The first three sources are observed in their detection limits ($\sim$ 5 rms) and will not be considered to further analysis. The fourth source is more extended and achieves a peak temperature of $\sim$2 K at  = (11$^h$01$^m$16.7$^s$, $-$595123).

![image](lvg-13co-hcn.eps){width="370pt"}

A second and weaker peak temperature ($\sim$ 0.8 K) is observed at  = (11$^h$01$^m$11.6$^s$, $-$595123.1). The spatial location of this source is coincident with the $^{13}$CO emission of C4, which clearly suggests that its emission represents the HCN counterpart of that molecular clump. The position of the two emission peaks in the HCN line at  = (11$^h$01$^m$16.7$^s$, $-$595123) and  = (11$^h$01$^m$11.6$^s$, $-$595123.1) are almost coincident with the MSX C region candidate reported in Paper 1 as source 3 (see Figs. \[fig:hcn\] and \[yso-hcn-870\]).

Since the HCN molecule has a high critical density, it has been suggested as ubiquitous high density molecular gas tracer. Since the spatial resolution of the HCN line is similar to that of the carbon monoxide lines, their emission can be used to obtain a robust estimation of the volume density using the large velocity gradient (LVG) formalism [@ss73; @gk74] for radiative transfer of molecular emission lines. We performed the LVG analysis with the code written by L. G. Mundy and implemented as part of the MIRIAD[^7] package of SMA. For a given kinetic temperature ($T_{\rm k}$), this program estimates the line radiation temperature of a molecular transition as a function of the molecular column density (normalized by the line width) and H$_2$ volume density. Considering that $T_{\rm k}$ $\approx$ $T_{\rm exc}$ at densities higher than 10$^{4}$ cm$^{-3}$ [@hay99], and that $T_{\rm exc}$ = 57.2 K (see Table \[tabla:propiedades\]) we adopted kinetic temperatures in the range 50 K $\leq$ $T_{\rm k}$ $\leq$ 80 K which are also typical temperatures derived for molecular clouds close to OB associations [@oh10]. Nevertheless, we further find that the derived densities are relatively insensitive to $T_{\rm k}$ in this range. We use the $^{13}$CO molecule adopting a canonical abundance \[$^{13}$CO\]/\[H$_2$\] = 2 $\times$ 10$^{-6}$ [@d78]. For the case of HCN, its abundance is less certain making it the main source of error. We adopted the abundance range derived for Orion-KL (\[HCN\]/\[H$_2$\] = 2 $\times$ 10$^{-8}$ - 2 $\times$ 10$^{-9}$; @sch92).

In Fig. \[fig:lvg\] we show the 50 $\times$ 50 model grids of , and  over a volume density range $n$(H$_2$) = 10$^3$ - 10$^8$ 3. For the  line, the normalized column density ranges are $N^{13}$(CO)/$\Delta v$ = 10$^{13}$ - 10$^{18}$ cm$^{-2}$ ()$^{-1}$. At these high densities, the $^{13}$CO line is in collisional equilibrium and is almost independent of $n$(H$_2$). The HCN, however, has higher critical density and its line strength can be used to diagnose $n$(H$_2$) in this range. The dotted line in each panel of Fig. \[fig:lvg\] shows the range of values where the solutions for both molecular species coincide, indicating that volume density in clump C4 is in the range 4 $\times$ 10$^{4}$ to 5.6 $\times$ 10$^{5}$ cm$^{-3}$. These values are higher than that obtained from the LTE assumption ($n$(H$_2$)$_{\rm LTE}$ $\sim$ 1 $\times$ 10$^4$ cm$^{-3}$), although the latter is highly dependant on distance and geometry (assumed to be spherical) and a comparison might not be conclusive. We have used in the LVG analysis HCN abundances obtained in Orion-KL, although lower abundances were derived for a number of galactic molecular clouds (e.g. 0.6 $\times$ 10$^{-10}$, @joh03; 7 $\times$ 10$^{-10}$, @ten06) which would highly shift the HCN lines up relative to $^{13}$CO, implying higher densities ($>$ 10$^{6}$ cm$^{-3}$). It is well accepted that volume densities higher than 10$^5$ cm$^{-3}$ are critical for the initial condition of stellar formation [@elme02]. Further, two candidate YSOs were identified close to the HCN emission peak and projected onto the center of C4 (see Fig. \[yso-hcn-870\] in Sect. 3.6) which suggests that star formation process may be occurring inside this dense clump. In order to obtain the $^{13}$CO column density, we multiplied the inferred value of $N^{13}$(CO)/$\Delta v$ by the line width, which yields to $N^{13}$(CO) $\sim$ 2.4 $\times$ 10$^{16}$ cm$^{-2}$.

Continuum dust emission
-----------------------

Optically thin sub-millimeter continuum emission at 870 $\mu$m is usually dominated by the thermal emission from cold dust, which is contained in dense material (e.g. dense molecular cores or filaments).

  --------------- ------------------ ----------------- --------------- ----------------- -------------------------------- -------------------------------- -------------------------------- -----------------------
    Dust clump            RA            Dec.(J2000)     $R_{\rm eff}$      $S_{870}$      $M_{\rm dust}$(20 K)$^{\ddag}$   $M_{\rm dust}$(30 K)$^{\ddag}$   $M_{\rm dust}$(40 K)$^{\ddag}$   $^{13}$CO counterpart
                   ($^h$ $^m$ $^s$)        (  )           (arcsec)           (Jy)                       ()                               ()                               ()                
       D1           11 : 01 : 17.4    $-$59 : 51 : 36        21         1.85 $\pm$ 0.22          1.59 $\pm$ 0.19                  0.91 $\pm$ 0.11                  0.63 $\pm$ 0.08                    C4
       D2           11 : 01 : 12.6    $-$59 : 51 : 36        23         2.13 $\pm$ 0.26          1.84 $\pm$ 0.22                  1.05 $\pm$ 0.13                  0.73 $\pm$ 0.09                    C4
       D3           11 : 01 : 05.2    $-$59 : 51 : 36        14         0.58 $\pm$ 0.09          0.50 $\pm$ 0.08                  0.29 $\pm$ 0.04                  0.20 $\pm$ 0.03                    C3
       D4           11 : 01 : 08.7    $-$59 : 50 : 30        14         0.56 $\pm$ 0.10          0.48 $\pm$ 0.09                  0.28 $\pm$ 0.05                  0.19 $\pm$ 0.03                    C7
       D5           11 : 01 : 13.5    $-$59 : 40 : 00        14         0.39 $\pm$ 0.10     2.59 $\pm$ 0.68$^{(\dag)}$       1.46 $\pm$ 0.37$^{(\dag)}$       1.02 $\pm$ 0.26$^{(\dag)}$              C10
       D6           11 : 01 : 05.5    $-$59 : 47 : 48         9         0.15 $\pm$ 0.04     1.00 $\pm$ 0.23$^{(\dag)}$      0.56 $\pm$ 0.15 $^{(\dag)}$       0.39 $\pm$ 0.10$^{(\dag)}$              C11
   \[table:870\]                                                                                                                                                                            
  --------------- ------------------ ----------------- --------------- ----------------- -------------------------------- -------------------------------- -------------------------------- -----------------------

[Notes: $({\dag})$ Values obtained adopting a distance of $\sim$ 8 kpc. [**$(\ddag)$ Dust mass derived from the continuum emission at 870 $\mu$m.**]{}]{}

In the upper panel of Fig. [\[870mi\]]{} we display the image at 870 $\mu$m extracted from ATLASGAL. The image shows an extended source centered at  = (11$^h$01$^m$11$^s$, $-$5951). The brightest section of the source extends along Dec.(J2000) = $-$595136, from RA. = 11$^h$01$^m$01$^s$ to 11$^h$01$^m$22$^s$.

![[*Upper panel:*]{} 870 $\mu$m continuum emission map from ATLASGAL. The grayscale goes from 50 to 500 . Contour levels correspond to 150 ($\sim$ 2.5 rms), 300, 500, and 700 . [*Bottom panel:*]{} Overlay of IRAC image at 8 $\mu$m and the coutour levels of the upper panel. The grayscale goes from 10 to 180 MJy ster$^{-1}$. []{data-label="870mi"}](870-irac.eps){width="250pt"}

Three clumps can be easily identified, being brighter the one to the east, and fainter the one to the west. Their positions are  = (11$^h$01$^m$17.4$^s$, $-$595136),  = (11$^h$01$^m$12.6$^s$, $-$595136), and  = (11$^h$01$^m$05.2$^s$, $-$595136), indicated in the upper panel of Fig. \[870mi\] as D1, D2, and D3, respectively. A fainter clump, named as D4, is placed at  = (11$^h$01$^m$08.7$^s$, $-$595030). The four dust clumps are immersed in a faint plateau of emission.

A comparison of the image at 870 $\mu$m and the $^{13}$CO images of Fig. 3 shows that D1 and D2 are the dust counterparts of C4, while D3 partially coincides with C3, and D4 with C7. Also note that the region of low molecular emission present at  = (11$^h$01$^m$06.3$^s$, $-$595055), between C3 and C7, does not show emission at 870 $\mu$m. Clearly, the dust emission is the counterpart of the molecular emission. D1 and D2 are also detected in the HCN line (see Fig.\[yso-hcn-870\] in Sect. 3.6).

Two additional patches of emission are detected at 870 $\mu$m: one at  = (11$^h$01$^m$13.5$^s$, $-$594900) (D5) and the other at  = (11$^h$01$^m$05.5$^s$, $-$594748) (D6). Clumps D5 and D6 seem to be the dust counterparts of the molecular clumps C10 and C11, detected in the velocity interval $+$18.7 to $+$21.7   (see Fig. \[fig:mosaico\]). In particular, D5 coincides with a faint source detected in the radio continuum at 4800 MHz and a bright source at 8 $\mu$m (see Fig. \[fig:ir-cont\]). The 2MASS candidate young stellar objects $\#$1 and $\#$12 from Paper I coincide with this region, suggesting that this is a candidate star forming region. Surprisingly, the emissions at both 870 $\mu$m and $^{13}$CO extend slightly to the north-west, opposite to the position of the central cavity of NGC3503 and the exciting stars, suggesting a relation to these object. However, as mentioned in Sect. 3.1, the circular galactic rotation model predicts for a velocity of $\sim$ +20  distances of about 8 kpc, far away from NGC 3503. As regards D6, it displays an arc-shaped morphology encircling a point like source detected at 4.5 and 8 $\mu$m (labeled in Sect. 3.6 as 2MASS candidate YSO \#22 and WISE candidate YSO \#65).

Table \[table:870\] lists flux densities and masses of the dust clumps. Dust mass estimates ($M_{\rm dust}$) were obtained using Eq. \[eq:mdust\] considering a conservative dust temperature range between 20 to 40 K. Values in the range 20 - 30 K are assumed for cold clumps and protostellar condensations [@joh06; @deh09], while the last value was derived from the emissions at 60 and 100 $\mu$m (see Paper I). For dust clumps D5 and D6 we have adopted a distance of 8 kpc, in common with molecular clumps C10 and C11, although a physical association with   cannot be discarded without having information of the velocity of the ionized gas of MSX source \#1.

A new identification of candidate YSOs 
---------------------------------------

As pointed out in Sect. 1, a search for candidate YSOs in the environs of   was performed in Paper I using the IRAS, MSX, and 2MASS point source catalogs. To accomplish a more complete study, we have extended the search to a larger area around the nebula and we have included new data.

![image](fuentes_y_emision3.eps){width="530pt"}

We intend to study the star formation in the vicinity of   by detecting all the candidate YSOs (intrinsically reddened) and analyzing their position with respect to the dust, ionized gas, and molecular gas.

In this new analysis we have used again the 2MASS catalog [@cu03], which provides detections in $J$, $H$ and $K_s$ bands, to search for point sources with infrared excess. We have selected only sources with signal-to-noise ratio (S/N) $>$ 10 (quality “AAA”) and followed the criteria of @co05, to determine the parameter . Then, sources with $q <$ $-$0.15 (i.e. sources with IR excess) were identified as candidate YSOs. In Table \[yso-2mass\] we have listed the 2MASS candidate YSOs identified with the method explained above. For completeness, we have also included in the table the three MSX Cregion candidates reported in Paper I. For the sake of clarity, the numerical identification up to source \#12 is compatible to that of Paper I. The spatial distribution of 2MASS and MSX sources is depicted in Fig. \[fig-yso\]. As can be seen from this figure, from a total of 61 sources detected in the field, only 10 sources are projected onto the IR counterpart of  or its central cavity, namely: \#4, \#5, \#6, \#7, \#8, \#9, \#11, \#12, and \#22 (for completeness, we have included in Fig. \[fig-yso\] all the 2MASS candidate YSOs detected in the studied area). Sources \#4 to \#12 were previously reported in Paper I, as possibly associated with the nebula. Sources \#4, \#5, \#6, \#7 are projected towards the central cavity of , and lay inside the radio continuum counterpart of the nebula (see Fig. \[fig:ir-cont\]) while sources \#8, \#9, and \#11 lay onto the IR arc towards the eastern and northern sections of IR the nebula. Low-intensity radio continuum emission is also seen projected onto these sources (see Fig. \[fig:ir-cont\]). Source \#22 is coincident with a point-like source detected in the four IRAC bands, and correlates with the emission at 870 $\mu$m of D6 (see Figs. \[870mi\] and \[fig-yso\]) and clump C11, suggesting that this candidate YSO is buried in a “cocoon” of cold dust and dense gas. Its WISE counterpart can be classified as a Class-I candidate (source \#65; see below). None of the above mentioned 2MASS sources are seen projected onto the molecular gas associated with the nebula at velocities between –27.8 to –23.7  or –17.1 to –15.3 (see Fig. \[fig-yso\]). 2MASS sources \#15, \#16, \#29, \#34, \#38, \#40, \#45, and \#50) appear projected onto the north-western and western borders of the molecular gas in the velocity range from –27.8 to –23.7 far from the nebula, thus, a physical association with the molecular environment of  is uncertain. Source \#32 is projected onto the molecular component at velocities between –17.1 to –15.3 (clump C9).

Candidate YSOs are usually classified, according to their evolutionary phase, into two standard categories: 1) Class-I YSO which are young protostellar sources embedded in dense infalling envelopes of gas and dust, and 2) Class-II YSO which are pre-main-sequence sources whose emission originates mainly in dense optically thick diks around the protostar (candidate T Tauri stars). These sources exhibit an infrared excess that cannot be attributed to the ISM along the line of sight, but rather to the envelope and/or the disk surrounding the protostar. Class-III sources are usually referred to as pre-main-sequence (or main-sequence) field stars. With the aim of identifying new candidate YSOs associated with , we have included in our analysis photometric data from the [*Wide-field Infrared Survey Explorer*]{} (WISE; @wr10) obtained from the IPAC database[^8]. This survey maps the whole sky in four bands centered at 3.4, 4.6, 12, and 22 $\mu$m. We have used the criteria of @koe12, biased against the youngest and least massive candidate YSOs, as follows:

  -------- -------------------------- ------------------- --------- ---------- ---------- ----------
   Source                                 MSX source        F$_8$    F$_{12}$   F$_{14}$   F$_{21}$
     No        ($^h$ $^m$ $^s$, )                           (Jy)       (Jy)       (Jy)       (Jy)
     1      11 01 10.77, –59 49 13.9   G289.4859+00.1420   1.49250    1.8145     0.9005     0.3180
     2      11 01 11.18, –59 50 34.3   G289.4993+00.1231   0.82699    1.4887     1.2237     4.3818
     3      11 01 13.57, –59 51 21.9   G289.5051+00.1161   0.87911    2.8632     1.6759     5.6330
                                         2MASS source        $J$       $H$        $K$     
    $\#$       ($^h$ $^m$ $^s$, )                           (mag)     (mag)      (mag)    
     4      11 01 19.34, -59 49 42.2   11011934-5949422    13.277     13.047     12.806   
     5      11 01 20.21, -59 49 12.3   11012021-5949123    15.084     14.837     14.506   
     6      11 01 19.33, -59 50 03.7   11011933-5950037    14.539     14.100     13.696   
     7      11 01 23.29, -59 49 26.9   11012329-5949269    14.862     14.380     14.027   
     8      11 01 35.91, -59 51 03.7   11013591-5951037    15.237     14.592     14.106   
     9      11 01 32.63, -59 51 14.0   11013263-5951140    15.633     14.946     14.404   
     10     11 01 22.38, -59 52 44.9   11012238-5952449    12.747     11.744     10.836   
     11     11 01 32.38, -59 49 33.9   11013238-5949339    14.754     14.243     13.873   
     12     11 01 13.90, -59 49 09.9   11011390-5949099    14.888     13.661     12.827   
     13     11 01 30.41, -59 52 00.2   11013041-5952001    13.110     13.006     12.857   
     14     11 00 35.64, -59 53 14.1   11003564-5953141    15.385     14.976     14.512   
     15     11 00 38.07, -59 46 06.3   11003807-5946063    14.239     13.649     13.229   
     16     11 00 41.91, -59 45 53.5   11004191-5945535    15.421     14.638     14.128   
     17     11 00 32.05, -59 50 12.6   11003205-5950126    14.676     14.123     13.693   
     18     11 01 42.92, -59 51 52.9   11014292-5951529    15.091     14.775     14.497   
     19     11 00 35.99, -59 45 56.6   11003599-5945566    15.347     14.712     14.238   
     20     11 00 57.48, -59 52 32.2   11005748-5952322    15.124     14.855     14.472   
     21     11 01 46.19, -59 52 17.1   11014619-5952171    15.145     14.657     14.284   
     22     11 01 05.71, -59 47 53.4   11010571-5947534    15.142     12.941     11.305   
     23     11 00 56.09, -59 47 03.5   11005609-5947035    15.796     15.042     14.527   
     24     11 00 35.10, -59 48 39.7   11003510-5948397    14.950     14.499     14.114   
     25     11 02 07.01, -59 52 17.1   11020701-5952170    12.534     12.421     12.269   
     26     11 00 35.21, -59 48 43.5   11003521-5948435    15.413     14.571     14.017   
     27     11 01 50.30, -59 53 55.2   11015030-5953552    13.501     13.435     13.264   
     28     11 01 33.09, -59 53 39.4   11013309-5953394    15.326     14.602     14.097   
     29     11 00 45.46, -59 48 42.5   11004546-5948425    14.941     14.390     13.905   
     30     11 01 31.53, -59 52 21.1   11013153-5952211    14.358     14.177     13.985   
     31     11 00 26.52, -59 49 11.2   11002652-5949111    15.785     15.019     14.418   
     32     11 01 36.49, -59 50 06.5   11013649-5950065    15.528     14.928     14.352   
     33     11 01 53.25, -59 52 19.4   11015325-5952194    14.362     14.091     13.856   
     34     11 00 57.88, -59 48 53.6   11005788-5948536    15.784     15.092     14.447   
     35     11 02 00.99, -59 50 41.7   11020099-5950417    15.593     14.831     14.246   
     36     11 00 31.45, -59 46 49.3   11003145-5946493    14.796     13.869     13.266   
     37     11 01 01.56, -59 52 41.3   11010156-5952413    15.020     14.670     14.388   
     38     11 00 46.66, -59 52 03.7   11004666-5952037    15.245     14.724     14.246   
     39     11 01 49.19, -59 53 02.6   11014916-5953026    14.233     14.117     13.899   
     40     11 00 46.77, -59 49 08.3   11004677-5949083    14.713     14.509     14.121   
     41     11 00 47.55, -59 52 53.9   11004755-5952539    15.062     14.778     14.451   
     42     11 02 09.58, -59 51 21.2   11020958-5951212    14.368     13.942     13.624   
     43     11 02 12.42, -59 53 19.2   11021242-5953192    14.666     14.477     14.227   
     44     11 01 33.21, -59 52 16.2   11013321-5952162    14.667     14.421     14.131   
     45     11 00 53.27, -59 50 14.1   11005327-5950141    15.318     14.585     14.094   
     46     11 02 13.44, -59 50 49.5   11021344-5950495    14.549     14.443     14.244   
     47     11 00 32.75, -59 45 52.0   11003275-5945520    14.577     14.018     13.628   
     48     11 00 45.93, -59 53 36.3   11004593-5953363    13.534     13.232     12.983   
     49     11 01 45.83, -59 52 41.9   11014583-5952419    14.245     13.271     12.580   
     50     11 00 40.21, -59 46 50.0   11004021-5946500    10.339     10.258     10.085   
     51     11 02 00.06, -59 52 12.1   11020005-5952121    15.819     14.957     14.206   
     52     11 00 29.66, -59 46 05.5   11002966-5946055    13.880     12.898     12.177   
     53     11 01 43.81, -59 46 41.8   11014381-5946418    15.025     14.770     14.478   
     54     11 01 35.32, -59 48 06.6   11013532-5948066    14.097     13.781     13.520   
     55     11 01 12.65, -59 46 57.3   11011265-5946573    14.795     14.540     14.239   
     56     11 01 46.02, -59 48 44.6   11014602-5948446    13.965     13.863     13.653   
     57     11 01 25.70, -59 45 51.8   11012570-5945518    14.413     14.381     14.241   
     58     11 01 30.72, -59 45 59.9   11013072-5945599    13.575     13.327     13.071   
     59     11 02 01.50, -59 47 35.2   11020150-5947352    13.610     13.485     13.333   
     60     11 02 00.12, -59 47 45.5   11020012-5947455    14.893     14.482     14.131   
     61     11 02 03.37, -59 48 47.4   11020336-5948474    15.734     15.127     14.512   
     62     11 01 18.81, -59 47 00.8   11011881-5947008    15.467     14.630     14.039   
     63     11 01 18.92, -59 46 14.8   11011892-5946148    15.156     14.839     14.500   
     64     11 01 25.91, -59 46 34.5   11012591-5946345    14.337     14.136     13.938   
  -------- -------------------------- ------------------- --------- ---------- ---------- ----------

  -------- ------- -------------------------- --------------------- --------- --------- ---------- ---------- --------------------
   Source   Class                                  WISE source       \[3.4\]   \[4.6\]   \[12.0\]   \[22.0\]     Matching with
     No               ( $^h$ $^m$ $^s$, )                             (mag)     (mag)     (mag)      (mag)          MSX and
                                                                                                                 2MASS sources
     65       I     11 01 05.73, -59 47 53.5   J110105.73-594753.5    9.574     8.360     5.252      2.904          $\#$ 22
     66       I     11 01 30.79, -59 50 11.6   J110130.79-595011.6   12.473    11.446     6.191      5.443    
     67       I     11 01 20.38, -59 50 10.9   J110120.35-595010.9   12.500    11.435     6.332      3.443    
     68       I     11 01 22.08, -59 50 17.1   J110122.01-595017.1   13.019    11.657     6.277      2.017    
     69       I     11 01 10.86, -59 48 57.3   J110110.86-594857.3   11.708    10.446     5.400      2.790    
     70       I     11 01 38.74, -59 50 04.4   J110138.74-595004.4   13.295    11.985     8.423      4.506    
     71       I     11 01 13.50, -59 51 23.8   J110113.50-595123.8    8.839     7.820     3.518      0.195       $\sim$ $\#$ 3
     72       I     11 01 14.33, -59 49 12.8   J110114.33-594912.8    9.465     8.460     3.535      0.440     $\#$ 12 and $\#$ 1
     73       I     11 01 13.81, -59 49 25.5   J110113.81-594925.5   10.826     9.792     5.056      5.581    
     74       I     11 01 21.30, -59 50 27.3   J110121.30-595027.3   13.165    11.902     7.195      4.070    
     75       I     11 01 19.13, -59 50 03.9   J110119.13-595003.9   12.388    11.244     6.155      2.784           $\#$ 6
     76       I     11 01 05.73, -59 51 39.2   J110105.73-595139.2   11.520    10.477     5.249      2.930    
   source   Class                                  WISE source       \[3.4\]   \[4.6\]   \[12.0\]   \[22.0\]  
    $\#$              ( $^h$ $^m$ $^s$, )                             (mag)     (mag)     (mag)      (mag)    
     77      II     11 01 33.82, -59 51 24.7   J110133.82-595124.7   11.083    10.696     6.622      3.169    
     78      II     11 01 24.80, -59 49 51.6   J110124.80-594951.6   13.012    12.209     7.309      3.537    
     79      II     11 01 37.36, -59 53 01.9   J110137.36-595301.9   13.755    13.277     8.518      6.706    
     80      II     11 01 23.27, -59 49 38.3   J110123.27-594938.3   12.510    11.852     7.009      3.912    
     81      II     11 01 22.49, -59 48 39.8   J110122.49-594839.8   12.270    11.805     7.058      3.972    
     82      II     11 01 21.60, -59 50 53.3   J110121.60-595053.3    8.906     8.003     5.084      1.147    
     83      II     11 01 31.44, -59 50 38.8   J110131.44-595038.8   11.325    10.853     6.003      3.915    
     84      II     11 01 22.38, -59 52 44.8   J110122.38-595244.8    9.631     9.046     5.964      3.402          $\#$ 10
     85      II     11 01 17.35, -59 51 22.1   J110117.35-595122.1    9.120     8.162     3.628      1.149    
     86      II     11 00 29.67, -59 46 05.5   J110029.67-594605.5   11.125    10.635     8.156      5.558          $\#$ 52
     87      II     11 01 30.65, -59 48 47.4   J110130.65-594847.4   13.315    12.992     10.876     6.954    
     88      II     11 01 25.62, -59 49 39.3   J110125.62-594939.3   11.862    11.122     7.408      4.760    
     89      II     11 01 24.25, -59 51 33.1   J110124.25-595133.1    9.862     9.281     4.447      4.481    
     90      II     11 01 28.54, -59 49 13.4   J110128.54-594913.4   11.943    11.598     6.835      4.735    
     91      II     11 01 17.62, -59 49 52.0   J110117.62-594952.0   10.820    10.080     6.688      1.612    
     92      II     11 01 33.53, -59 53 37.5   J110133.53-595337.5   11.280    10.437     5.819      3.931          $\#$ 28
     93      II     11 01 15.75, -59 50 29.3   J110115.75-595029.3    9.352     8.891     4.124      1.614    
     94      II     11 01 31.93, -59 49 59.2   J110131.93-594959.2   10.784    10.340     6.082      4.162    
     95      II     11 01 11.41, -59 49 09.6   J110111.41-594909.6   10.341     9.843     5.005      2.870    
     96      II     11 01 17.84, -59 50 52.4   J110117.84-595052.4    8.748     8.193     5.036      5.552    
     97      II     11 01 45.82, -59 52 42.0   J110145.82-595242.0   11.722    11.118     9.209      6.565          $\#$ 49
     98      II     11 01 15.46, -59 50 12.4   J110115.46-595012.4    9.949     9.430     4.714      1.120    
     99      II     11 01 43.54, -59 48 06.1   J110143.54-594806.1   12.745    11.920     7.143      5.265    
    100      II     11 01 45.01, -59 48 06.0   J110145.01-594806.0   12.976    12.318     7.502      5.598    
  -------- ------- -------------------------- --------------------- --------- --------- ---------- ---------- --------------------

after removing contamination arising from background objects like galaxies (very red in \[4.6\]$-$\[12\]), broad-line active galactic nuclei (of similar colours as YSOs, but distinctly fainter) and resolved PAH emission regions (redder than the majority of YSOs), we identified infrared excess sources demanding that $$\begin{array}{ll}
[3.4] - [4.6] - \sigma_1 & > 0.25\\

[4.6] - [12.0] - \sigma_2  & > 1.0

\end{array}$$ where \[3.4\], \[4.6\], and \[12.0\] are the WISE bands 1, 2, and 3 magnitudes, respectively, and $\sigma_1 = \sigma([3.4]-[4.6])$ and $\sigma_2 = \sigma([4.6]-[12.0])$ indicate the combined errors of \[3.4\]$-$\[4.6\] and \[4.6\]$-$\[12.0\] colors, added in quadrature. Class I sources are a sub-sample of this defined by $$\begin{array}{ll}
[3.4] - [4.6] & > 1.0 \\

[4.6] - [12.0] & > 2.0

\end{array}$$ (the rest are Class II objects). For the method explained above, we have considered sources with error in magnitudes lower than 0.2 mag in bands 1, 2, and 3.

![ WISE band 1,2, and 3 color-color diagram showing the distribution of Class-I (in red) and Class-II (in green) candidate YSOs. []{data-label="hist-cc"}](wise-classI-II.eps){width="240pt"}

In Table \[yso-wise\] we list the candidate YSOs identified with the method explained above. For the sake of clarity, the numerical identification is succedent to that of Table \[yso-2mass\]. We found a total of 36 sources (12 Class-I and 24 Class-II candidates). In Fig.\[hist-cc\] we show the WISE band 1,2, and 3 color-color diagram depicting the distribution of sources listed in Table \[yso-wise\]. The spatial location of Class-I and Class-II candidates is illustrated in Fig. \[fig-yso\]. Unlike 2MASS sources, the spatial distribution of WISE sources is rather concentrated toward the nebula. Sources \#67, \#68, \#74, \#75 (which is coincident with 2MASS source \#6), \#78, \#80, \#88, and \#91 are projected toward the center of the nebula and its radio continuum emission (see Fig.\[fig:ir-cont\]), while sources \#66, \#77, \#81, \#83, \#90, and \#94 are seen projected onto the IR arc. Sources \#72 and \#73 are almost coincident with the MSX C region candidate \#1 and 2MASS candidate YSO \#12. Source \#70 is projected close to the peak of emission of the molecular component at velocities between –17.1 to –15.3  (clump C9)

None of the WISE candidate YSOs sources mentioned so far are observed in the direction of any molecular clump detected in the velocity range from $-$27.8 to $-$23.7 . Differently, three protostellar sources, \#71, \#76, and \#85, are observed in the direction of $^{13}$CO emission peaks corresponding to clumps C4 and C3, which are the densest clumps (see Sect. 3.2). In order to better trace the spatial distribution of the gas/dust and candidate YSOs in this region, we present in Fig.\[yso-hcn-870\] a composite image of HCN, C$^{18}$O, and 870 $\mu$m emission, superimposed on the location of WISE sources \#71, \#76, and \#85. Sources \#71 and \#85 are projected onto dust clumps D2 and D1, respectively, and outwardly a C$^{18}$O peak emission of seen at  = (11$^h$01$^m$15$^s$, $-$595120) (C$^{18}$O counterpart of clump C4). Source \#85 is almost projected onto the HCN peak at  = (11$^h$01$^m$16.7$^s$, $-$595123), while the position of source \#71 is close to the HCN peak seen at  = (11$^h$01$^m$11.6$^s$, $-$595123.1) and the central position of the MSX C region candidate \#3. This very likely suggests that these sources were formed inside high density molecular cores. As for source \#76, this object is seen projected close to the direction of a C$^{18}$O peak seen at  = (11$^h$01$^m$2.5$^s$, $-$595015) (very likely the C$^{18}$O counterpart of clump C3) and coincident with the dust clump D3 seen at 870 $\mu$m. This protostellar source is also projected onto the weaker HCN structure located at  = (11$^h$01$^m$05$^s$, $-$595140). As suggested in Sect. 3.2, clump C4 is likely exposed to extra compression, heating, and ionization from  and , which might have triggered/enhanced the formation of protostellar candidates \#71 and \#85 (and probably \#76) in the densest regions of the molecular gas. In that matter, it is also worth to stress the location of WISE sources \#69, \#82, \#89, \#93, \#95, \#96, and \#98, which are spatially aligned perfectly following the eastern border of molecular clumps C4, C6, and C7. The location of these protostellar sources suggests that they are linked to the collected layers of molecular gas due to the expansion of the ionization front of  over its molecular environment, which has probably aided the stellar formation activity along the external borders of the molecular gas.

To determine whether the fragmentation of the collected layer via “collect and collapse” process might have triggered the star formation in the environs of , in Paper I we made use of the analytical model of @wi94 for expanding  regions. We obtained that $t_{\rm frag}$ $\sim$ 3.5 10$^6$ yr and $R_{\rm frag}$ $\sim$ 7.5 pc, which are considerably larger than the age, $t_{\rm dyn}$, and size, $R_{\rm  HII}$, of   (derived assuming a classical expanding  region; @dy97). This lead us to conclude that fragmentation of the collected layer triggered by the expansion of the nebula is doubtful. We keep in mind, however, that newer simulations have shown that accretion and ionization may occur simultaneously for compact  regions, which makes their size unrelated on their age until late in their lifetimes [@pet10].

![Composite image showing the emission of the  line (green contours), the  line (white/black contours), and the 870 $\mu$m continuum emission (grayscale) in the central region of . The velocity ranges are $-$26.2 to $-$25.2  and $-$26.5 to $-$25.5  for the C$^{18}$O and HCN lines, respectively. Red triangles depict the position of WISE candidate YSOs \#71, \#76, and \#85. The C$^{18}$O couterpart of clumps C3, C4, and C7 are indicated. The beam sizes are shown in the lower left corner.[]{data-label="yso-hcn-870"}](ysos-hcn-870.eps){width="260pt"}

Given the lack of certainty of the latter scenario, we have considered alternative approaches: probably, the formation of YSOs lying at the border of the  region (sources \#69, \#82, \#89, \#93, \#95, \#96, and \#98) and inside clumps C4 and C3 (sources \#71, \#76, and \#85) results from an interaction of the ionization front of  (and ) with pre-existing gravitationally bound molecular condensations (C3, C4, and C7; see Sect. 3.2), which has enhanced the formation of the protostellar objects (RDI process; @lela94). Also, the formation of the protostellar candidates may be the result of small-scale Jeans gravitational instabilities in the collected layers of molecular gas (e.g. [@pom09; @par11]). Higher spatial resolution molecular observations might help to shed some light on these issues.

Summary
=======

Using APEX , , , and  line data, and ATLASGAL 870 $\mu$m images, we carried out a multifrequency study of the molecular gas and dust associated with the  region/star forming region . To analyze the star formation process in the region we made use of the WISE and 2MASS data obtained from the IPAC archive. This work is a follow-up study of @dvcca12. The main results can be summarized as follows:

1.  The ionized gas of   is expanding against the molecular gas component in the velocity range from $\sim$ $-$28 to $-$23 . The morphology of the molecular gas close to the nebula, the location of the PDR, and the shape of radio continuum emission confirm the “champagne flow” scenario proposed in Paper I.

2.  New APEX observations allowed the small scale structure of the molecular gas associated with the nebula (previously reported in Paper I) to be fully imaged. We identified several molecular clumps (C1 to C11) and studied their physical and dynamical properties to investigate the impact of the expanding nebula and/or the southern bright rimmed cloud  onto the molecular gas.

3.  We found that warmer clumps (C3, C4, and C7) are close to the  region, which is indicative of an external heating source, most probably by photoionization of their surface molecular layers by the intense UV field of Pis 17. Warmer clumps are also denser, which suggests that they are submitted to an external compression due to the expansion of . These clumps are likely molecular gas of the parental cloud that has been swept up by the expansion of the ionization front and has been condensed. Clump C4 is also adjacent to , which might explain its highest temperature and density. A noticeable velocity gradient is detected in C4. This gradient may indicative of a kinematical disturbance, or else, be the consequence of two molecular cores at slightly different velocity that are not fully resolved in the carbon monoxide emissions.

4.  Clumps located near to  also have the highest LTE and virialized masses. They also exhibit the highest $M_{\rm LTE}/M_{\rm vir}$ ratio (predominantly $>$1), which, according to the classical interpretation, indicates that they are gravitationally bound.

5.  All molecular clumps exhibit line broadening beyond the thermal width, which possibly indicates that in some cases the line widths of the composite spectrum of the clumps have different velocities, rather than showing turbulence effects. Also, spatially unresolved process like ouflows, infall, accretion, etc. might be contributing in the broadening.

6.  We have analyzed the 870 $\mu$m emission, characteristic of filaments and dense molecular cores. We detected emission only in the direction of clumps C3, C4, and C7. This is also indicative of high density gas.

7.  We have presented some evidence of stellar formation in the region by detecting sources with IR excess. Unlike 2MASS candidate YSOs, WISE Class-I and Class-II candidates show a spatial distribution concentrated to the IR nebula. Several sources are detected along the external border of two of the densest molecular clumps (C4 and C7) which suggests that they might be formed in the compressed layers of molecular gas. Three sources are projected close to HCN, C$^{18}$O, and 870 $\mu$m emission peaks (coincident with the position of clumps C4 and C3). This very likely indicates that they were born inside high density molecular cores, making clumps C4 and C3 excellent candidates to further investigate star formation with higher spatial resolution instruments like ALMA.

8.  Since the dynamical age and fragmentation time derived for the molecular layer differ to the age and radius of the nebula (Paper I) we have excluded the “collect and collapse” scenario for the YSO formation. Instead, we have proposed here some alternative mechanisms, such as “radiative-driven implosion” in pre-existing gravitationally bound clumps (C3, C4, and C7), or small-scale Jeans gravitational instabilities in the swept-up layers of molecular gas.

We acknowledge the anonymous referee for his/her helpful comments that improved the presentation of this paper. This project was partially financed by CONICET of Argentina under projects PIP 112-800201-01299 and PIP 02488 and, UNLP under project 11/G120, and CONICYT Proyect PFB06. This research has made use of the NASA/ IPAC Infrared Science Archive, which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. This work is based \[in part\] on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA. This publication makes use of data products from the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation. The MSX mission is sponsored by the Ballistic Missile Defense Organization (BMDO).

[^1]: Radial velocities in this paper are always referred to the local standard of rest ([LSR]{})

[^2]: APEX is a collaboration between the Max-Planck-Institut fur Radioastronomie, the European Southern Observatory, and the Onsala Space Observatory

[^3]: http://www-wfau.roe.ac.uk/sss/index.html

[^4]: http://www.iram.fr/IRAMFR/GILDAS

[^5]: http://sha.ipac.caltech.edu/applications/Spitzer/SHA/

[^6]: Obtained with ATCA (see Paper I)

[^7]: http://www.cfa.harvard.edu/sma/miriad/packages/

[^8]: http://irsa.ipac.caltech.edu/cgi-bin/Gator/nph-dd