Carbon-Chain Chemistry around Massive Young Stellar Objects– Revealing the Origins of Chemical Diversity –Kotomi Taniguchi,1 Kei E. I. Tanaka,2,3 & Masao Saito3,4
1

Department of Physics, Faculty of Science, Gakushuin University, 1–5–1 Mejiro, Toshima-ku, Tokyo 171–8588, Japan
2
Center for Astrophysics and Space Astronomy, University of Colorado Boulder, Boulder, CO 80309
3
National Astronomical Observatory of Japan, 2–21–1 Osawa, Mitaka-shi, Tokyo 181–8588
4
Department of Astronomical Science, School of Physical Science, SOKENDAI (The Graduate University for Advanced Studies), 2–21–1
Osawa, Mitaka-shi, Tokyo 181–8588, Japan
kotomi.taniguchi@gakushuin.ac.jp

Abstract
Recent radio astronomical observations have found chemical diversity around massive young stellar objects(MYSOs). Such chemical diversity is likely related to the variety of environments and/or processes during thestarless core phase or the later protostellar phase. Hence, it is important for the understanding of the formation processes of massive stars to investigate the characteristics and origins of the chemical diversity around MYSOs. High−1
angular-resolution (<∼ 1000 mas) and high velocity-resolution (∼ 0.2 km s ) observations of unsaturated carbonchain species and saturated complex organic molecules (COMs) by the next generation Very Large Array (ngVLA)
will draw their spatial distributions of abundance and excitation condition, kinematics of envelopes and inner hot gas,
and the heritage of the chemical composition from envelopes to disk structures around MYSOs. Such observations
will shed light on puzzles about complex chemistry around MYSOs.
Key words: Astrochemistry — ISM: molecules — stars: massive
1. Introduction
1.1. The diversity of carbon chemistry around massive youngstellar objects
Approximately 200 molecules have been detected in the interstellar medium and circumstellar shells so far. The chemicalcomposition is a useful tool to investigate star formation processes, and evolutionary stages and physical conditions in starforming regions (e.g., Caselli & Ceccarelli 2012; Jørgensen etal. 2020). Hence, it is important to reveal relationships between
the chemical composition and physics in star-forming regions.
Notable, carbon is one of the essential elements in the universe,
as it is the pivotal element of organic molecules including prebiotic molecules.
Saturated complex organic molecules (COMs) have beenfound to be abundant around massive young stellar objects(MYSOs), namely hot core chemistry (e.g., Herbst & van
Dishoeck 2009). A similar chemical character was foundaround low-mass young stellar objects (YSOs) and named hot
corinos. These COMs are mainly formed on dust surface during the cold starless and lukewarm protostellar phases (e.g.,
Garrod & Herbst 2006).
Another type of molecules containing carbon abundantly isunsaturated carbon-chain species. Their chemistry has beenstudied in detail in low-mass star-forming regions. Carbonchain molecules are known as early-type species; they areabundant in young starless cores, and their abundances decrease in star-forming cores (e.g., Suzuki et al. 1992; Benson
et al. 1998). Carbon-chain molecules are efficiently formed byion-molecule reactions or exothermic neutral-neutral reactions
without reaction barriers involving C+ and/or C in the cold gas
in starless cores (∼ 10 K).

In contrast to the classic carbon-chain chemistry in cold starless cores, it was found that carbon-chain molecules are anew
formed around low-mass YSOs from CH4 sublimated from ice
mantles at the warm temperatures around 25 K. This carbonchain formation around low-mass YSOs was named “warmcarbon-chain chemistry” (WCCC; Sakai & Yamamoto 2013).
In WCCC sources, COMs are relatively deficient compared tothose in hot corino sources, and it was suggested that there ischemical diversity around low-mass YSOs.
On the other hand, the carbon-chain chemistry in high-massstar-forming regions has been revealed only fairly recently(e.g., Taniguchi et al. 2017; Taniguchi et al. 2018a; Taniguchi
et al. 2018b; Taniguchi et al. 2018c; Taniguchi et al. 2019b).
It was found that HC5 N, the second shortest member of the
cyanopolyyne family (HC2n+1 N), is abundant in hot cores
and/or warm envelopes of their target MYSOs (Taniguchi etal. 2017; Taniguchi et al. 2018b). Cyanopolyynes are one of
the unsaturated carbon-chain series. In addition, Taniguchi etal. (2018b) found chemical diversity around MYSOs; COMpoor cores are surrounded by carbon-chain-rich envelopes,
while COM-rich cores, namely hot cores, are encompassedwith CH3 OH-rich envelopes. A subsequent study with theNobeyama 45-m radio telescope (Taniguchi et al. 2020) reported the CCH/HC5 N column density ratios around MYSOs
of ∼ 15, which is significantly lower than the ratio around theL1527 low-mass WCCC source (∼ 625). These results indicatethat carbon-chain molecules around MYSOs at least partiallyexist in higher temperature regions than those around low-mass
WCCC sources.
In order to investigate the formation mechanisms of
cyanopolyynes around MYSOs, chemical network simulations
have been conducted (Taniguchi et al. 2019a). The models un-

2[Vol. ,
Cold : ! 10 K

Warm : " 25-100 K(Formation in the gas phase,accretion onto dust grains,accumulation in ice mantles)
Hot : # 100 K(Ice-mantle sublimation andcyanopolyyne-rich gas)

Fig. 1. A schematic view of HCCC (Taniguchi et al. 2019a).

veiled a new intriguing path of the cyanopolyyne chemistry
in the warm and hot gas phases. Cyanopolyynes first formin the warm gas (25–100 K), but most of them are once absorbed and stored in ice mantles of dust grains. Later, as
the temperature increases to the ice sublimation temperatureof 100 K, the cyanopolyynes are released and show their gasphase peak abundances. Fig.1 shows a schematic view of thiscyanopolyyne chemistry around MYSOs. We call the carbonchain chemistry around MYSOs “hot carbon-chain chemistry(HCCC)” in this document.
In summary, carbon-chain molecules are formed anew
around MYSOs, and there is the chemical diversity around
MYSOs.
1.2. Possible factors producing the chemical diversity around
MYSOs
The chemical diversity should be related to the variety ofenvironments and/or formation processes when starless cores
form and after YSOs are born. If we consider a condition withidentical elemental abundances, there are three possible factors
producing the chemical diversity; (1) formation and destruction
mechanisms of each molecule, (2) different amounts of illumination from the interstellar radiation field (ISRF; Spezzano
et al. 2016) or cosmic-ray ionization, and (3) different heating timescale (Taniguchi et al. 2019a). In order to disentangle
these factors observationally, it is necessary to carry out highspatial-resolution and high velocity-resolution observations ofboth carbon-chain molecules and COMs toward MYSOs. The
next generation VLA (ngVLA) will be a particularly ideal observational facility, because it will enable us to study the above
factors.
2. Observations of carbon-chain molecules and COMs
around MYSOs with the ngVLA
2.1. General observational strategy
In order to investigate the characteristics of the chemical
diversity around MYSOs, it is essential to reveal spatial distributions of molecular abundance and excitation condition ofboth carbon-chain species and COMs around MYSOs. Suchobservations will be useful for the investigation of the effects
of formation and destruction mechanisms of each species on
the chemical diversity. It is generally known that carbonchain molecules are formed in the gas phase, whereas COMs
are formed mainly on dust surfaces. Some COMs could beformed in the gas phase (e.g., Balucani et al. 2015). Hence, thespatial distributions of abundance and excitation condition are
expected to be different among species with different formation/destruction mechanisms. Comparisons of their spatial distributions among different types of species in various MYSOs
could provide information about the carbon-chain chemistryaround MYSOs, or the HCCC mechanism (Taniguchi et al.
2019a).
Observations with the ngVLA will answer the effect of thedifferent heating timescale. The heating timescale (th ) depends
not only on spectral type of the central star but also on the infall velocity (Vinfall ) and the size of the warm region (Rwarm )(Taniguchi et al. 2019a). Here, “warm” means temperatures
around 25 – 100 K (see Fig.1). It can be considered that thwarm
is proportional to RVinfall(Aikawa et al. 2008). The ngVLA is
expected to draw the gas motion, i.e., Vinfall , even in dense regions where it is opaque in ALMA observations. The ngVLA
will also cover the NH3 lines at the 23 GHz band, a useful temperature probe. We can derive spatial distributions of the gas
kinetic temperature using the NH3 data. Combining these results, we can evaluate how long materials pass in warm regions
when they accrete onto the central hot regions. By this means,
we will assess the effects of the heating timescale on the chemical diversity. Furthermore, observations of the magnetic field
will be related to this study, because the magnetic field controls the infalling motion (Commerçon et al. 2011; Rosen &Krumholz 2020).
One good target source as a carbon-chain-rich source is
the G28.28–0.36 MYSO. The HC5 N/CH3 OH ratio towardG28.28–0.36 is higher than the other MYSOs by one order ofmagnitude (Taniguchi et al. 2018b). A unique characteristic of
this source is that the cosmic-ray ionization does not seem towork efficiently compared to other MYSOs and carbon-chain
species are formed by bottom-up mechanisms (Taniguchi et al.
2020). This feature enables us to investigate temperature dependences of formation and destruction mechanisms of carbonchain species and COMs and confirm the HCCC mechanism.
Cyanopolyynes (HC2n+1 N) are good candidates as target
carbon-chain molecules. For assessment of the effects of the
ISRF on the chemical diversity, it is necessary to carry out observations of carbon-chain species and COMs toward several
MYSOs located inside and outside of the same giant molecular clouds (GMCs) or clusters. If the ISRF is important for thechemical diversity around MYSOs, as suggested by Spezzano
et al. (2016), it is expected that MYSOs located outside of
GMCs are rich in carbon-chain species. However, we needto distinguish whether carbon-chain molecules are formed by
the HCCC mechanism or by the bottom-up formation involving C/C+ produced in photodominated regions (PDRs) in order to confirm the chemical diversity. It is necessary to confirm
that carbon-chain molecules are formed by the HCCC mechanism in order to claim the chemical diversity around MYSOs.
Although CCH traces PDRs, long cyanopolyynes, such as
HC7 N and HC9 N, are probably not enhanced in PDRs (Le Gal
et al. 2017). Hence, we will be able to distinguish whether
carbon-chain species are enhanced by the HCCC mechanism
or not with observations of long cyanopolyynes. As shown
later, the ngVLA can cover many lines of long cyanopolyynes.
For example, rotational lines of HC9 N from J = 36 − 35
(Eup = 18.5 K) to J = 86 − 85 (Eup = 104.3 K) are covered

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Fig. 2. Estimations column densities of cyanopolyynes and CH3 OH
based on hot core models with the fast warm-up period (5 × 104 yr)model (Taniguchi et al. 2019a). The orange covered range is used for
determining column densities.

by the ngVLA Bands 4 and 5.
In order to achieve these science goals, we need highangular-resolution and high velocity-resolution observations.
Because the typical size of hot core is ≈ 0.1 pc with the distance of 2–5 kpc, a required angular resolution is estimated at< 1000 mas. The velocity resolution of ∼ 0.2 km s−1 is needed
to investigate gas kinematics of envelopes and cores around
MYSOs.
2.2. Feasibility of observations with the ngVLA
We study the feasibility of observations of carbon-chain
molecules around MYSOs with the ngVLA. Regarding COMs,
other authors have already studied (Beltrán & Rivilla 2018).
Thus, we evaluate the observability of only cyanopolyynes
here.
In addition to the main isotopologues of cyanopolyynes,
we investigate the rare isotopologues. Since the deuterium
fractionation is an important factor to investigate evolutionary
stages and conditions of star-forming cores, we investigate thedeuterium species. The deuterium factor of HC3 N may provide
a hint of star-formation history from before cores form to after
protostars are born (Bianchi et al. 2019; Rivilla et al. 2020).
Our procedures for estimations of line intensities are as
follows. We derived the column densities of molecules using the results of chemical network simulations by Taniguchi
et al. (2019a), assuming that the H2 column density is 1022
cm−2 . Since we are interested in cyanopolyynes in hot regions (T > 100 K), we determined the column densities by
the time average while the temperature is 100 – 200 K (orange
covered range in Fig.2). We assumed that the HC3 N/DC3 N is100. The column densities of HC9 N and DC3 N are derived tobe 5.9 × 1013 cm−2 and 1.9 × 1012 cm−2 , respectively.
Using the CASSIS1 software, we estimated line intensities
of rotational lines of HC9 N and DC3 N with the above col1

http://cassis.irap.omp.eu

Fig. 3. Estimations of line intensities for HC9 N (upper panel) andDC3 N (lower panel).

umn densities and a fixed excitation temperature (100 K) and afixed line width (FWHM; 2.0 km s−1 ). The results are shownin Fig.3. We also indicate the 5σ limits of observations by the
ngVLA with setups and integration times shown in the figure.
As seen in the upper panel of Fig.3, the HC9 N lines can beeasily detected in Band 4 and Band 5.
Regarding DC3 N, we assumed that observations with thevelocity resolution of 0.5 km s−1 instead of 0.2 km s−1 , because its detection is only a matter. In Band 6 lower, threelines (J = 9 − 8; Eup = 18.2 K, J = 10 − 9; Eup = 22.3 K,
J = 11 − 10; Eup = 26.7 K) are expected to be detected byobservations with an integration time of 4.5 hours. Moreover,rotational lines of HC5 N (from J = 27 − 26; Eup = 48.3 K to
J = 35 − 34; Eup = 80.5 K) and HC7 N (from J = 63 − 62;
Eup = 109.1 K to J = 84 − 83; Eup = 193.3 K) lie in thisfrequency band, and most of them can be expected to detect
simultaneously within the short observing time. Therefore, we
can trace the deuterium fractionation and warm and hot gas simultaneously with the ngVLA. Such observations will be benefit for studying evolutionary stage and environments in hotcore regions.
Observations with the ngVLA toward hot cores will provide us a new insight into the chemistry of both carbon-chain
species and COMs (see also Beltrán & Rivilla 2018). Using
radio astronomical facilities which are available currently orin the near future, we can prepare for future observations with
the ngVLA. For instance, survey observations of carbon-chain
molecules and COMs in the 7 mm band toward MYSOs will

4
provide us good target source lists. The ngVLA will boost up
studies about characteristics of the chemical diversity around
MYSOs.
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