adsabs/FOCAL huggingface dataset:
For a star that moves with respect to its surrounding medium, the stellar motion adds an asymmetry to the wind velocity profile, since different parts of the wind face the ISM with different relative velocities. If the motion is supersonic, a bow shock arises at the interface where the ram pressure of the ISM and the stellar wind balance. The stand-off distance, i.e. the distance of the star to the apex of the shock front, is given by (1)\begin{equation} \label{R0_eq} R_0=\sqrt{\frac{\dot{M} v_w}{4 \pi \rho_0 v_*^2}}, \end{equation}R0=Ṁvw4πρ0v∗2,where vw is the terminal wind velocity, v∗ the stellar velocity with respect to the ISM, Ṁ the mass-loss rate, and ρ0 the density of the surrounding medium (Baranov et al. 1971). The density can be expressed in number density of hydrogen atoms (mH = 1.6727 × 10-27 kg), which follows roughly (2)\begin{equation} \label{density} n_{\rm H} = 2.0 \; {\rm e}^{-\frac{|z|}{100\,{\rm pc}}}, \end{equation}nH=2.0e−|z|100 pc,where z is the galactic height (Mihalas & Binney 1981) and nH is given in atoms per cm3. Wilkin (1996) demonstrated that the shape of the bow shock only depends on the stand-off distance, while Cox et al. (2012) showed that this assumption remains valid for viewing angles up to 70°. Above this value, the bow shock cone becomes broader. Therefore, we were able to use Eq. (1) to estimate the mass-loss rate from the binary system by measuring the stand-off distance. Generally, the ISM density and stellar velocity can be determined following Eq. (2) and Johnson & Soderblom (1987), respectively. While the error of the space motion is negligible, the ISM density value is only an estimate since the star could move through a dense cloud, which is not considered by Eq. (2). The respective values of these quantities for the three objects are given in Table 1. To obtain the space motion (v∗ ,LSR) with respect to the local standard of rest, we corrected the heliocentric motions from the solar motion vector (U,V,W)⊙ = (8.50 ± 0.29,13.38 ± 0.43,6.49 ± 0.26) km s-1 (Coşkunoǧlu et al. 2011). However, since the proper motions are quite small (a few mas yr-1), the correction for the solar motion has a large impact, especially on the PA of the motion. Interestingly, the PA of the corrected LSR motion is a worse match to the bow-shock orientation than the PA of the uncorrected motion (see Figs. 1, 3, and 5). Using (U,V,W)⊙ = (10.00 ± 0.36,5.25 ± 0.62,7.17 ± 0.38) km s-1 determined from the Hipparcos data by Dehnen & Binney (1998) leads to vLSR velocities which are slightly closer to the better matching heliocentric values. A similar discrepancy between bow-shock orientation and LSR motion (and less so with heliocentric motion) is found by Peri et al. (2012, 2015) for a large number of O- and B-type stars with bow shocks, as collected in the WISE E-BOSS survey. For this reason, we overplotted both the heliocentric and LSR motions on the WISE images in Figs. 1, 3, and 5.
Citation: Mihalas & Binney 1981
Citation function: Uses; Uses
A few possibilities are explored for XCl: (1) The most straightforward explanation is that the Cl radical itself is present in the coma. Although it seems unlikely due to its reactivity, the CN radical was discovered in the coma of 67P (Hänni et al. 2020); therefore, the Cl radical cannot be excluded as a possible candidate. (2) Next to CH3Cl (Fayolle et al. 2017), which is too low in abundance to be a suitable neutral candidate to explain the observations, no other volatile Cl-bearing species (e.g. like Cl2) have been identified in the DFMS mass spectra. However, as all results show that EII product ion fractions decrease as the complexity of the molecules increases (Schuhmann et al. 2019) and since DFMS spectra for masses >100 have not been thoroughly analysed for the whole mission, there is still a possibility for other Cl-bearing species to be present within the DFMS mass range, albeit with low abundances. (3) One cannot exclude a priori that there would be heavier (semi)volatile Cl-bearing parents at higher m/z outside the DFMS mass range that sublimate and are present in the coma. However, the fragmentation of such parents, e.g. of the form CxHyCl, in the DFMS ion source would create Cl-bearing fragments at lower m/z. Alternatively, heavier semivolatile chlorine-bearing neutrals might undergo photolysis rather than sublimation, releasing intermediate chlorine-bearing neutrals at lower m/z into the coma, which in turn fragment into smaller ions in the DFMS ion source. Both processes are improbable since apart from CH3Cl no Cl-bearing fragments at lower m/z have been found and the effect of photodissociation should be much more pronounced during perihelion when Rosetta was farther away from the comet and this was not observed. (4) A source of Cl may lie in compounds that decompose upon sublimation and/or ionization. As an example, it is known that ammonium perchlorate (NH4ClO4) releases HCl upon warming (Boldyrev 2006). Other perchlorates or compounds with oxidized states of Cl may provide relatively more Cl+ than HCl+ upon dissociation. According to Schauble, Rossman & Taylor (2003), molecules with oxidized Cl are enriched with 37Cl relative to non-oxidized species, which could also explain the somewhat higher coma isotopic values observed.
Citation: Schuhmann et al. 2019
Citation function: Motivation
When dipolar species, such as CO, are allowed to deposit in the laboratory on a cold surface under vacuum to form a layer of approximately 4–5 monolayers (ML), the molecules may spontaneously orient throughout the layer such that their positive or negative ends protrude from the surface, creating a polarization potential (Balog et al. 2009; Cassidy et al. 2012; Field et al. 2013; Lasne et al. 2015; Rosu-Finsen et al. 2016). We have developed a semi-empirical description of this so-called spontelectric effect, which describes major features of the phenomenon. This description is based on a physical model that invokes spontaneous orientation of the dipolar molecules of which the film is composed. The analysis, described in detail in Field et al. (2013), is remarkably successful in describing important characteristics of spontelectrics, such as the deposition temperature dependence of the surface polarization charge, including the counterintuitive behavior of solid methyl formate (Plekan et al. 2011). In the case of CO, the positive oxygen end protrudes on average (Collings et al. 2014). The spontelectric effect is exemplified by a powerful spontaneous electric field in the film. It was first discovered for nitrous oxide using a direct measurement of surface potential, employing high-resolution, low-energy electron beams (Balog et al. 2009). Studies have now been extended to CO using RAIR (reflection-absorption infrared) spectroscopy (Rosu-Finsen et al. 2016). RAIR spectra were recorded of CO laid down to a thickness of 20 ML, dosed at 21, 22, and 24 K, on 50 ML of compact amorphous solid water (cASW) formed by dosing at 110 K, in order to reproduce as far as possible the nature of ices on grains in a prestellar core. The presence of spontaneous electric fields in the film of CO was demonstrated by measurements of Stark frequency shifts in RAIR spectra, the values of which are film deposition temperature dependent. These observations were used to derive the electric field in the film, yielding a temperature-averaged surface potential, ϕ, of 6.7 ± 0.5 mV per ML of CO (Rosu-Finsen et al. 2016). We note that experimental observations show that the surface potential for any spontelectric material depends on the temperature of the substrate at the moment of deposition, which is ≤26 K in the ISM. The subsequent cooling of the substrate to 10 K in the ISM has no effect on the surface potential.
Citation: Plekan et al. 2011
Citation function: Compare/Contrast
Dramatic changes in the optical broad emission lines (BELs) of CLQs can also probe the physical origin of the lines themselves. Optical spectra of AGN are characterized primarily by a power-law continuum, BELs, and narrow emission lines (e.g., Vanden Berk et al. 2001). BELs are emitted from the broad-line region (BLR), which is believed to be the high velocity gas gravitationally bound to the central supermassive black hole (Peterson & Wandel 2000; Peterson et al. 2004). Results from reverberation mapping (Denney et al. 2009; De Rosa et al. 2018), and study of the quasar orientation and observed width of BEL profiles (Shen & Ho 2014; Storchi-Bergmann et al. 2017) imply that the geometry of the BLR is likely to be disk-like. Furthermore, study of BEL profiles suggests the BLR gas is structured as a smooth continuous flow (Laor et al. 2006). However, the exact origin of the BLR gas is still unclear (Elvis 2017). The disk-wind model suggests that the BLR gas comes from winds produced by the accretion disk (Emmering et al. 1992; Murray et al. 1995; Murray & Chiang 1997; Elitzur & Ho 2009; Elitzur et al. 2014; Elitzur & Netzer 2016). When the mass accretion rate drops below a certain limit (corresponding to a luminosity of ∼




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), winds can no longer be sustained, such that the observed BELs “disappear” (Elitzur & Ho 2009). Below the critical luminosity, AGN are expected to be “true” Type 2 AGN (i.e., Type 2 AGN intrinsically absent of BLRs). Although this critical luminosity is dependent on the black hole mass, the disk-wind model predicts that more detections of BEL disappearance are expected below the 1% Eddington ratio (Elitzur & Netzer 2016), and a double-peaked BEL profile, the signature of a rotating disk, should emerge in the quasar spectrum when the accretion rate drops (Elitzur et al. 2014). Single-epoch observations show that Eddington ratios of bright quasars with prominent BELs, are always higher than 1% (Kollmeier et al. 2006; Steinhardt & Elvis 2010). Yet it is still unknown whether Eddington ratios of individual quasars varying around 1% will display the appearance/disappearance of broad emission lines. By studying the Eddington ratios of the single changing-look AGN UGC 3223, Wang et al. (2020) find this object crosses the 1% Eddington ratio when its AGN type changes. However, it is still necessary to investigate whether this 1% Eddington ratio is associated with appearance/disappearance of BELs with multiepoch observations of CLQs, and with a larger data set. In this work, we also study the changes in the Eddington ratio distributions from the brightest to the faintest optical spectra of CLQs, investigate the possible connection with the 1% Eddington ratio, and study the BEL profiles of those CLQs.
Citation: Murray et al. 1995
Citation function: Background
Low redshift (z ≲ 3) LyC emitters have proven difficult to find (e.g., Leitherer et al. 1995; Bergvall et al. 2006; Siana et al. 2007, 2015; Vanzella et al. 2010; Leitet et al. 2011, 2013) due to the attenuation of LyC photons by the intergalactic medium (IGM). For decades, the only confirmed, directly detected LyC escape from a low-redshift galaxy was that in Haro 11 (Bergvall et al. 2006; Leitet et al. 2011), with Östlin et al. (2015) providing the first spatially resolved kinematic study of a confirmed LyC emitter. Only recently has significant progress been made in detecting local LyC emitters. Green peas (GPs) are the most prolific class of local LyC-leaking galaxies, showing a very high LyC detection rate. So far, 11 out of 11 GPs have been confirmed as leaking LyC by direct detection with HST/COS (Izotov et al. 2016a,b, 2018a,b). GPs are compact galaxies which appear green in SDSS g, r, i composite images due to their extremely high equivalent width of [O III] λλ4959, 5007 and their redshifts of only z ∼ 0.2 − 0.4. They have subsolar metallicities, low masses, and high specific star-formation rate, and represent good analogs to high redshift SFGs (e.g., Cardamone et al. 2009; Izotov et al. 2011; Bian et al. 2016). While much closer than their high-z cousins, GPs are nevertheless at distances that still present severe difficulties for a spatially resolved analysis of their properties. Pioneering work with integral field unit (IFU) observations of four GPs (Lofthouse et al. 2017) reveals that two are rotationally supported, with seemingly undisturbed morphology, and two are dispersion-dominated, leading the authors to conclude that mergers may not be a necessary driver of their star formation properties, and by extension, for their LyC emission. At the same time, Amorín et al. (2012) find complex, multicomponent Hα line profiles, with implied high-velocity gas in all five of their observed GPs. While these GPs are not confirmed LyC emitters, and therefore their kinematical properties cannot be taken as a necessary condition for LyC escape, outflows nevertheless seem to play a role in enabling LyC leakage. For example, Chisholm et al. (2017) find that outflows can be detected in all LyC emitters they examine, albeit with velocities not statistically different from a control sample of non-leakers. Further, Martin et al. (2015) find that ULIRG galaxies with the strongest outflows manifest the lowest column densities of neutral gas. Simulations also support the scenario of outflows enabling LyC escape. Trebitsch et al. (2017) find that mechanical feedback from supernovae explosions has a vital role in disrupting dense star-forming regions and in clearing low-density escape paths for the LyC photons. While IFU observations of a large sample of GPs would help settle some of these issues, the large distances to these galaxies would likely prohibit the detailed identification and characterization of the exact escape mechanisms of LyC. Much closer galaxies, showing the same characteristics as GPs, are required for such an endeavor.
Citation: Lofthouse et al. 2017
Citation function: Background; Background
The density structure and infall kinematics discussed here are probably dominated by the most massive object clearly identified in Fig. 2b. The envelope profile defined above (see Sect. 3.5) and their corresponding values given in Table 3 have been used in Eq. (A.2) to estimate a mass of ~3500 M⊙ in a 2.5 pc radius for the central UCH ii region. This region exhibits a steeper density gradient in its outer envelope, with qout ≃ −2.5, also illustrated by the cloud structure studied with probability density function (Schneider et al. 2015; Rayner et al., in prep.). It suggests compression by external forces, as shown in numerical simulations of Hennebelle et al. (2003) and outlined in Appendix A. This compressive process could be associated with stellar winds and ionisation shocks of nearby stars such as those observed in the M 16 massive YSO (young stellar object) by Tremblin et al. (2014b) or with globally infalling gas driven by the dynamic formation of Mon R2 cloud through force fall. Such infalling motions have in fact been observed in CO by Loren (1977), and clumps with similar masses generally exhibit active global infall. This is the case of SDC335, which has a mass of ~5500 M⊙ in 2.5 pc (Peretto et al. 2013) and contains a protostellar object of similar type B1 (Avison et al. 2015), as well as of DR21, Clump-14 (DR21-south, ~4900 M⊙), and Clump-16 (DR21-north, ~3350 M⊙) (Schneider et al. 2010a), all of which show supersonic infall (V = 0.5−0.7 km s-1). Moreover, the hourglass morphology of the magnetic field is thought to be a signature of global infall (Carpenter & Hodapp 2008), and Koch et al. (2014) suggest that Mon R2 is a super critical, quickly collapsing cloud. The expected infall velocity gradient, as observed in SDC13 (Peretto et al. 2014), seems to be a crucial ingredient for generating the filament crossing (Dobashi et al. 2014) characteristic of all the regions mentioned here and commonly observed in many places.
Citation: Schneider et al. 2015
Citation function: Similarities
Typical corotating high-speed streams (HSS), i.e. those streams coming from the equatorial extension of polar coronal holes, are characterized by different regions. Within these regions, extending on daily scales, field and plasma parameters assume different average values and fluctuations have a different character. The first region is the stream interface (SI), located between fast and slow stream, right where the dynamical interaction between the two flows is stronger (Schwenn & Marsch 1990; Bruno & Carbone 2016). The SI is characterized by high level of pressure, both kinetic and magnetic, high temperature and low Alfvénicity. This region is followed by the trailing edge (TE), which is characterized by the highest wind speed and temperature, the lowest compressibility and the highest level of Alfvénicity. The TE is then followed by a wind that progressively slows down and becomes cooler. This region was first named rarefaction region by Hundhausen (1972). Interesting enough, these two portions of the stream are separated by a very narrow region across which Alfvénicity changes abruptly, from high to low (Bruno & Telloni 2015). Most of the time, the rarefaction region is followed by the heliospheric current sheet crossing characterized by low Alfvénicity, low temperature but higher field and plasma compressibility. Thus, moving from fast to slow wind, one observes a different turbulence, not only relatively to its power level, but also to its Alfvénic character, although the spectral slope at fluid scales clearly shows a persistent Kolmogorov-like scaling. In addition, besides the above-cited differences within the fluid regime, there are also clear differences at proton kinetic scales. Bruno, Trenchi & Telloni (2014), investigating the behaviour of the spectral slope at proton scales, up to frequencies of a few Hz, beyond the high-frequency break separating fluid from kinetic scales, recorded a remarkable variability of the spectral index (Smith et al. 2006; Sahraoui et al. 2010) within the following frequency decade or so. The steepest spectra corresponded to the TEs of fast streams, while the flattest ones were found within the subsequent slow wind regions. The same authors found an empirical relationship between the power associated with the inertial range and the spectral slope observed within this narrow region, which allowed us to estimate that this slope approaches the Kolmogorov scaling within the slowest wind and reaches a limiting value of roughly 4.4 within the fast wind. In addition, Bruno et al. (2014) suggested the possible role played also by Alfvénicity in this spectral dependence. Recent theoretical results seem to move in this direction attributing a relevant role to Alfvénic imbalance within the inertial range (Voitenko & De Keyser 2016).
Citation: Bruno & Telloni 2015
Citation function: Background
The AR NOAA 12673 was highly flare productive.5

5

https://www.swpc.noaa.gov/products/solar-region-summary

 It appeared in the eastern limb of the Sun on August 28 as a simple α-type AR and gradually evolved into complex βγ-type on September 4. It became an even more complex βγδ-type on September 5 and remained so until its disappearance over the western limb of the Sun on September 10. In total, it produced 27 M-class and 4 X-class flares between September 4–10. Following the occurrence of the two X-class flares on September 6, reported in this article, it went on to produce an X1.3-class flare on September 7 and an X8.2-class flare on September 10 besides several M-class major eruptive events. During the occurrence of X-class flares, the complex AR had shown δ-sunspots, which are identified with a complex distribution of sunspot groups in which the umbrae of positive and negative polarities share a common penumbra (Künzel 1960). Such complex ARs are known to produce powerful flares (see e.g., Zirin & Liggett 1987; Sammis et al. 2000; Takizawa & Kitai 2015). It is noteworthy that AR 12673 was a rather compact region (Figure 9) that displayed more spatial extension in the north–south direction than the usual east–west direction. The δ-sunspots were concentrated at the central part of the AR where magnetic fields were very strong, and the magnetic field gradient across the PIL was extremely high (∼2.4 × 103 G Mm−1; see Figure 9(c)). Earlier studies have shown a close relationship between major flare activities and strong magnetic field, especially those with a high gradient and those that are highly sheared across the PIL (Hagyard et al. 1984; Zirin & Wang 1993; Schrivjer 2007; Barnes et al. 2016). The reported eruptive activities in AR 12673, thus, represent the capability of the AR in the rapid generation and storage of huge amount of excess magnetic energy in the corona. In this context, the evolution of normalized free magnetic energy during the X-class flares is noteworthy (Figure 12). We found that the free magnetic energy stored in the AR before the flaring activity was ∼82% of the potential magnetic energy. After the two X-class flares, it reduced to ∼70%. Our analysis, therefore, implies that a large amount of free magnetic energy was already stored in the AR before the flaring activities and that the large X-class flares essentially released only a small fraction of it.
Citation: Künzel 1960
Citation function: Background
A likely scenario for the geometry of the entire system is presented in Fig. 15. The locations of systems A–C are fairly well constrained by their metal absorption. System D is harder to place, as there is little to be learned from its negligibly low metal content. It is highly improbable that it is an outflow similar to C but with a higher velocity, or indeed that it has originated inside the galaxy at all. Instead we believe it to trace the metal-poor gas that acts as a replenishment mechanism of galaxies throughout the universe. It has indeed been suggested that GRB hosts in general are fuelled by recent metal-poor gas inflow (Michałowski et al. 2015, 2016). The −320 km s-1 blueshift implies that it is not accreting onto galaxy B – if B hosted the GRB, then D must be in front of it, moving away at that velocity. The degeneracy between local velocities and cosmological redshift allows for a range of locations of system D. One possibility is that it is located between the two galaxies and is currently falling towards galaxy A, or has fallen from behind galaxy B and is being slowed by its gravity. Another possibility is that it has fallen from a large distance into the potential well of the complex and is now in the foreground, having initially passed both galaxies without being accreted, having joined in a disk-like structure (e.g. Bouché et al. 2013). For any of these scenarios, system D represents a rare detection of such a metal-poor inflow in a GRB galaxy, or indeed in absorption at such a small projected distance to any galaxy. Systems with positive velocities relative to the gas local to GRBs, and thus inferred to be falling in towards the star-forming regions of their hosts, have been detected previously (e.g. Prochaska et al. 2008), but these have tended to be richer in metals than the gas observed here and may represent recycled gas that has previously been expelled from the galaxy. Ly-α emitters are thought to trace accretion in large halos around quasars (Cantalupo et al. 2014; Hennawi et al. 2015), and have been detected at impact parameters of ≥70 kpc from an LLS (Fumagalli et al. 2016). If system D is indeed bound to the host system, then it could be much closer to the galaxies than that. Finally, as discussed when defining the host complex in Sect. 3.1, it could in fact be a foreground system located ~1.5 Mpc from the interacting complex. While this would exclude it from being a direct accretion flow onto the complex, it represents at the very least a reservoir of cool metal-poor gas residing in the IGM near the dark matter halo, available for star formation at some future epoch.
Citation: Michałowski et al. 2015
Citation function: Compare/Contrast