arXiv subjects

E. Roueff

Publications and source records attributed to E. Roueff.

At least 19 recordsLinked to original sources

arXiv2026

New Estimate for the Cosmic Ray-Induced $\rm H_2$ Photodissociation Rate in the Interstellar Medium

In the interstellar medium, cosmic rays (CRs) generate a field of ultraviolet (UV) photons via the excitation and subsequent radiative decay of $\rm H_2$ molecules. This UV field is a major agent of ionization and dissociation in the inner regions of molecular clouds that are shielded from the effects of the interstellar radiation field. In particular, the dissociation of $\rm H_2$, by far the most abundant molecule in interstellar clouds, leads to the production of atomic hydrogen which then takes part in the production of a multitude of molecules, in particular complex organics on the surfaces of interstellar dust grains. Precise knowledge of the rates of CR-induced dissociation processes is thus crucial for constructing reliable chemical models. For the present paper, we have derived a new value of $k_{\rm diss, CR}(\mbox{$\rm H_2$})=0.831\zeta$ for the rate of $\rm H_2$ dissociation, where $\zeta$ is the CR ionization rate of $\rm H_2$. This prediction contrasts a previous value from the Leiden database which overestimated the rate due to an inconsistent treatment of the $\rm H_2$ abundances and photodissociation cross sections. By running a series of chemical models, we show that the overestimated dissociation rate has a large effect on the results of chemical simulations, with the abundance of methanol being overestimated by over one order of magnitude. Hence, we strongly recommend the adoption of our new estimate $k_{\rm diss, CR}(\mbox{$\rm H_2$})=0.831\zeta$ in all chemical models that include this process. Our newly derived value corresponds to $\rm H_2$ being purely in the para form ($J^{\prime\prime} = 0$). However, in the interiors of molecular clouds the $\rm H_2$ ortho-to-para ratio is low and using the rate for para-$\rm H_2$ is an adequate approximation.

astro-ph.GA↗

arXiv2025

CHemical Evolution in MassIve star-forming COres (CHEMICO). I. Evolution of the temperature structure

Increasing evidence shows that most stars in the Milky Way, including the Sun, are born in star-forming regions containing also high-mass stars, but due to both observational and theoretical challenges, our comprehension of their chemical evolution is far less clear than that of their low-mass counterparts. We present the project "CHemical Evolution of MassIve star-froming COres" (CHEMICO). The project aims at investigating any aspect of the chemical evolution of high-mass star-forming cores by observing representatives of the three main evolutionary categories: high-mass starless cores, high-mass protostellar objects, and ultra-compact HII regions. We carried out an unbiased spectral line survey of the entire bandwidth at 3, 2, and 1.2 mm with the 30m telescope of the Insitut de Radioastronomie millimetrique towards three targets that represent the three evolutionary stages. The number of lines and species detected increases with evolution. In this first work, we derive the temperature structure of the targets through the analysis of the carbon-bearing species C2H, c-C3H, c-C3H2, C4H, CH3CCH, HC3N, CH3CN, and HC5N. The excitation temperature, Tex, increases with evolution in each species, although not in the same way. Hydrocarbons tend to be associated with the smallest Tex values and enhancements with evolution, while cyanides are associated with the highest Tex values and enhancements. In each target, the higher the number of atoms in the molecule, the higher Tex tends to be. The temperature structure evolves from a cold, uniform envelope traced by simple hydrocarbons in the high-mass starless core stage, to a more stratified envelope in the protostellar stage (made by a hot core, a shell with intermediate Tex, and a larger cold envelope), to finally a hot core surrounded only by a cold envelope in the Ultracompact HII stage.

astro-ph.GA↗

arXiv2025

PDRs4All XV: CH radical and H$_3^+$ molecular ion in the irradiated protoplanetary disk d203-506

Most protoplanetary disks experience a phase in which they are subjected to strong ultraviolet radiation from nearby massive stars. This UV radiation can substantially alter their chemistry by producing numerous radicals and molecular ions. In this Letter we present detailed analysis of the JWST-NIRSpec spectrum of the d203-506 obtained as part of the PDRs4All Early Release Science program. Using state-of-the-art spectroscopic data, we searched for species using a multi-molecule fitting tool, PAHTATmol, that we developed for this purpose. Based on this analysis, we report the clear detection of ro-vibrational emission of the CH radical and likely detection of the H$_3^+$ molecular ion, with estimated abundances of a few times 10$^{-7}$ and approximately 10$^{-8}$, respectively. The presence of CH is predicted by gas-phase models and well explained by hydrocarbon photochemistry. H$_3^+$ is usually formed through reactions of H$_2$ with H$_2^+$ originating from cosmic ray ionization of H$_2$. However, recent theoretical studies suggest that H$_3^+$ also forms through UV-driven chemistry in strongly irradiated ($G_0>$10$^3$), dense ($n_{\rm H} >10^{6}$ cm$^{-3}$) gas. The latter is favored as an explanation for the presence of ``hot'' H$_3^+$ ($T_{\rm ex}\gtrsim$1000 K) in the outer disk layers of d203-506, coinciding with the emission of FUV-pumped H$_2$ and other ``PDR species'', such as CH$^+$, CH$_3^+$, and OH. Our detection of infrared emission from vibrationally excited H$_3^+$ and CH raises questions about their excitation mechanisms and, underscore that UV radiation can have a profound impact on the chemistry of planet forming disks. They also demonstrate the power of JWST pushing the limit for the detection of elusive species in protoplanetary disks.

astro-ph.GA↗

arXiv2025

Rotational equilibrium of C$_2$ in diffuse interstellar clouds

Context. Recent spectroscopic measurements have revealed absorption from higher rotational levels in C$_2$ than previous observations. These improvements are accompanied by the availability of updated radiative and collisional data. Aims. We revisit the density and radiation field intensity diagnostics provided by the observations of many rotational levels of inter- stellar C$_2$ and extensive molecular information. Methods. We built an excitation model of C2 without spatial structure, including levels up to J= 34 where updated radiative and collisional excitation data are introduced as well as excitation by chemical formation. Results. We confirm the importance of the recent collisional excitation rate coefficients of C$_2$ by molecular H$_2$. We show that the new higher level observations cannot be explained by the standard balance between collisional excitation and radiative transitions. We propose that chemical excitation at formation provides a plausible mechanism to explain the observed high excitation of C$_2$. In addition, it allows us to lift the degeneracy of the density over radiation field strength parameter in the excitation model. Conclusions. A 0D model remains limited and it is highly desirable to use a full Photon Dominated Region (PDR) model, which includes all excitation processes introduced here and full chemical and thermal balance.

astro-ph.GA↗

arXiv2024

PDRs4All. X. ALMA and JWST detection of neutral carbon in the externally irradiated disk d203-506: Undepleted gas-phase carbon

The gas-phase abundance of carbon, x_C = C/H, and its depletion factors are essential parameters for understanding the gas and solid compositions that are ultimately incorporated into planets. The majority of protoplanetary disks are born in clusters and, as a result, are exposed to external FUV radiation. These FUV photons potentially affect the disk's evolution, chemical composition, and line excitation. We present the first detection of the [CI]609um fine-structure line of neutral carbon (CI), achieved with ALMA, toward one of these disks, d203-506, in the Orion Nebula Cluster. We also report the detection of CI forbidden and permitted lines (from electronically excited states up to 10 eV) observed with JWST in the IR. These lines trace the irradiated outer disk and photo-evaporative wind. Contrary to the common belief that these IR lines are C+ recombination lines, we find that they are dominated by FUV-pumping of CI followed by fluorescence cascades. They trace the transition from atomic to molecular gas, and their intensities scale with G0. The lack of outstanding IR OI fluorescent emission, however, implies a sharper attenuation of external FUV radiation with E > 12 eV (~Lyman-beta). This is related to a lower effective FUV dust absorption cross section compared to that of interstellar grains, implying a more prominent role for FUV shielding by the CI photoionization continuum. The [CI]609um intensity is proportional to N(CI) and can be used to infer x_C. We derive x_C ~ 1.4E-4. This implies that there is no major depletion of volatile carbon compared to x_C measured in the natal cloud, hinting at a young disk. We also show that external FUV radiation impacts the outer disk and wind by vertically shifting the water freeze-out depth, which results in less efficient grain growth and settling. This shift leads to nearly solar gas-phase C/O abundance ratios in these irradiated layers.

astro-ph.GA↗

arXiv2024

Dissociative recombination of NS+ in collisions with slow electrons

Cross sections and rate coefficients for the Dissociative Recombination (DR) of the NS+ ion induced by collisions with low-energy electrons are reported for temperatures between 10 and 1000 K, relevant to a large range of interstellar cloud temperatures. Uncertainties are discussed for these rates. Comparisons are made with DR rates for the isovalent NO+ molecular ion which are found to be much faster. The present findings lead to a moderate dissociative reaction rate coefficient, smaller by a factor of 2 than the current estimates reported in the different kinetic databases for a temperature of 10 K. We consider that our rate coefficients obtained through multichannel quantum defect theory for NS+ are likely to be better than those displayed in the different kinetic databases.

astro-ph.IM↗

arXiv2024

Doubly substituted isotopologues of HCCCN in TMC-1: Detection of D13CCCN, DC13CCN, DCC13CN, DCCC15N, H13C13CCN, H13CC13CN, HC13C13CN, HCC13C15N, and HC13CC15N

We report the first detection in space of a complete sample of nine doubly substituted isotopologues of HCCCN towards the cyanopolyyne peak of TMC-1 using observations of the QUIJOTE line survey taken with the Yebes 40 m telescope. We detected D13CCCN, DC13CCN, DCC13CN, DCCC15N, H13C13CCN, H13CC13CN, HC13C13CN, HCC13C15N, and HC13CC15N through their J=4-3 and J=5-4 lines in the 7 mm window. In addition, we present an extensive analysis of the emission of HCCCN and its singly substituted isotopologues through a large velocity gradient model of the lines detected at 7 mm and 3 mm using the Yebes 40 m and the IRAM 30 m telescopes, respectively. The derived column densities for all the isotopologues are consistent in the two spectral bands for an H2 volume density of 1e4 cm-3 and a kinetic temperature of 10 K. Whereas we observed a 13C fractionation for HCC13CN and other double isotopologues with a 13C atom adjacent to the nitrogen atom, we derived similar C/13C abundance ratios for the three 13C substituted species of DCCCN. This suggests additional chemical discrimination for deuterated isotopologues of HCCCN. Finally, we present the spatial distribution of the J=4-3 and J=5-4 lines from the singly substituted species observed with the Yebes 40 m telescope. The emission peak of the spatial distribution of DCCCN appears to be displaced by 40'' with respect to that of HCCCN and the 13C and 15N isotopologues. In addition to a different formation route for the deuterated species, we could also expect that this differentiation owing to the deuterium fractionation is more efficient at low temperatures, and therefore, that deuterated species trace a colder region of the cloud.

astro-ph.GA↗

arXiv2024

Protonated acetylene in the z=0.89 molecular absorber toward PKS1830-211

We report the first interstellar identification of protonated acetylene, C2H3+, a fundamental hydrocarbon, in the z=0.89 molecular absorber toward the gravitationally lensed quasar PKS1830-211. The molecular species is identified from clear absorption features corresponding to the 2_12-1_01 (rest frequency 494.034 GHz) and 1_11-0_00 (431.316 GHz) ground-state transitions of ortho and para forms of C2H3+, respectively, in ALMA spectra toward the southwestern image of PKS1830-211, where numerous molecules, including other hydrocarbons, have already been detected. From the simple assumption of local thermodynamic equilibrium (LTE) with cosmic microwave background photons and an ortho-to-para ratio of three, we estimate a total C2H3+ column density of 2 x 10^12 cm^-2 and an abundance of 10^-10 compared to H_2. However, formation pumping could affect the population of metastable states, yielding a C2H3+ column density higher than the LTE value by a factor of a few. We explore possible routes to the formation of C2H3+, mainly connected to acetylene and methane, and find that the methane route is more likely in PDR environment. As one of the initial hydrocarbon building blocks, C2H3+ is thought to play an important role in astrochemistry, in particular in the formation of more complex organic molecules.

astro-ph.GA↗

arXiv2023

The evolution of sulphur-bearing molecules in high-mass star-forming cores

To understand the chemistry of sulphur (S) in the interstellar medium, models need to be tested by observations of S-bearing molecules in different physical conditions. We analyse observations obtained with the IRAM 30m telescope towards 15 well-known cores classified in the three main evolutionary stages of the high-mass star-formation process: high-mass starless cores,high-mass protostellar objects, and ultracompact HII regions. We detected rotational lines of SO, SO+, NS, C34S, 13CS, SO2, CCS, H2S, HCS+, OCS, H2CS, and CCCS. We also analyse for the first time lines of the NO molecule to complement the analysis. From a local thermodynamic equilibrium approach, we have derived column densities of each species and excitation temperatures. Based on a statistical analysis on the line widths and the excitation temperatures, we find that: NS, C34S, 13CS, CCS, and HCS+ trace cold, quiescent, and likely extended material; OCS, and SO2 trace warmer, more turbulent, and likely denser and more compact material; SO and perhaps SO+ trace both quiescent and turbulent material depending on the target. The abundances of SO, SO2, and H2S show the strongest positive correlations with the kinetic temperature, believed to be an evolutionary indicator. Moreover, the sum of all molecular abundances show an enhancement of gaseous S from the less evolved to the more evolved stages. These trends could be due to the increasing amount of S sputtered from dust grains owing to the increasing protostellar activity with evolution. The average abundances in each evolutionary group increase especially in the oxygen-bearing molecules, perhaps due to the increasing abundance of atomic oxygen with evolution owing to photodissociation of water in gas phase. Our observational work represents a test-bed for theoretical studies aimed at modelling the chemistry of sulphur during the evolution of high-mass star-forming cores

astro-ph.GA↗

arXiv2023

HCN emission from translucent gas and UV-illuminated cloud edges revealed by wide-field IRAM 30m maps of Orion B GMC: Revisiting its role as tracer of the dense gas reservoir for star formation

We present 5 deg^2 (~250 pc^2) HCN, HNC, HCO+, and CO J=1-0 maps of the Orion B GMC, complemented with existing wide-field [CI] 492 GHz maps, as well as new pointed observations of rotationally excited HCN, HNC, H13CN, and HN13C lines. We detect anomalous HCN J=1-0 hyperfine structure line emission almost everywhere in the cloud. About 70% of the total HCN J=1-0 luminosity arises from gas at A_V < 8 mag. The HCN/CO J=1-0 line intensity ratio shows a bimodal behavior with an inflection point at A_V < 3 mag typical of translucent gas and UV-illuminated cloud edges. We find that most of the HCN J=1-0 emission arises from extended gas with n(H2) ~< 10^4 cm^-3, even lower density gas if the ionization fraction is > 10^-5 and electron excitation dominates. This result explains the low-A_V branch of the HCN/CO J=1-0 intensity ratio distribution. Indeed, the highest HCN/CO ratios (~0.1) at A_V < 3 mag correspond to regions of high [CI] 492 GHz/CO J=1-0 intensity ratios (>1) characteristic of low-density PDRs. Enhanced FUV radiation favors the formation and excitation of HCN on large scales, not only in dense star-forming clumps. The low surface brightness HCN and HCO+ J=1-0 emission scale with I_FIR (a proxy of the stellar FUV radiation field) in a similar way. Together with CO J=1-0, these lines respond to increasing I_FIR up to G0~20. On the other hand, the bright HCN J=1-0 emission from dense gas in star-forming clumps weakly responds to I_FIR once the FUV radiation field becomes too intense (G0>1500). The different power law scalings (produced by different chemistries, densities, and line excitation regimes) in a single but spatially resolved GMC resemble the variety of Kennicutt-Schmidt law indexes found in galaxy averages. As a corollary for extragalactic studies, we conclude that high HCN/CO J=1-0 line intensity ratios do not always imply the presence of dense gas.

astro-ph.GA↗

arXiv2023

Combined model for $\rm ^{15}N$, $\rm ^{13}C$, and spin-state chemistry in molecular clouds

We present a new gas-grain chemical model for the combined isotopic fractionation of carbon and nitrogen in molecular clouds, in which the isotope chemistry of carbon and nitrogen is coupled with a time-dependent description of spin-state chemistry. We updated the rate coefficients of some isotopic exchange reactions considered in the literature, and present here a set of new exchange reactions involving molecules substituted in $\rm ^{13}C$ and $\rm ^{15}N$ simultaneously. We apply the model to a series of zero-dimensional simulations representing a set of physical conditions across a prototypical prestellar core, exploring the deviations of the isotopic abundance ratios in the various molecules from the elemental isotopic ratios as a function of physical conditions and time. We find that the $\rm ^{12}C/^{13}C$ ratio can deviate from the elemental ratio by up to a factor of several depending on the molecule, and that there are highly time-dependent variations in the ratios. The $\rm HCN/H^{13}CN$ ratio, for example, can obtain values of less than 10 depending on the simulation time. The $\rm ^{14}N/^{15}N$ ratios tend to remain close to the assumed elemental ratio within $\sim$ ten per cent, with no clear trends as a function of the physical conditions. Abundance ratios between $\rm ^{13}C$-containing molecules and $\rm ^{13}C$+$\rm ^{15}N$-containing molecules show somewhat increased levels of fractionation due to the newly included exchange reactions, though still remaining within a few tens of per cent of the elemental $\rm ^{14}N/^{15}N$ ratio. Our results imply the existence of gradients in isotopic abundance ratios across prestellar cores, suggesting that detailed simulations are required to interpret observations of isotopically substituted molecules correctly, especially given that the various isotopic forms of a given molecule do not necessarily trace the same gas layers.

astro-ph.GA↗

arXiv2023

The extremely sharp transition between molecular and ionized gas in the Horsehead nebula

(Abridged) Massive stars can determine the evolution of molecular clouds with their strong ultraviolet (UV) radiation fields. Moreover, UV radiation is relevant in setting the thermal gas pressure in star-forming clouds, whose influence can extend from the rims of molecular clouds to entire star-forming galaxies. Probing the fundamental structure of nearby molecular clouds is therefore crucial to understand how massive stars shape their surrounding medium and how fast molecular clouds are destroyed, specifically at their UV-illuminated edges, where models predict an intermediate zone of neutral atomic gas between the molecular cloud and the surrounding ionized gas whose size is directly related to the exposed physical conditions. We present the highest angular resolution (~$0.5$", corresponding to $207$ au) and velocity-resolved images of the molecular gas emission in the Horsehead nebula, using CO J=3-2 and HCO$^+$ J=4-3 observations with ALMA. We find that CO and HCO$^+$ are present at the edge of the cloud, very close to the ionization (H$^+$/H) and dissociation fronts (H/H$_2$), suggesting a very thin layer of neutral atomic gas (<$650$ au) and a small amount of CO-dark gas ($A_V=0.006-0.26$ mag) for stellar UV illumination conditions typical of molecular clouds in the Milky Way. The new ALMA observations reveal a web of molecular gas filaments with an estimated thermal gas pressure of $P_{\mathrm{th}} = (2.3 - 4.0) \times 10^6$ K cm$^{-3}$, and the presence of a steep density gradient at the cloud edge that can be well explained by stationary isobaric PDR models with pressures consistent with our estimations. However, in the HII region and PDR interface, we find $P_{\mathrm{th,PDR}} > P_{\mathrm{th,HII}}$, suggesting the gas is slightly compressed. Therefore, dynamical effects cannot be completely ruled out and even higher angular observations will be needed to unveil their role.

astro-ph.GA↗

arXiv2022

Linking the dust and chemical evolution: Taurus and Perseus -- New collisional rates for HCN, HNC, and their C, N, and H isotopologues

HCN, HNC, and their isotopologues are ubiquitous molecules that can serve as chemical thermometers and evolutionary tracers to characterize star-forming regions. Despite their importance in carrying information that is vital to studies of the chemistry and evolution of star-forming regions, the collision rates of some of these molecules have not been available for rigorous studies in the past. We perform an up-to-date gas and dust chemical characterization of two different star-forming regions, TMC 1-C and NGC 1333-C7, using new collisional rates of HCN, HNC, and their isotopologues. We investigated the possible effects of the environment and stellar feedback in their chemistry and their evolution. With millimeter observations, we derived their column densities, the C and N isotopic fractions, the isomeric ratios, and the deuterium fractionation. The continuum data at 3 mm and 850 $\mu$m allowed us to compute the emissivity spectral index and look for grain growth as an evolutionary tracer. The H$^{13}$CN/HN$^{13}$C ratio is anticorrelated with the deuterium fraction of HCN, thus it can readily serve as a proxy for the temperature. The spectral index $(\beta\sim 1.34-2.09)$ shows a tentative anticorrelation with the H$^{13}$CN/HN$^{13}$C ratio, suggesting grain growth in the evolved, hotter, and less deuterated sources. Unlike TMC 1-C, the south-to-north gradient in dust temperature and spectral index observed in NGC 1333-C7 suggests feedback from the main NGC 1333 cloud. With this up-to-date characterization of two star-forming regions, we found that the chemistry and the physical properties are tightly related. The dust temperature, deuterium fraction, and the spectral index are complementary evolutionary tracers. The large-scale environmental factors may dominate the chemistry and evolution in clustered star-forming regions.

astro-ph.GA↗

arXiv2022

Gas phase Elemental abundances in Molecular cloudS (GEMS) VI. A sulphur journey across star-forming regions: study of thioformaldehyde emission

In the context of the IRAM 30m Large Program GEMS, we present a study of thioformaldehyde in several starless cores located in star-forming filaments of Taurus, Perseus, and Orion. We investigate the influence of the environmental conditions on the abundances of these molecules in the cores, and the effect of time evolution. We have modelled the observed lines of H2CS, HDCS, and D2CS using the radiative transfer code RADEX. We have also used the chemical code Nautilus to model the evolution of these species depending on the characteristics of the starless cores. We derive column densities and abundances for all the cores. We also derive deuterium fractionation ratios, Dfrac, to determine and compare the evolutionary stage between different parts of each star-forming region. Our results indicate that the north region of the B213 filament in Taurus is more evolved than the south, while the north-eastern part of Perseus presents an earlier evolutionary stage than the south-western zone. Model results also show that Dfrac decreases with the cosmic-ray ionisation rate, while it increases with density and with the degree of sulphur depletion. In particular, we only reproduce the observations when the initial sulphur abundance in the starless cores is at least one order of magnitude lower than the solar elemental sulphur abundance. The progressive increase in HDCS/H2CS and D2CS/H2CS with time makes these ratios powerful tools for deriving the chemical evolutionary stage of starless cores. However, they cannot be used to derive the temperature of these regions, since both ratios present a similar evolution at two different temperature ranges (7-11 K and 15-19 K). Regarding chemistry, (deuterated) thioformaldehyde is mainly formed through gas-phase reactions (double-replacement and neutral-neutral displacement reactions), while surface chemistry plays an important role as a destruction mechanism.

astro-ph.SR↗

arXiv2022

Dissociative recombination and rotational transitions of D$_2^+$ in collisions with slow electrons

Rate coefficients for dissociative recombination and state-to-state rotational transitions of the D$_{2}^{+}$ ion induced by collisions with very low-energy electrons have been reported following our previous studies on HD$^{+}$ and H$_{2}^{+}$ [9,10]. The same molecular structure data sets, excitations ($N_{i}^{+} \rightarrow$ $N_{f}^{+}=N_{i}^{+}+2$ for $N_{i}^{+}=0$ to $10$) and de-excitations ($N_{i}^{+}$ $\rightarrow$ $N_{f}^{+}=N_{i}^{+}-2$, for $N_{i}^{+}=2$ to $10$) were used for collision energies ranging from $0.01$ meV to $0.3$ eV. Isotopic effects for dissociative recombination and rotational transitions of the vibrationally relaxed targets are presented.

physics.atom-ph↗

arXiv2021

New deuterated species in TMC-1: Detection of CH2DC4H with the QUIJOTE line survey

We report the first detection in space of the single deuterated isotopologue of methyldiacetylene, CH2DC4H. A total of 12 rotational transitions, with J = 8-12 and Ka = 0 and 1, were identified for this species in TMC-1 in the 31.0-50.4 GHz range using the Yebes 40m radio telescope. The observed frequencies allowed us to obtain, for the first time, the spectroscopic parameters of this deuterated isotopologue. We derived a column density of (5.5+/-0.2)e11 cm-2. The abundance ratio between CH3C4H and CH2DC4H is 24+/-2. This ratio is similar to that found for the CH3C3N/CH2DC3N analogue system, which is 22+/-2.We did not detect the deuterated species CH3C4D, which has already been observed in laboratory experiments. The detection of deuterated CH3C4H allows us to extend the discussion on the chemical mechanisms of deuterium fractionation at work in TMC-1 using a new gas-phase chemical model with multiply deuterated molecules. Introducing a possible deuterium exchange reaction between CH3CCH and atomic deuterium allows us to account for the CH3C4H/CH2DC4H abundance ratio.

astro-ph.GA↗

arXiv2021

Detection of deuterated methylcyanoacetylene, CH$_2$DC$_3$N, in TMC-1

We report the first detection in space of the single deuterated isotopologue of methylcyanoacetylene, CH$_2$DC$_3$N. A total of fifteen rotational transitions, with $J$ = 8-12 and $K_a$ = 0 and 1, were identified for this species in TMC-1 in the 31.0-50.4 GHz range using the Yebes 40m radio telescope. The observed frequencies were used to derive for the first time the spectroscopic parameters of this deuterated isotopologue. We derive a column density of $(8.0\pm 0.4) \times 10^{10}$ cm$^{-2}$. The abundance ratio between CH$_3$C$_3$N and CH$_2$DC$_3$N is $\sim$22. We also theoretically computed the principal spectroscopic constants of $^{13}$C isotopologues of CH$_3$C$_3$N and CH$_3$C$_4$H and those of the deuterated isotopologues of CH$_3$C$_4$H for which we could expect a similar degree of deuteration enhancement. However, we have not detected either CH$_2$DC$_4$H nor CH$_3$C$_4$D nor any $^{13}$C isotopologue. The different observed deuterium ratios in TMC-1 are reasonably accounted for by a gas phase chemical model where the low temperature conditions favor deuteron transfer through reactions with H$_2$D$^+$.

astro-ph.GA↗

arXiv2021

Influence of the nano-grain depletion in photon-dominated regions: Application to the gas physics and chemistry in the Horsehead

The large disparity in physical conditions from the diffuse interstellar medium (ISM) to denser clouds such as photon-dominated regions (PDRs) triggers an evolution of the dust properties (i.e. composition, size, and shape). The gas physics and chemistry are tightly connected to these dust properties and are therefore affected by dust evolution and especially the nano-grain depletion in the outer irradiated part of PDRs. We highlight the influence of nano-grain depletion on the gas physics and chemistry in the Horsehead nebula, a prototypical PDR. We used a model for atomic and molecular gas in PDRs, the Meudon PDR code, using diffuse ISM-like dust and Horsehead-like dust to study the influence of nano-grain depletion on the gas physics and chemistry, focusing on the impact on photoelectric heating and H2 formation and, therefore, on the H2 gas lines. We find that nano-grain depletion in the Horsehead strongly affects gas heating through the photoelectric effect and thus the gas temperature and the H2 formation, hence the H -> H2 position. Consequently, the first four pure rotational lines of H2 (e.g. 0-0 S(0), S(1), S(2), and S(3)) vary by a factor of 2 to 14. The 0-0 S(3) line that is often underestimated in models is underestimated even more when taking nano-grain depletion into account due to the decrease in gas heating through the photoelectric effect. This strongly suggests that our understanding of the excitation of H2 and/or of heating processes in the Horsehead, and more generally in PDRs, is still incomplete. Nano-grain depletion in the outer part of the Horsehead has a strong influence on several gas tracers that will be prominent in JWST observations of irradiated clouds. We therefore need to take this depletion into account in order to improve our understanding of the Horsehead, and more generally PDRs, and to contribute to the optimal scientific return of the mission.

astro-ph.GA↗