Recent observations have revealed the existence of complex organic molecules (COMs) in cold dense cores and pre-stellar cores. The presence of these molecules in such cold conditions is not well ...understood and remains a matter of debate since the previously proposed 'warm-up' scenario cannot explain these observations. In this paper, we study the effect of Eley-Rideal and complex induced reaction mechanisms of gas-phase carbon atoms with the main ice components of dust grains on the formation of COMs in cold and dense regions. Based on recent experiments, we use a low value for the chemical desorption efficiency (which was previously invoked to explain the observed COM abundances). We show that our introduced mechanisms are efficient enough to produce a large amount of COMs in the gas phase at temperatures as low as 10 K.
A wide variety of molecules have recently been detected in the Horsehead nebula photodissociation region (PDR) suggesting that: (i) gas-phase and grain chemistries should both contribute to the ...formation of organic molecules; and (ii) far-ultraviolet (FUV) photodesorption may explain the release into the gas phase of grain surface species. In order to tackle these specific problems and more generally in order to better constrain the chemical structure of these types of environments we present a study of the Horsehead nebula gas-grain chemistry. To do so we used the 1D astrochemical gas-grain code Nautilus with an appropriate physical structure computed with the Meudon PDR code and compared our modeled outcomes with published observations and with previously modeled results when available. The use of a large set of chemical reactions coupled with the time-dependent code Nautilus allows us to reproduce most of the observations well, including those of the first detections in a PDR of the organic molecules HCOOH, CH2CO, CH3CHO and CH3CCH, which are mostly associated with hot cores. We also provide some abundance predictions for other molecules of interest. Understanding the chemistry behind the detection of these organic molecules is crucial to better constrain the environments these molecules can probe.
Aims. Ionized carbon is the main gas-phase reservoir of carbon in the neutral diffuse interstellar medium (ISM) and its 158 μm fine structure transition C ii is the most important cooling line of the ...diffuse ISM. We combine C ii absorption and emission spectroscopy to gain an improved understanding of physical conditions in the different phases of the ISM. Methods. We present high-resolution C ii spectra obtained with the Herschel/HIFI instrument towards bright dust continuum regions in the Galactic plane, probing simultaneously the diffuse gas along the line of sight and the background high-mass star forming regions. These data are complemented by single pointings in the 492 and 809 GHz fine structure lines of atomic carbon and by medium spectral resolution spectral maps of the fine structure lines of atomic oxygen at 63 and 145 μm with Herschel/PACS. Results. We show that the presence of foreground absorption may completely cancel the emission from the background source in medium spectral resolution PACS data and that high spectral resolution spectra are needed to interpret the C ii and O i emission and the C ii/FIR ratio. This phenomenon may explain part of the C ii/FIR deficit seen in external luminous infrared galaxies where the bright emission from the nuclear regions may be partially canceled by absorption from diffuse gas in the foreground. The C+ and C excitation in the diffuse gas is consistent with a median pressure of ~5900 K cm-3 for a mean kinetic temperature of ~100 K. A few higher pressure regions are detected along the lines of sight, as emission features in both fine structure lines of atomic carbon. The knowledge of the gas density allows us to determine the filling factor of the absorbing gas along the selected lines of sight. The derived median value of the filling factor is 2.4%, in good agreement with the properties of the Galactic cold neutral medium. The mean excitation temperature is used to derive the average cooling due to C+ in the Galactic plane : 9.5 × 10-26 erg-1H-1. Along the observed lines of sight, the gas phase carbon abundance does not exhibit a strong gradient as a function of Galacto-centric radius and has a weighted average of C/H = 1.5 ± 0.4 × 10-4.
ABSTRACT
Molecular oxygen has been the subject of many observational searches as chemical models predicted it to be a reservoir of oxygen. Although it has been detected in two regions of the ...interstellar medium, its rarity is a challenge for astrochemical models. In this paper, we have combined the physical conditions computed with smoothed particle hydrodynamics simulations with our full gas–grain chemical model Nautilus, to study the predicted O2 abundance in interstellar material forming cold cores. We thus follow the chemical evolution of gas and ices in parcels of material from the diffuse interstellar conditions to the cold dense cores. Most of our predicted O2 abundances are below 10−8 (with respect to the total proton density) and the predicted column densities in simulated cold cores are at maximum a few 10−14 cm−2, in agreement with the non-detection limits. This low O2 abundance can be explained by the fact that, in a large fraction of the interstellar material, the atomic oxygen is depleted on to the grain surface (and hydrogenated to form H2O) before O2 can be formed in the gas-phase and protected from ultraviolet photodissociations. We could achieve this result only because we took into account the full history of the evolution of the physical conditions from the diffuse medium to the cold cores.
Aims.
Interstellar molecules form early in the evolutionary sequence of interstellar material that eventually forms stars and planets. To understand this evolutionary sequence, it is important to ...characterize the chemical composition of its first steps.
Methods.
In this paper, we present the result of a 2 and 3 mm survey of five cold clumps identified by the
Planck
mission. We carried out a radiative transfer analysis on the detected lines in order to put some constraints on the physical conditions within the cores and on the molecular column densities. We also performed chemical models to reproduce the observed abundances in each source using the gas-grain model Nautilus.
Results.
Twelve molecules were detected: H
2
CO, CS, SO, NO, HNO, HCO
+
, HCN, HNC, CN, CCH, CH
3
OH, and CO. Here, CCH is the only carbon chain we detected in two sources. Radiative transfer analyses of HCN, SO, CS, and CO were performed to constrain the physical conditions of each cloud with limited success. The sources have a density larger than 10
4
cm
−3
and a temperature lower than 15 K. The derived species column densities are not very sensitive to the uncertainties in the physical conditions, within a factor of 2. The different sources seem to present significant chemical differences with species abundances spreading over one order of magnitude. The chemical composition of these clumps is poorer than the one of Taurus Molecular Cloud 1 Cyanopolyyne Peak (TMC-1 CP) cold core. Our chemical model reproduces the observational abundances and upper limits for 79–83% of the species in our sources. The ‘best’ times for our sources seem to be smaller than those of TMC-1, indicating that our sources may be less evolved and explaining the smaller abundances and the numerous non-detections. Also, CS and HCN are always overestimated by our models.
Abstract
Methyl isocyanate (CH3NCO) is one of the important complex organic molecules detected on the comet 67P/Churyumov–Gerasimenko by Rosetta’s Philae lander. It was also detected in hot cores ...around high-mass protostars along with a recent detection in the solar-type protostar IRAS 16293−2422. We propose here a gas-grain chemical model to form CH3NCO after reviewing various formation pathways with quantum chemical computations. We have used nautilus three-phase gas-grain chemical model to compare observed abundances in the IRAS 16293−2422. Our chemical model clearly indicates the ice phase origin of CH3NCO.
ABSTRACT
We present a study of the elemental depletion in the interstellar medium. We combined the results of a Galactic model describing the gas physical conditions during the formation of dense ...cores with a full-gas-grain chemical model. During the transition between diffuse and dense medium, the reservoirs of elements, initially atomic in the gas, are gradually depleted on dust grains (with a phase of neutralization for those which are ions). This process becomes efficient when the density is larger than 100 cm−3. If the dense material goes back into diffuse conditions, these elements are brought back in the gas phase because of photo-dissociations of the molecules on the ices, followed by thermal desorption from the grains. Nothing remains on the grains for densities below 10 cm−3 or in the gas phase in a molecular form. One exception is chlorine, which is efficiently converted at low density. Our current gas–grain chemical model is not able to reproduce the depletion of atoms observed in the diffuse medium except for Cl, which gas abundance follows the observed one in medium with densities smaller than 10 cm−3. This is an indication that crucial processes (involving maybe chemisorption and/or ice irradiation profoundly modifying the nature of the ices) are missing.
The ketenyl radical (HCCO) has recently been discovered in two cold dense clouds with a non-negligible abundance of a few 10−11 (compared to H2). Until now, no chemical network has been able to ...reproduce this observation. We propose here a chemical scheme that can reproduce HCCO abundances together with HCO, H2CCO and CH3CHO in the dark clouds Lupus-1A and L486. The main formation pathway for HCCO is the OH + CCH → HCCO + H reaction as suggested by Agúndez et al. but with a much larger rate coefficient than used in current models. Since this reaction has never been studied experimentally or theoretically, this larger value is based on a comparison with other similar systems.
Abstract
Sulphur-bearing species are often used to probe the physical structure of star-forming regions of the interstellar medium, but the chemistry of sulphur in these regions is still poorly ...understood. In dark clouds, sulphur is supposed to be depleted under a form that is still unknown despite numerous observations and chemical modelling studies that have been performed. In order to improve the modelling of sulphur chemistry, we propose an enhancement of the sulphur chemical network using experimental and theoretical literature. We test the effect of the updated network on the outputs of a three-phases gas–grain chemical model for dark cloud conditions using different elemental sulphur abundances. More particularly, we focus our study on the main sulphur reservoirs as well as on the agreement between model predictions and the abundances observed in the dark cloud TMC-1 (CP). Our results show that depending on the age of the observed cloud, the reservoir of sulphur could either be atomic sulphur in the gas phase or HS and H2S in icy grain bulks. We also report the first chemical model able to reproduce the abundances of observed S-bearing species in TMC-1 (CP) using as elemental abundance of sulphur its cosmic value.
Context. The H3+ molecule has been detected in many lines of sight within the central molecular zone (CMZ) with exceptionally large column densities and unusual excitation properties compared to ...diffuse local clouds. The detection of the (3, 3) metastable level has been suggested to be the signature of warm and diffuse gas in the CMZ. Aims. We aim to determine the physical conditions and processes in the CMZ that explain the ubiquitous properties of H3+ in this medium and to constrain the value of the cosmic-ray ionization rate. Methods. We use the Meudon photodissociation region (PDR) code in which H3+ excitation has been implemented. We re-examine the relationship between the column density of H3+ and the cosmic-ray ionization rate, ζ, up to large values of ζ in the frame of this full chemical model. We study the impact of the various mechanisms that can excite H3+ in its metastable state. We produce grids of PDR models exploring different parameters (ζ, size of clouds, metallicity) and infer the physical conditions that best match the observations toward ten lines of sight in the CMZ. For one of them, Herschel observations of HF, OH+, H2O+, and H3O+ can be used as additional constraints. We check that the results found for H3+ also account for the observations of these molecules. Results. We find that the linear relationship between N(H3+) and ζ only holds up to a certain value of the cosmic-ray ionization rate, which depends on the proton density. A value ζ ~ 1−11 × 10-14 s-1 explains both the large observed H3+ column density and its excitation in the metastable level (3, 3). This ζ value agrees with that derived from synchrotron emission and Fe Kα line. It also reproduces N(OH+), N(H2O+) and N(H3O+) detected toward Sgr B2(N). We confirm that the CMZ probed by H3+ is diffuse, nH≲ 100 cm-3 and warm, T ~ 212−505 K. This warm medium is due to cosmic-ray heating. We also find that the diffuse component probed by H3+ must fill a large fraction of the CMZ. Finally, we suggest the warm gas in the CMZ enables efficient H2 formation via chemisorption sites as in PDRs. This contributes to enhance the abundance of H3+ in this high cosmic-ray flux environment.
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