31 research outputs found
Evolution of Chemistry in the envelope of Hot Corinos (ECHOS). I. Extremely young sulphur chemistry in the isolated Class 0 object B335
Within the project Evolution of Chemistry in the envelope of HOt corinoS
(ECHOS), we present a study of sulphur chemistry in the envelope of the Class 0
source B335 through observations in the spectral range 7, 3, and 2 mm. We have
modelled observations assuming LTE and LVG approximation. We have also used the
code Nautilus to study the time evolution of sulphur species. We have detected
20 sulphur species with a total gas-phase S abundance similar to that found in
the envelopes of other Class 0 objects, but with significant differences in the
abundances between sulphur carbon chains and sulphur molecules containing
oxygen and nitrogen. Our results highlight the nature of B335 as a source
especially rich in sulphur carbon chains unlike other Class 0 sources. The low
presence or absence of some molecules, such as SO and SO+, suggests a chemistry
not particularly influenced by shocks. We, however, detect a large presence of
HCS+ that, together with the low rotational temperatures obtained for all the S
species (<15 K), reveals the moderate or low density of the envelope of B335.
We also find that observations are better reproduced by models with a sulphur
depletion factor of 10 with respect to the sulphur cosmic elemental abundance.
The comparison between our model and observational results for B335 reveals an
age of 10t10 yr, which highlights the particularly early
evolutionary stage of this source. B335 presents a different chemistry compared
to other young protostars that have formed in dense molecular clouds, which
could be the result of accretion of surrounding material from the diffuse cloud
onto the protostellar envelope of B335. In addition, the analysis of the
SO2/C2S, SO/CS, and HCS+/CS ratios within a sample of prestellar cores and
Class 0 objects show that they could be used as good chemical evolutionary
indicators of the prestellar to protostellar transition
Photoprocessing of H2S on dust grains: building S chains in translucent clouds and comets
Stars and planetary system
H2S observations in young stellar disks in Taurus
Stars and planetary system
Gas Phase Elemental Abundances in Molecular CloudS (GEMS) V. Methanol in Taurus
Context. Methanol, one of the simplest complex organic molecules in the interstellar medium, has been shown to be present and extended in cold environments such as starless cores. Studying the physical conditions at which CH3OH starts its efficient formation is important to understand the development of molecular complexity in star-forming regions. Aims. We aim to study methanol emission across several starless cores and investigate the physical conditions at which methanol starts to be efficiently formed, as well as how the physical structure of the cores and their surrounding environment affect its distribution. Methods. Methanol and C18O emission lines at 3 mm have been observed with the IRAM 30 m telescope within the large programme Gas phase Elemental abundances in Molecular CloudS towards 66 positions across 12 starless cores in the Taurus Molecular Cloud. A non-LTE (local thermodynamic equilibrium) radiative transfer code was used to compute the column densities in all positions. We then used state-of-the-art chemical models to reproduce our observations. Results. We have computed N(CH3OH)/N(C18O) column density ratios for all the observed offsets, and the following two different behaviours can be recognised: the cores where the ratio peaks at the dust peak and the cores where the ratio peaks with a slight offset with respect to the dust peak (∼10 000 AU). We suggest that the cause of this behaviour is the irradiation on the cores due to protostars nearby which accelerate energetic particles along their outflows. The chemical models, which do not take irradiation variations into account, can reproduce the overall observed column density of methanol fairly well, but they cannot reproduce the two different radial profiles observed. Conclusions. We confirm the substantial effect of the environment on the distribution of methanol in starless cores. We suggest that the clumpy medium generated by protostellar outflows might cause a more efficient penetration of the interstellar radiation field in the molecular cloud and have an impact on the distribution of methanol in starless cores. Additional experimental and theoretical work is needed to reproduce the distribution of methanol across starless cores. © S. Spezzano et al. 2021.Acknowledgements. The authors are grateful to the anonymous referee for insightful comments. A large part of the data analysis described in this paper was performed during the spring of 2020, in the beginning of the COVID pandemic and during a hard lockdown. S.S. wishes to thank the Max Planck Society for the flexibility that was allowed during the pandemic, because it contributed to maintaining a clear and focus mind during the hours that she could dedicate to her work, and overall to keep calm, while waiting for the ‘storm’ to pass. Based on analysis carried out with the CASSIS software (http://cassis.irap. omp.eu) and CDMS and JPL spectroscopic databases and LAMDA molecular databases. CASSIS has been developed by IRAP-UPS/CNRS. S.S. wishes to thank the Max Planck Society for the Independent Max Planck Research Group funding. A.F., D.N.A. and M.R.B. are funded by Spanish MICINN through PID2010-106235GB-I00 national research project. V.W. acknowledges the CNRS program Physique et Chimie du Milieu Interstellaire (PCMI) co-funded by the Centre National d’Etudes Spatiales (CNES). A.V. and A.P. are the members of the Max Planck Partner Group at the Ural Federal University. A.V. and A.P. acknowledge the support of the Russian Ministry of Science and Education via the State Assignment Contract no. FEUZ-2020-0038
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.Comment: 31 pages, 26 figure
Gas-phase Elemental abundances in Molecular cloudS (GEMS) III. Unlocking the CS chemistry: the CS+O reaction
Context. Carbon monosulphide (CS) is among the most abundant gas-phase S-bearing molecules in cold dark molecular clouds. It is easily observable with several transitions in the millimeter wavelength range, and has been widely used as a tracer of the gas density in the interstellar medium in our Galaxy and external galaxies. However, chemical models fail to account for the observed CS abundances when assuming the cosmic value for the elemental abundance of sulfur.
Aims. The CS+O → CO + S reaction has been proposed as a relevant CS destruction mechanism at low temperatures, and could explain the discrepancy between models and observations. Its reaction rate has been experimentally measured at temperatures of 150−400 K, but the extrapolation to lower temperatures is doubtful. Our goal is to calculate the CS+O reaction rate at temperatures <150 K which are prevailing in the interstellar medium.
Methods. We performed ab initio calculations to obtain the three lowest potential energy surfaces (PES) of the CS+O system. These PESs are used to study the reaction dynamics, using several methods (classical, quantum, and semiclassical) to eventually calculate the CS + O thermal reaction rates. In order to check the accuracy of our calculations, we compare the results of our theoretical calculations for T ~ 150−400 K with those obtained in the laboratory.
Results. Our detailed theoretical study on the CS+O reaction, which is in agreement with the experimental data obtained at 150–400 K, demonstrates the reliability of our approach. After a careful analysis at lower temperatures, we find that the rate constant at 10 K is negligible, below 10−15 cm3 s−1, which is consistent with the extrapolation of experimental data using the Arrhenius expression.
Conclusions. We use the updated chemical network to model the sulfur chemistry in Taurus Molecular Cloud 1 (TMC 1) based on molecular abundances determined from Gas phase Elemental abundances in Molecular CloudS (GEMS) project observations. In our model, we take into account the expected decrease of the cosmic ray ionization rate, ζH2, along the cloud. The abundance of CS is still overestimated when assuming the cosmic value for the sulfur abundance
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 m allowed us
to compute the emissivity spectral index and look for grain growth as an
evolutionary tracer. The HCN/HNC ratio is anticorrelated with the
deuterium fraction of HCN, thus it can readily serve as a proxy for the
temperature. The spectral index shows a tentative
anticorrelation with the HCN/HNC 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.Comment: 25 pages, 20 figure
Gas phase Elemental abundances in Molecular cloudS (GEMS) VII. Sulfur elemental abundance
Gas phase Elemental abundances in molecular CloudS (GEMS) is an IRAM 30m
large program aimed at determining the elemental abundances of carbon (C),
oxygen (O), nitrogen (N), and sulfur (S) in a selected set of prototypical
star-forming filaments. In particular, the elemental abundance of S remains
uncertain by several orders of magnitude and its determination is one of the
most challenging goals of this program. We have carried out an extensive
chemical modeling of the fractional abundances of CO, HCO, HCN, HNC, CS,
SO, HS, OCS, and HCS to determine the sulfur depletion toward the 244
positions in the GEMS database. These positions sample visual extinctions from
A 3 mag to 50 mag, molecular hydrogen densities ranging from a
few 10~cm to 310~cm, and T 1035 K.
Most of the positions in Taurus and Perseus are best fitted assuming early-time
chemistry, t=0.1 Myr, (0.51)10 s,
and [S/H]1.510. On the contrary, most of the positions in
Orion are fitted with t=1~Myr and 10 s.
Moreover, 40% of the positions in Orion are best fitted assuming the
undepleted sulfur abundance, [S/H]1.510. Our results
suggest that sulfur depletion depends on the environment. While the abundances
of sulfur-bearing species are consistent with undepleted sulfur in Orion, a
depletion factor of 20 is required to explain those observed in Taurus
and Perseus. We propose that differences in the grain charge distribution in
the envelopes of the studied clouds might explain these variations. The shocks
associated with past and ongoing star formation could also contribute to
enhance [S/H] in Orion.Comment: 22 pages, 15 figures, Astronomy and Astrophysics, in pres
Gas phase Elemental abundances in Molecular cloudS (GEMS) : II. On the quest for the sulphur reservoir in molecular clouds: the H2S case
Context. Sulphur is one of the most abundant elements in the Universe. Surprisingly, sulphuretted molecules are not as abundant as expected in the interstellar medium and the identity of the main sulphur reservoir is still an open question.Aims. Our goal is to investigate the H2S chemistry in dark clouds, as this stable molecule is a potential sulphur reservoir.Methods. Using millimeter observations of CS, SO, H2S, and their isotopologues, we determine the physical conditions and H2S abundances along the cores TMC 1-C, TMC 1-CP, and Barnard 1b. The gas-grain model NAUTILUS is used to model the sulphur chemistry and explore the impact of photo-desorption and chemical desorption on the H2S abundance.Results. Our modeling shows that chemical desorption is the main source of gas-phase H2S in dark cores. The measured H2S abundance can only be fitted if we assume that the chemical desorption rate decreases by more than a factor of 10 when n(H) > 2 x 10(4). This change in the desorption rate is consistent with the formation of thick H2O and CO ice mantles on grain surfaces. The observed SO and H2S abundances are in good agreement with our predictions adopting an undepleted value of the sulphur abundance. However, the CS abundance is overestimated by a factor of 5-10. Along the three cores, atomic S is predicted to be the main sulphur reservoir.Conclusions. The gaseous H2S abundance is well reproduced, assuming undepleted sulphur abundance and chemical desorption as the main source of H2S. The behavior of the observed H2S abundance suggests a changing desorption efficiency, which would probe the snowline in these cold cores. Our model, however, highly overestimates the observed gas-phase CS abundance. Given the uncertainty in the sulphur chemistry, we can only conclude that our data are consistent with a cosmic elemental S abundance with an uncertainty of a factor of 10.Peer reviewe