arXiv subjects

P. Riviére-Marichalar

Publications and source records attributed to P. Riviére-Marichalar.

7 recordsLinked to original sources

arXiv2026

Evolution of Chemistry in the envelope of HOt corinoS (ECHOS). III. Sulphur chemistry in the Class 0 objects HH 212 and NGC 1333 IRAS 4A

Our goal is to find chemical diagnostics to determine the physical conditions in protostellar envelopes and help establish the development of matter during the formation of a low-mass star, as well as investigating a possible variation of sulphur depletion during the star formation process at the scale of the cold envelope. With observations with the Yebes-40m and IRAM-30m telescopes, we estimate column densities of sulphur-bearing species in the Class 0 objects HH212 and NGC1333 IRAS4A. A neural emulator of the chemical code Nautilus is used to constrain the chemical time, density, gas temperature, cosmic ray ionization rate, and sulphur elemental abundance in the cold envelope of these objects. We compare the resulting abundances of these species with those towards the Class 0 object B335. While sulphur-bearing species containing carbon chains are between 3 and 7 times more abundant in B335 than in the other two objects, sulphur oxides and nitrogen-bearing species are 3 times more abundant in NGC1333 IRAS4A. Our chemical modelling shows that, while the chemistry of HH212 and NGC1333 IRAS4A is well reproduced considering a gas temperature of 25 K and a sulphur depletion of a factor of 100 in their envelopes, significant differences are found in their average density and cosmic ray ionization rate. Comparing with similar studies in pre-stellar and protostellar cores, we derive an increase in the SO/CS and SO$_2$/C$_2$S ratios of about two orders of magnitude and a potential decrease in the HCS$^+$/CS ratio of a factor of 10 in the transition from the pre-stellar to the Class 0 phase. Sulphur compounds are good evolutionary tracers of the pre- to protostellar phase transition, with oxygen-bearing species being more abundant than those containing carbon in the evolved sources. Nonetheless, sulphur depletion in the cold envelope of Class 0 objects remains similar to that in starless cores.

astro-ph.GA↗

arXiv2026

Chemical study of two starless cores in the B213/L1495 filament

The chemical evolution of pre-stellar cores during their transition to a protostellar stage is not yet fully understood. Detailed chemical characterizations of these sources are needed to better define their chemistry during star formation. Our goal is to characterize the chemistry of the starless cores C2 and C16 in the B213/L1495 filament of the Taurus Molecular Cloud, and to understand how it relates to the environmental conditions and the evolutionary state of the cores. We made use of two complete spectral surveys at 7 mm of these sources, carried out using the Yebes 40-m telescope. Derived molecular abundances were compared with those of other sources in different evolutionary stages and with values computed by chemical models. Including isotopologs, 22 molecules were detected in B213-C2, and 25 in B213-C16. The derived rotational temperatures have values of between $\sim$ 5 K and $\sim$ 9 K. A comparison of the two sources shows lower abundances in C2, except for l-C$_{3}$H and HOCO$^{+}$, which have similar values in both cores. Model results indicate that both cores are best fit assuming early-time chemistry, and point to C2 being in a more advanced evolutionary stage, as it presents a higher molecular hydrogen density and sulfur depletion, and a lower cosmic-ray ionization rate. Our chemical modeling successfully accounts for the abundances of most molecules, including complex organic molecules and long cyanopolynes (HC$_{5}$N, HC$_{7}$N), but fails to reproduce those of the carbon chains CCS and C$_{3}$O. Chemical differences between C2 and C16 could stem from the evolutionary stage of the cores, with C2 being closer to the pre-stellar phase. Both cores are better fit assuming early-time chemistry of t $\sim$ 0.1 Myr. The more intense UV radiation in the northern region of B213 could account for the high abundances of l-C$_{3}$H and HOCO$^{+}$ in C2.

astro-ph.GA↗

arXiv2024

Grain growth and its chemical impact in the first hydrostatic core phase

The first hydrostatic core (FHSC) phase is a brief stage in the protostellar evolution that is difficult to detect. Our goal is to characterize the chemical evolution of gas and dust during the formation of the FHSC. Moreover, we are interested in analyzing, for the first time with 3D magnetohydrodynamic (MHD) simulations, the role of grain growth in its chemistry. We postprocessed $2\times10^{5}$ tracer particles from a $\texttt{RAMSES}$ non-ideal MHD simulation using the codes $\texttt{NAUTILUS}$ and $\texttt{SHARK}$ to follow the chemistry and grain growth throughout the simulation. A great chemical inheritance is seen, as gas-phase abundances of most of the C, O, N, and S reservoirs in the hot corino at the end of the simulation match the ice-phase abundances from the prestellar phase. Additionally, interstellar complex organic molecules (iCOMs) such as methyl formate, acetaldehyde, and formamide are formed during the warm-up process. The typical grain size in the hot corino $(n_{\rm H}>10^{11}\ {\rm cm^{-3}})$ increases forty-fold during the last 30 kyr, with negligible effects on its chemical composition. At moderate densities $(10^{10}<n_{\rm H}<10^{11}\ {\rm cm^{-3}})$ and cool temperatures $15<T<50$ K, increasing grain sizes delay molecular depletion. Finally, at low densities $(n_{\rm H}\sim10^{7}\ {\rm cm^{-3}})$, grains do not grow significantly. We also compared our results with a two-step model that reproduces well the abundances of C and O reservoirs, but not the N and S reservoirs. We conclude that the chemical composition of the FHSC is heavily determined by that of the parent prestellar core, chemo-MHD computations are needed for an accurate prediction of the abundances of the main N and S elemental reservoirs, and that the impact of grain growth in moderately dense areas delaying depletion permits the use of abundance ratios as grain growth proxies.

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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 $μ$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 $(β\sim 1.34-2.09)$ 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.

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arXiv2021

Photoprocessing of H2S on dust grains: building S chains in translucent clouds and comets

Context. Sulfur is used as a tracer of the evolution from interstellar clouds to stellar systems. However, most of the expected sulfur in molecular clouds remains undetected. Sulfur disappears from the gas phase in two steps. One first depletion occurs during the translucent phase, reducing the gas phase sulfur by 7-40 times, while the following freeze-out step occurs in molecular clouds, reducing it by another order of magnitude. This long-standing dilemma awaits an explanation. Aims. The aim of this study is to understand under which form the missing sulfur is hiding in molecular clouds. Depletion onto dust grains is considered. Methods. Experimental simulations mimicking H2S ice UV-photoprocessing in molecular clouds were conducted. The ice was monitored using infrared spectroscopy and the desorbing molecules were measured by quadrupole mass spectrometry. Theoretical Monte Carlo simulations were performed for interpretation of the experimental results and extrapolation to astrophysical conditions. Results. H2S2 formation was observed during irradiation at 8 K. Molecules H2Sx with x > 2 were also identified and found to desorb during warm-up, along with S2 to S4 species. Larger Sx molecules up to S8 are refractory at room temperature and remained on the substrate forming a residue. Monte Carlo simulations were able to reproduce the molecules desorbing during warming up, and found that residues are chains or sulfur consisting of 6-7 atoms. Conclusions. We propose that S+ in translucent clouds contributes notoriously to S depletion in denser regions by forming long S-chains on dust in few times 10^4 years. We suggest that the S2 to S4 molecules observed in comets are not produced by fragmentation of these large chains. Instead, they probably come either from UV-photoprocessing of H2S-bearing ice produced in molecular clouds or from short S chains formed during the translucent cloud phase

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arXiv2021

Gas phase Elemental abundances in Molecular cloudS (GEMS). IV. Observational results and statistical trends

Gas phase Elemental abundances in Molecular CloudS (GEMS) is an IRAM 30m Large Program designed to estimate the S, C, N, and O depletions and gas ionization degree, X(e-), in a set of star-forming filaments of Taurus, Perseus and Orion. Our immediate goal is to build up a complete database of molecular abundances that can serve as an observational basis for estimating X(e-) and the C, O, N, and S depletions through chemical modeling. We observed and derived the abundances of 14 species (13CO, C18O, HCO+, H13CO+, HC18O+, HCN, H13CN, HNC, HCS+, CS, SO, 34SO, H2S, and OCS) in 244 positions, covering the AV 3 to 100 mag, n(H2) a few 10$^{3}$ to 10$^6$ cm$^{-3}$, and Tk 10 to 30 K ranges in these clouds, avoiding protostars, HII regions, and outflows. A statistical analysis is carried out to identify general trends between different species and with physical parameters. Relations between molecules reveal strong linear correlations which define three different families: (1) 13CO and C18O; (2) H13CO+, HC18O+, H13CN, and HNC; and (3) the S-bearing molecules. The abundances of the CO isotopologs increase with the gas kinetic temperature until TK 15 K. For higher temperatures, the abundance remains constant with a scatter of a factor of 3. The abundances of H13CO+, HC18O+, H13CN, and HNC are well correlated with each other, and all of them decrease with molecular hydrogen density, following the law n(H2)$^{-0.8\pm0.2}$. The abundances of S-bearing species also decrease with n(H2) at a rate of (S-bearing/H)gas n(H2)$^{-0.6\pm0.1}$. The abundances of molecules belonging to groups 2 and 3 do not present any clear trend with gas temperature. At scales of molecular clouds, the C18O abundance is the quantity that better correlates with the cloud mass. We discuss the utility of the 13CO/C18O, HCO+/H13CO+, and H13CO+/H13CN abundance ratios as chemical diagnostics of star formation in external galaxies.

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arXiv2019

Gas phase Elemental abundances in Molecular cloudS (GEMS) I. The prototypical dark cloud TMC 1

GEMS is an IRAM 30m Large Program whose aim is determining the elemental depletions and the ionization fraction in a set of prototypical star-forming regions. This paper presents the first results from the prototypical dark cloud TMC 1. Extensive millimeter observations have been carried out with the IRAM 30m telescope (3mm and 2mm) and the 40m Yebes telescope (1.3cm and 7mm) to determine the fractional abundances of CO, HCO+, HCN, CS, SO, HCS+, and N2H+ in three cuts which intersect the dense filament at the well-known positions TMC 1-CP, TMC 1-NH3, and TMC 1-C, covering a visual extinction range from Av~3 to ~20mag. Two phases with differentiated chemistry can be distinguished: i) the translucent envelope with molecular hydrogen densities of (1-5)x10$^3$ cm$^{-3}$; and ii) the dense phase, located at Av>10mag, with molecular hydrogen densities >10$^4$ cm$^{-3}$. Observations and modeling show that the gas phase abundances of C and O progressively decrease along the C+/C/CO transition zone where C/H~8x10$^{-5}$ and C/O~0.8-1, until the beginning of the dense phase at Av~10 mag. This is consistent with the grain temperatures being below the CO evaporation temperature in this region. In the case of sulfur, a strong depletion should occur before the translucent phase where we estimate a S/H~(0.4 - 2.2) x10$^{-6}$, an abundance ~7-40 times lower than the solar value. A second strong depletion must be present during the formation of the thick icy mantles to achieve the values of S/H measured in the dense cold cores (S/H~8x10$^{-8}$). Based on our chemical modeling, we constrain the value of $ζ_{\rm H_2}$ to ~(0.5 - 1.8) x10$^{-16}$ s$^{-1}$ in the translucent cloud.

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