arXiv subjects

P. Fernández-Ruiz

Publications and source records attributed to P. Fernández-Ruiz.

2 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↗

arXiv2024

Evolution of Chemistry in the envelope of HOt CorinoS (ECHOS) II. The puzzling chemistry of isomers as revealed by the HNCS/HSCN ratio

The observational detection of some metastable isomers in the interstellar medium with abundances comparable to those of the most stable isomer, or even when the stable isomer is not detected, highlights the importance of non-equilibrium chemistry. This challenges our understanding of the interstellar chemistry. We present a chemical study of isomers through the sulphur isomer pair HNCS and HSCN, since HSCN has been observed in regions where its stable isomer has not been detected, and the observed HNCS/HSCN ratio seems to significantly vary from cold to warm regions. We have used the Nautilus chemical code to model the formation and destruction paths of HNCS and HSCN in different astrochemical scenarios, and the time evolution of the HNCS/HSCN ratio. We have also analysed the influence of the environmental conditions on their chemical abundances. We present an observational detection of the metastable isomer HSCN in the Class I object B1-a, but not of the stable isomer HNCS, despite HNCS lying 3200 K lower in energy than HSCN. Our results show an HNCS/HSCN ratio sensitive to the gas temperature and the evolutionary time, with the highest values obtained at early stages (t<10^4 yr) and low (Tg<20 K) temperatures. The results suggest a different efficiency of the isomerisation processes depending on the source temperature. The progressive decrease of HNCS/HSCN with gas temperature at early evolutionary times indicates that this ratio may be used as a tracer of cold young objects. This work also demonstrates the key role of grain surface chemistry in the formation of the isomer pair HNCS and HSCN in cold regions, and the importance of the ions H2NCS+ and HNCSH+ in warm/hot regions. Since most of the interstellar regions where HSCN is detected are cold regions, a larger sample including sources characterised by high temperatures are needed to corroborate the theoretical results.

astro-ph.GA↗