The GUAPOS project. VI: the chemical inventory of shocked gas

TL;DR

This paper investigates the chemical inventory of shocked gas in the G31.41+0.31 protocluster as a proxy for icy mantle composition, contrasting it with the neighboring hot core and with other shocks. Using an unbiased ALMA Band 3 spectral survey (GUAPOS‑VI), LTE line fitting with SLIM/MADCUBA and isotopologue analyses, the authors derive $N$ and $N/N_{ extrm{H}_2}$ for 30 species (plus 18 isotopologues) in the G31.41 shock, obtaining $N( extrm{H}_2) \

Abstract

The study of the chemical composition of star-forming regions is key to understand the chemical ingredients available during the formation of planetary systems. Given that the chemical inventory on interstellar dust grains in the prestellar phases might be altered due to the prostostellar warm-up, an alternative to infer the chemical composition on the grains could be to observe regions affected by shocks associated with molecular outflows. Such shocks can desorb the molecules, and might produce less chemical processing due to shorter timescales. We present here a detailed study of the chemical reservoir of a shocked region located in the G31.41+0.31 protocluster using GUAPOS data (G31.41+0.31 Unbiased ALMA sPectral Observational Survey). We report here the detection of 30 molecular species (plus 18 isotopologues). We performed a comparison of the molecular ratios in the shocked region with those derived towards the hot core of G31.41+0.31, finding that they are poorly correlated, excepting N-bearing species. Our results confirm observationally that a different level of chemical alteration is present in hot cores and in shocks. While the former likely alter the molecular ratios due to thermal processing during longer timescales, the latter might represent freshly desorbed material that constitutes a better proxy of the icy mantle composition. The similarity of molecular ratios between the N-bearing species in the G31.41 shock and the hot core suggests that these species are desorbed at early evolutionary stages. Interestingly, we have found that the abundances in the G31.41 shock show better correlations with other shock-dominated regions (two protostellar outflows and a Galactic Center molecular cloud). This suggests a negligible gas-phase chemistry after shock-induced ejection from grains, and that the ice-mantle composition is similar regardless of the Galactic environment.

Figures (12)

• Figure 1: Spatial distribution towards the G31.41 star-forming region of the molecular emission of CH$_3$CN, the broad component of CH$_3$OH, C$_2$H$_5$OH, the narrow component of OCS (upper panels from left to right) and HNCO, CH$_3$OCH$_3$ and NS (lower panels from left to right). In the all panels it is showed in white the molecule name, the molecular transition, the energy of the upper level and the velocity range. In addition, in green is labeled the G31.41 shock region according to region 2 of Fig. 1 in Fontani2024. The ultracompact HII region and the hot core of G31.41 are also labeled. The 3 and 20 $\sigma$ contour levels of the 3mm continuum map at 98.5 GHz from Mininni2020 are plotted in black. The minimum flux level of the colorbar of each panel is indicated by white contours.
• Figure 2: Transitions of CH$_3$CHO detected towards the G31.41 shock position. The black histogram and its gray shadow are the observed spectrum. The red curve is the best LTE fit of the individual species and the blue curve is the cumulative fit considering all detected species. The red dashed lines indicate the frequency of the transitions that we are fitting. The plots are sorted by decreasing line intensity of the transitions.
• Figure 3: Transitions of NH$_2$CHO detected towards the G31.41 shock position. The black histogram and its gray shadow are the observed spectrum. The red curve is the best LTE fit of the individual species and the blue curve is the cumulative fit considering all detected species. The red dashed lines indicate the frequency of the transitions that we are fitting. The plots are sorted by decreasing line intensity of the transitions.
• Figure 4: Transitions of CH$_3$SH detected towards the G31.41 shock position. The black histogram and its gray shadow are the observed spectrum. The red curve is the best LTE fit of the individual species and the blue curve is the cumulative fit considering all detected species. The red dashed lines indicate the frequency of the transitions that we are fitting. The plots are sorted by decreasing line intensity of the transitions.
• Figure 5: Comparison of the linewidths and velocities of the molecules detected towards the G31.41 shock (from Table \ref{['tab:Poperties_molecules_on_G31_shock']} and from GUAPOS IV) with those in the G31.41 core (from Table \ref{['tab:Poperties_molecules_on_G31_core']}, GUAPOS I, II, III and V). The species identified in the G31.41 shock are separated in two groups (see Sect. \ref{['sec:mols_arise_shock']} for details): those directly related with the shock (blue dots) and those likely related with an extended PDR in the line of sight (green dots). The molecules detected in the core are indicated with orange dots. Only the molecules for which FWHM and v were left free in the fits are plotted.