Gas phase Elemental abundances in Molecular cloudS (GEMS)

Abstract

Gas phase Elemental abundances in Molecular CloudS (GEMS) is an IRAM 30 m Large Program designed to provide estimates of the S, C, N, and O depletions and gas ionization degree, X(e−), in a selected set of star-forming filaments of Taurus, Perseus, and Orion. Our immediate goal is to build up a complete and large 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 103 to 106 cm−3, and Tk ~10 to ~30 K ranges in these clouds, and avoiding protostars, HII regions, and bipolar outflows. A statistical analysis is carried out in order to identify general trends between different species and with physical parameters. Relations between molecules reveal strong linear correlations which define three different families of species: (1) 13CO and C18O isotopologs; (2) H13CO+, HC18O+, H13 CN, 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 H13 CO+, HC18 O+, H13 CN, and HNC are well correlated with each other, and all of them decrease with molecular hydrogen density, following the law ∝ n(H2)−0.8 ± 0.2. The abundances of S-bearing species also decrease with molecular hydrogen density at a rate of (S-bearing/H)gas ∝ n(H2)−0.6 ± 0.1. 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 H13 CO+/H13CN abundance ratios as chemical diagnostics of star formation in external galaxies.

Gas phase Elemental abundances in Molecular cloudS (GEMS) V. Methanol in Taurus

“…Figure 2 shows the column density ratio of methanol and C 18 O in all observed positions in our sample plotted against the visual extinction and the dust temperature derived from Herschel and Planck data (Palmeirim et al 2013;Rodríguez-Baras et al 2021), as well as against the volume density derived from the methanol MCMC+RADEX analysis, presented here in Sect. 4.…”

“…Since methanol lines have a relatively high critical density (between 0.7 and 3 × 10 4 cm −3 , see Table 1), the correspondent column density is very sensitive to variations in the H 2 volume density. The H 2 volume density towards all observed positions within the GEMS catalogue has been calculated by modelling the emission of CS, C 34 S, and 13 CS with the radiative transfer code RADEX (van der Tak et al 2007) in Rodríguez-Baras et al (2021). However, the lines of methanol and CS could come from different velocity components and/or different layers within the same cloud.…”

“…The visual extinction tabulated in Table B.1 has been calculated from the H 2 column density maps reported in Rodríguez- Baras et al (2021), using the formula Bohlin et al 1978).…”

“…Two velocity components are present for methanol, while a more complex structure is present in C 18 O, see Figure A.3. Figure A.14 shows in the upper panel the observed offsets within TMC-1 NH 3 on the H 2 column density map derived from Herschel and Planck data (Rodríguez-Baras et al 2021), and the variation of the CH 3 OH and C 18 O column density ratio, as well as the single column densities N(CH 3 OH) and N(C 18 O) in the observed cut across the core for both velocity components, in the lower panel. In the component at 5.7 km s −1 , the column densities of both molecules have a peak at the offset 3, and then decreases moving towards the dust peak.…”

“…As an example of our dataset, Fig. 1 shows to the left the H 2 column density map of the TMC-1 molecular cloud (upper panel) and the B213 filament (lower panel) computed from Herschel and Planck data (Palmeirim et al 2013;Rodríguez-Baras et al 2021), and on the right a zoom-in into the H 2 column density map of B213-C1 where the observed offsets are marked (upper panel) and the spectra of the 2 1,2 -1 1,1 (E 2 ) transition of methanol are overlaid with the 1-0 transition of C 18 O (lower panel). The A10, page 2 of 36…”