Gas phase Elemental abundances in Molecular cloudS (GEMS)

IV. Observational results and statistical trends

¹ Observatorio Astronómico Nacional (OAN), Alfonso XII, 3, 28014 Madrid, Spain
e-mail: m.rodriguez@oan.es
² Centre for Astrochemical Studies, Max-Planck-Institute for Extraterrestrial Physics, Giessenbachstrasse 1, 85748 Garching, Germany
³ Observatoire de Paris, PSL Research University, CNRS, École Normale Supérieure, Sorbonne Universités, UPMC Univ. Paris 06, 75005 Paris, France
⁴ Institut de Radioastronomie Millimétrique, 300 rue de la Piscine, Domaine Universitaire, 38406 Saint Martin d’Hères, France
⁵ LERMA, Observatoire de PARIS, PSL Research University, CNRS, Sorbonne Université, 92190 Meudon, France
⁶ Laboratoire d’astrophysique de Bordeaux, Univ. Bordeaux, CNRS, B18N, allée Geoffroy Saint-Hilaire, 33615 Pessac, France
⁷ Harvard-Smithsonian Center for Astrophysics, 60 Garden St., Cambridge, MA 02138, USA
⁸ Faculty of Aerospace Engineering, Delft University of Technology, Delft, The Netherlands
⁹ University of Leiden, PO Box 9513, NL, 2300 RA Leiden, The Netherlands
¹⁰ École Normale Supérieure de Lyon, CRAL, UMR CNRS 5574, Université Lyon I, 46 allée d’Italie, 69364 Lyon Cedex 07, France
¹¹ Instituto de Física Fundamental (CSIC), Calle Serrano 123, 28006 Madrid, Spain
¹² Institut des Sciences Moléculaires (ISM), CNRS, Univ. Bordeaux, 351 cours de la Libération, 33400 Talence, France
¹³ Chalmers University of Technology, Department of Space, Earth and Environment, 412 93 Gothenburg, Sweden
¹⁴ Centro de Astrobiología (CSIC-INTA), Ctra. de Ajalvir, km 4, Torrejón de Ardoz, 28850 Madrid, Spain
¹⁵ University of Vienna, Department of Astrophysics, Tuerkenschanzstrasse 17, 1180 Vienna, Austria
¹⁶ Jeremiah Horrocks Institute, University of Central Lancashire, Preston PR1 2HE, UK
¹⁷ Observatorio de Yebes (IGN). Cerro de la Palera s/n, 19141 Yebes, Spain
¹⁸ Department of Physics, University of Helsinki, PO Box 64, 00014 Helsinki, Finland
¹⁹ Institute of Physics I, University of Cologne, Cologne, Germany
²⁰ National Radio Astronomy Observatory, 520 Edgemont Rd., Charlottesville VA 22901, USA

Received: 10 December 2020
Accepted: 10 February 2021

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 A_V ~3 to ~100 mag, n(H₂) ~ a few 10³ to 10⁶ cm⁻³, and T_k ~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 T_K ~ 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.