Gas phase Elemental abundances in Molecular cloudS (GEMS)

I. The prototypical dark cloud TMC 1

¹ Observatorio Astronómico Nacional (OAN), Alfonso XII, 3, 28014 Madrid, Spain
e-mail: a.fuente@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 Université Paris 06, 75005 Paris, France
⁴ Instituto Radioastronomía Milimétrica (IRAM), Av. Divina Pastora 7, Nucleo Central, 18012 Granada, Spain
⁵ Sorbonne Université, Observatoire de Paris, Université PSL, CNRS, LERMA, 92190 Meudon, France
⁶ Faculty of Aerospace Engineering, Delft University of Technology, Delft, The Netherlands
⁷ University of Leiden, PO Box 9513, 2300 RA Leiden, The Netherlands
⁸ École Normale Supérieure de Lyon, CRAL, UMR CNRS 5574, Université Lyon I, 46 Allée d’Italie, 69364 Lyon Cedex 07, France
⁹ National Radio Astronomy Observatory, 520 Edgemont Rd., Charlottesville, VA 22901, USA
¹⁰ Instituto de Física Fundamental (CSIC), Calle Serrano 123, 28006 Madrid, Spain
¹¹ Laboratoire d’astrophysique de Bordeaux, Université Bordeaux, CNRS, B18N, allée Geoffroy Saint-Hilaire, 33615 Pessac, France
¹² Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
¹³ Centro de Astrobiología (CSIC-INTA), Ctra. de Ajalvir, km 4, Torrejón de Ardoz, 28850 Madrid, Spain
¹⁴ Department of Physics, University of Warwick, Coventry, CV4 7AL, UK
¹⁵ Institut des Sciences Moléculaires (ISM), CNRS, Université Bordeaux, 351 cours de la Libération, 33400 Talence, France
¹⁶ Department of Physics, University of Helsinki, PO Box 64, 00014 Helsinki, Finland
¹⁷ Institute of Physics I, University of Cologne, Cologne, Germany
¹⁸ Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138, USA
¹⁹ Jeremiah Horrocks Institute, University of Central Lancashire, Preston, PR1 2HE, UK
²⁰ Chalmers University of Technology, Department of Space, Earth and Environment, 412 93 Gothenburg, Sweden
²¹ Departamento de Física, Universidade Federal de Juiz de Fora-UFJF, Juiz de Fora, MG 36036-330, Brazil

Received: 15 November 2018
Accepted: 28 February 2019

Abstract

GEMS is an IRAM 30 m 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 Taurus molecular cloud (TMC) 1. Extensive millimeter observations have been carried out with the IRAM 30 m telescope (3 and 2 mm) and the 40 m Yebes telescope (1.3 cm and 7 mm) 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 A_V ~ 3 to ~20 mag. Two phases with differentiated chemistry can be distinguished: (i) the translucent envelope with molecular hydrogen densities of 1−5 × 10³ cm⁻³; and (ii) the dense phase, located at A_V > 10 mag, with molecular hydrogen densities >10⁴ cm⁻³. Observations and modeling show that the gas phase abundances of C and O progressively decrease along the C⁺/C/CO transition zone (A_V ~ 3 mag) where C/H ~ 8 × 10⁻⁵ and C/O ~ 0.8−1, until the beginning of the dense phase at A_V ~ 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 an S∕H ~ (0.4−2.2) × 10⁻⁶, 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 ~ 8 × 10⁻⁸). Based on our chemical modeling, we constrain the value of ζ_H2 to ~(0.5–1.8) × 10⁻¹⁶ s⁻¹ in the translucent cloud.