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Gas phase Elemental abundances in Molecular cloudS (GEMS): I. The prototypical dark cloud TMC 1.
- Source :
-
Astronomy and astrophysics [Astron Astrophys] 2019 May 17; Vol. 624. Date of Electronic Publication: 2019 Apr 19. - Publication Year :
- 2019
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Abstract
- GEMS is an IRAM 30m 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 TMC 1. Extensive millimeter observations have been carried out with the IRAM 30m telescope (3 mm and 2 mm) and the 40m Yebes telescope (1.3 cm and 7 mm) to determine the fractional abundances of CO, HCO <superscript>+</superscript> , HCN, CS, SO, HCS <superscript>+</superscript> , and N <subscript>2</subscript> H <superscript>+</superscript> 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 <subscript>V</subscript> ~ 3 to ~20 mag. Two phases with differentiated chemistry can be distinguished: i) the translucent envelope with molecular hydrogen densities of 1-5×10 <superscript>3</superscript> cm <superscript>-3</superscript> ; and ii) the dense phase, located at A <subscript>V</subscript> > 10 mag, with molecular hydrogen densities >10 <superscript>4</superscript> cm <superscript>-3</superscript> . Observations and modeling show that the gas phase abundances of C and O progressively decrease along the C <superscript>+</superscript> /C/CO transition zone (A <subscript>V</subscript> ~ 3 mag) where C/H ~ 8×10 <superscript>-5</superscript> and C/O~0.8-1, until the beginning of the dense phase at A <subscript>V</subscript> ~ 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 a S/H ~ (0.4 - 2.2) ×10 <superscript>-6</superscript> , 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 <superscript>-8</superscript> ). Based on our chemical modeling, we constrain the value of ζ <subscript>H <subscript>2</subscript> </subscript> to ~ (0.5 - 1.8) ×10 <superscript>-16</superscript> s <superscript>-1</superscript> in the translucent cloud.
Details
- Language :
- English
- ISSN :
- 0004-6361
- Volume :
- 624
- Database :
- MEDLINE
- Journal :
- Astronomy and astrophysics
- Publication Type :
- Academic Journal
- Accession number :
- 31156252
- Full Text :
- https://doi.org/10.1051/0004-6361/201834654