Volume 575, March 2015
|Number of page(s)||34|
|Section||Interstellar and circumstellar matter|
|Published online||02 March 2015|
The role of surface chemistry
INAF – Osservatorio Astrofisico di Arcetri, L.go E. Fermi 5,
2 Instituto de Astrofísica de Andalucía, CSIC, Glorieta de la Astronomía, 18008 Granada, Spain
3 INAF – Istituto di Astrofisica e Planetologia Spaziali, via Fosso del Cavaliere 100, 00133 Roma, Italy
4 Centro de Radioastronomía y Astrofísica, Universidad Nacional Autónoma de México, PO Box 3-72, 58090 Morelia, Michoacán, Mexico
5 Max-Planck-Institut für extraterrestrische Physik (MPE), Giessenbachstr., 85741 Garching, Germany
6 I. Physikalisches Institut der Universität zu Köln, Zülpicher Strasse 77, 50937 Köln, Germany
7 Department of Astronomy, University of Florida, Gainesville, FL 32611, USA
8 Department of Physics, University of Florida, Gainesville, FL 32611, USA
9 Department of Astronomy, University of Geneva, Ch. d’Ecogia 16, 1290 Versoix, Switzerland
Received: 5 August 2014
Accepted: 27 October 2014
Context. An ever growing number of observational and theoretical evidence suggests that the deuterated fraction (column density ratio between a species containing D and its hydrogenated counterpart, Dfrac) is an evolutionary indicator both in the low- and the high-mass star formation process. However, the role of surface chemistry in these studies has not been quantified from an observational point of view.
Aims. Because many abundant species, such as NH3, H2CO, and CH3OH, are actively produced on ice mantles of dust grains during the early cold phases, their Dfrac is expected to evolve differently from species formed only (or predominantly) in the gas, such as N2H+, HNC, HCN, and their deuterated isotopologues. The differences are expected to be relevant especially after the protostellar birth, in which the temperature rises, causing the evaporation of ice mantles.
Methods. To compare how the deuterated fractions of species formed only in the gas and partially or uniquely on grain surfaces evolve with time, we observed rotational transitions of CH3OH, 13CH3OH, CH2DOH, and CH3OD at 3 mm and 1.3 mm, of NH2D at 3 mm with the IRAM-30 m telescope, and the inversion transitions (1, 1) and (2, 2) of NH3 with the GBT, towards most of the cores already observed in N2H+, N2D+, HNC, and DNC.
Results. NH2D is detected in all but two cores, regardless of the evolutionary stage. Dfrac(NH3) is on average above 0.1 and does not change significantly from the earliest to the most evolved phases, although the highest average value is found in the protostellar phase (~0.3). Few lines of CH2DOH and CH3OD are clearly detected, and then only towards protostellar cores or externally heated starless cores. In quiescent starless cores, we have only one doubtful detection of CH2DOH.
Conclusions. This work clearly confirms an expected different evolutionary trend of the species formed exclusively in the gas (N2D+ and N2H+) and those formed partially (NH2D and NH3) or totally (CH2DOH and CH3OH) on grain mantles. It also reinforces the idea that Dfrac(N2H+) is the best tracer of massive starless cores, while high values of Dfrac(CH3OH) seem fairly good tracers of the early protostellar phases, where the evaporation or sputtering of the grain mantles is most efficient.
Key words: stars: formation / molecular data / submillimeter: ISM / ISM: molecules
Tables 3–6, 8, and Appendices are available in electronic form at http://www.aanda.org
IRAM 30 m data (final reduced data used in the paper, in FITS format) are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (22.214.171.124) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/575/A87
© ESO, 2015
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