EDP Sciences
Free access
Volume 415, Number 3, March I 2004
Page(s) 1065 - 1072
Section Interstellar and circumstellar matter
DOI http://dx.doi.org/10.1051/0004-6361:20031668

A&A 415, 1065-1072 (2004)
DOI: 10.1051/0004-6361:20031668

The NH $\mathsf{_3}$ / N $\mathsf{_2}$H + abundance ratio in dense cores

S. Hotzel1, J. Harju1 and C. M. Walmsley2

1  Observatory, PO Box 14, 00014 University of Helsinki, Finland
2  Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy

(Received 19 June 2003 / Accepted 18 December 2003 )

We have observed the dense cores Barnard 217 (B217) and LDN 1262 (L1262) in the (J,K)=(1,1) and (2,2) inversion lines of ammonia and in the $J=1\mbox{--}0$ rotational line of diazenylium. The two cores are morphologically similar, in the sense that both contain a Class-I young stellar object (YSO) 1-2´ away from the core centre. The NH 3 and N 2H + column densities show the same pattern in both cores: the average NH 3/N 2H + abundance ratios are 140-190 in the starless main bodies of the cores, while they drop to about 60-90 in the regions around the YSOs. Comparison with the dust continuum emission of B217 suggests that this pattern is due to an enhanced fractional ammonia abundance in the quiescent part of the core, where we find $N(\mbox{NH$_3$ })/N(\mbox{H$_2$ })=\ensuremath{{5}\times10^{-8}} $. On the outskirts of the core the fractional ammonia abundance is about $\ensuremath{{3}\times10^{-8}} $, in accordance with our previous results. We discuss these findings in the light of recent chemical models including molecular depletion.

Key words: ISM: individual objects: Barnard 217 -- ISM: individual objects: LDN 1262 -- ISM: abundances -- ISM: molecules

Offprint request: S. Hotzel, hotzel@astro.helsinki.fi

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