Issue |
A&A
Volume 505, Number 3, October III 2009
|
|
---|---|---|
Page(s) | 1199 - 1211 | |
Section | Interstellar and circumstellar matter | |
DOI | https://doi.org/10.1051/0004-6361/200912547 | |
Published online | 18 August 2009 |
C2H in prestellar cores
1
Università di Firenze, Dipartimento di Astronomia e Scienza dello Spazio, Largo E. Fermi 2, 50125 Firenze, Italy e-mail: padovani@arcetri.astro.it
2
INAF-Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy e-mail: [walmsley;galli]@arcetri.astro.it
3
Observatorio Astronómico Nacional, Alfonso XII 3, 28014 Madrid, Spain e-mail: m.tafalla@oan.es
4
I. Physikalisches Institut, Universität zu Köln, Zülpicher Straße 77, 50937 Köln, Germany e-mail: hspm@ph1.uni-koeln.de
Received:
20
May
2009
Accepted:
6
August
2009
Aims. We study the
abundance of C2H in prestellar
cores both because of its role in the chemistry and because it
is a potential probe of the magnetic field. We also consider
the non-LTE behaviour
of the -0 and
-1 transitions of C2H and improve current estimates of the spectroscopic
constants of C2H.
Methods. We used the IRAM 30 m radiotelescope to map the N = 1-0 and N = 2-1 transitions of C2H towards the prestellar cores L1498 and CB246. Towards CB246, we also mapped the 1.3 mm dust emission, the J = 1-0 transition of N2H+ and the J = 2-1 transition of C18O. We used a Monte Carlo radiative transfer program to analyse the C2H observations of L1498. We derived the distribution of C2H column densities and compared with the H2 column densities inferred from dust emission.
Results. We find that while non-LTE intensity ratios of different components
of the -0 and
-1 lines are present, they are
of minor importance and do not impede C2H column density
determinations based upon
LTE analysis. Moreover, the comparison of our Monte-Carlo calculations
with
observations suggest that the non-LTE deviations can be qualitatively
understood. For extinctions less than 20 visual magnitudes, we
derive toward these two cores (assuming LTE)
a relative abundance [ C2H] /[ H2] of
(1.0±0.3)
10-8 in L1498 and (0.9±0.3)
10-8 in CB246 in reasonable agreement with our Monte-Carlo estimates. For L1498, our observations in conjunction with the Monte Carlo
code imply a C2H depletion hole of radius 9
1016 cm similar
to that found for other C-containing species. We briefly discuss
the significance of the observed C2H abundance distribution. Finally,
we used our observations
to provide improved estimates for the rest frequencies of all
six components of the C2H (1-0) line and seven components of
C2H (2-1). Based on these results, we compute improved
spectroscopic constants for C2H. We also give a brief discussion
of the prospects for measuring magnetic field strengths using C2H.
Key words: ISM: abundances / ISM: clouds / ISM: molecules / ISM: general / radio lines: ISM / molecular data
© ESO, 2009
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