Issue |
A&A
Volume 494, Number 3, February II 2009
|
|
---|---|---|
Page(s) | 977 - 985 | |
Section | Interstellar and circumstellar matter | |
DOI | https://doi.org/10.1051/0004-6361:200810933 | |
Published online | 22 December 2008 |
HCO mapping of the Horsehead: tracing the illuminated dense molecular cloud surfaces *,**
1
LERMA–LRA, UMR 8112, CNRS, Observatoire de Paris and École Normale Supérieure, 24 Rue Lhomond, 75231 Paris, France e-mail: maryvonne.gerin@lra.ens.fr; jrgoicoechea@fis.ucm.es
2
IRAM, 300 rue de la Piscine, 38406 Grenoble cedex, France e-mail: pety@iram.fr
3
Laboratoire d'Astrophysique, Observatoire de Grenoble, BP 53, 38041 Grenoble Cedex 09, France e-mail: pierre.hilyblant@obs.ujf-grenoble.fr
Received:
8
September
2008
Accepted:
14
November
2008
Context. Far-UV photons (FUV) strongly affect the physical and chemical state of molecular gas in the vicinity of young massive stars.
Aims. Finding molecular tracers of the presence of FUV radiation fields in the millimeter wavelength domain is desirable because IR diagnostics (for instance PAHs) are not easily accessible along high extinction line-of-sights. Furthermore, gas phase diagnostics provide information on the velocity fields.
Methods. We have obtained maps of the HCO and H13CO+ ground state lines
towards the Horsehead edge at angular resolution with a
combination of Plateau de Bure Interferometer (PdBI) and
the IRAM-30 m telescope observations. These maps have been
complemented with IRAM-30 m observations of several excited transitions
at two different positions.
Results. Bright formyl radical emission delineates the illuminated edge of
the nebula, with a faint emission remaining towards the
shielded molecular core. Viewed from the illuminated star, the HCO
emission almost coincides with the PAH and CCH emission.
HCO reaches a similar abundance to HCO+ in the photon dissociation
region (PDR), 1–
with respect to H2. To our
knowledge, this is the highest HCO abundance ever measured.
Pure gas-phase chemistry models fail to reproduce the observed
HCO abundance by ~2 orders of magnitude, except if reactions of
atomic oxygen with carbon radicals abundant in the PDR (i.e., CH2)
play a significant role in the HCO formation.
Alternatively, HCO could be produced
in the PDR by non-thermal processes such as photo-processing of
ice mantles and subsequent photo-desorption of either HCO or H2CO, and
further gas phase photodissociation.
Conclusions. The measured HCO/H13CO+ abundance ratio is large towards the PDR
(50), and much lower toward the gas shielded from FUV radiation
(≲1). We propose that high HCO abundances (≳10-10)
together with large HCO/H13CO+ abundance ratios (≳1) are
sensitive diagnostics of the presence of active photochemistry induced
by FUV radiation.
Key words: astrochemistry / ISM: clouds / ISM: molecules / ISM: individual objects: Horsehead nebula / radiative transfer / radio lines: ISM
© ESO, 2009
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