Issue |
A&A
Volume 448, Number 3, March IV 2006
|
|
---|---|---|
Page(s) | 1043 - 1060 | |
Section | Interstellar and circumstellar matter | |
DOI | https://doi.org/10.1051/0004-6361:20053580 | |
Published online | 03 March 2006 |
Neutral carbon and CO emission in the core and the halo of dark cloud Barnard 5
1
Radioastronomisches Institut, Universität Bonn (RAIUB), Auf dem Hügel 71, 53121 Bonn, Germany e-mail: fbensch@astro.uni-bonn.de
2
Harvard-Smithsonian Center for Astrophysics (CfA), 60 Garden Street, Cambridge, MA 02138, USA
Received:
6
June
2005
Accepted:
11
October
2005
Aims.The physical conditions and chemical structure in the dark cloud of
Barnard 5 and its surrounding atomic halo is studied.
The impact of the halo on the line emission emerging from
the molecular cloud is investigated.Methods.We present observations of the [CI] 3P 3P0 transition
of neutral carbon and the low-J transitions of 12CO and 13CO.
The CO maps extend from the core (
) to the northern
cloud edge and into the halo (
). They are complemented
by deeply integrated [CI] spectra made along a 1D cut of similar extent.
Escape probability and photon-dominated region (PDR)
models are employed to interpret the observations.Results.12CO and 13CO are detected in the cloud and the halo, while
[CI] is detected only toward the molecular cloud.
This occurs even though the neutral carbon column density
is
5 times larger than the CO column density in the halo, but it can
be understood in terms of excitation.
The [CI] excitation is governed by collisions even at the low halo
densities, while the CO excitation
is dominated by the absorption of line photons emitted by the nearby molecular
cloud. The upper limit on the neutral carbon column density
in the halo is
cm-2.
The PDR studies show that even small column densities of H2 and CO,
such as those in the B5 halo, can significantly change
the [CI] and CO line emission (pre-shielding). Since this effect
decreases the [CI] intensity and increases the CO intensity, the largest
impact is noted for the [CI]/CO line ratios.
For the B5 cloud, a PDR model with a molecular
hydrogen column density of ~
cm-2 in the
halo matches the observed [CI]/CO line ratios best.
Models with no pre-shielding, in contrast, suggest high gas densities
that are in conflict with independently derived densities.
The PDR models with a
demonstrate that the [CI]/CO ratios cannot be
attributed solely to a reduced FUV field.
© ESO, 2006
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