| Issue |
A&A
Volume 375, Number 2, August IV 2001
|
|
|---|---|---|
| Page(s) | 553 - 565 | |
| Section | The Sun | |
| DOI | https://doi.org/10.1051/0004-6361:20010769 | |
| Published online | 15 August 2001 | |
Interstellar C
and CN toward
the Cyg OB2 association
A case study of X-ray induced chemistry
1
Max-Planck Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany e-mail: gredel@caha.es
2
Onsala Space Observatory, Chalmers University of Technology, 43992 Onsala, Sweden e-mail: jblack@oso.chalmers.se
3
Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
Corresponding author: R. Gredel, gredel@mpia-hd.mpg.de
Received:
7
February
2001
Accepted:
23
May
2001
An analysis of deep optical echelle spectra towards six stars in the
Cyg OB2 (VI Cygni) association is presented. Interstellar absorption
lines up to 
in the (2,0) and (3,0) bands of the C2
A
-X
system are detected towards Cyg OB2 No. 12. The large number
of rotational lines accurately constrains the
gas-kinetic temperature T and the density
n to 
K and 
cm-3.
The inferred C2
column density is N(C
cm-2.
The detection of various lines in the (1,0) and (2,0) band of the
CN A
-X
red system suggest a column density of N(CN
-
cm-2.
C2 absorption lines
are also detected towards Cyg OB2 No. 5 and No. 9. Inferred parameters
are 
K,
cm-3,
N(C
cm-2 towards No. 5, and 
K, 
cm-3,
N(C
cm-2 towards No. 9. Marginal detections of C2 towards Cyg OB2 No. 8A
indicate N(C
cm-2 and 
K. Upper
limits are N(C
cm-2 toward Cyg OB2 No. 7 and No. 11.
The C2 observations eliminate the possibility that the molecular material along
the line of sight towards Cyg OB2 No. 12 is spread over a pathlength of
several hundred parsecs of very low density 
cm-3. The
observations provide some support to a recent chemical model
which assumes a nested structure of the molecular gas. Alternatively,
the C2 and CN abundances obtained towards Cyg OB2 No. 12 are in agreement
with the predictions of an X-ray induced chemistry driven by an ionisation
rate of 
-
s-1. Calculated equilibrium
temperatures of 
-50 K agree with temperatures inferred from
C2. The model also reproduces the observed column densities of CO and CH,
and that of H
to within a factor of two. We predict a H2O+
column density of 
cm-2 towards Cyg OB2 No. 12 and
H2O+ absorption lines which are detectable by optical absorption
line techniques.
We report the detection of interstellar Rubidium towards Cyg OB2 No. 12
and No. 5. Inferred column densities are N(Rb
cm-2 towards No. 12 and N(Rb cm-2 towards No. 5.
Key words: ISM: abundances / ISM: clouds / ISM: molecules / X-rays: general
© ESO, 2001
Current usage metrics show cumulative count of Article Views (full-text article views including HTML views, PDF and ePub downloads, according to the available data) and Abstracts Views on Vision4Press platform.
Data correspond to usage on the plateform after 2015. The current usage metrics is available 48-96 hours after online publication and is updated daily on week days.
Initial download of the metrics may take a while.