Issue |
A&A
Volume 370, Number 2, May I 2001
|
|
---|---|---|
Page(s) | 576 - 585 | |
Section | The Sun | |
DOI | https://doi.org/10.1051/0004-6361:20010260 | |
Published online | 15 May 2001 |
Comparative chemistry of diffuse clouds
II. CN, HCN, HNC, CH3CN & N2H^+
1
National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903-2475, USA
2
Institut de Radioastronomie Millimétrique, 300 rue de la Piscine, 38406 Saint Martin d'Hères, France
Corresponding author: H. S. Liszt, hliszt@nrao.edu
Received:
18
October
2000
Accepted:
26
January
2001
Using the Plateau de Bure interferometer, we observed the λ3 mm
absorption lines of CN, HCN and HNC from some of the diffuse clouds
which lie toward our well-studied sample of compact extragalactic
mm-wave continuum sources. The column densities of these species all
vary by a factor of about fifty and are prominent in only a limited
subset of the clouds seen in the most ubiquitous species such as
OH, HCO+, C2H and C3H2. We searched unsuccessfully for CH3CN
and N2H+, which are underabundant compared to dark clouds, by factors of
at least 10 and 100, respectively.
The CN-HCN-HNC column densities vary strongly and non-linearly with
N(HCO+), for example, which probably best represents their variation
with H2 as well. But their abundances are very tightly and linearly
coupled to each other, varying in fixed proportion, as is the
case for OH and HCO+, and (only slightly more loosely) for C2H and
C3H2. Having measured one, it is hardly necessary to observe the two
others in this group. We find
,
.
Such a small
ratio is typical of warmer gas in darker,
denser environments, and is consistent with the notion of molecular
formation in warmer media. The 6.8:1:0.21 ratio in diffuse gas is
very different from TMC-1 (6.8:4.5:4.5) where HCN and HNC are relatively
much more abundant.
It seems likely that the sequence of features with increasing column
densities of the CN-HCN family or CnHm-family molecules in diffuse gas
actually represents a series of gas parcels of increasingly higher
H2-fraction, number density, and molecular abundances,
occurring over a relatively narrow interval of total hydrogen column density.
Our experiment seems to have caught many molecules in the act of turning
on -and turning each other on -in the diffuse interstellar medium.
Key words: ISM: molecules; abundances; clouds / ISM: structure Radio lines: ISM
© ESO, 2001
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