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ABSTRACT: Chemical models are presented for the photon dominated region (PDR) IC
63, a small isolated molecular cloud located close to the B0.5 IVpe star
γ Cas for which we have presented observations of various
molecular species in an earlier paper, and for which observations of
ionized and neutral atomic carbon, as well as additional CO
observations, are available. The models treat the photoexcitation and
photodissociation processes in detail, and use the actual γ Cas
radiation field as measured by the S2-68 and IUE satellites. The heating
and cooling balance of the cloud is taken explicitly into account. The
computed H_2_ ultraviolet fluorescent spectrum agrees well with
observations, thus providing support for the inferred physical
structure. Effects of the radiation field, elemental depletions, density
and cosmic ray ionization rate on the chemistry are discussed. Several
distinct chemical zones are found with depth into the cloud. At the
edge, most species are in ionized atomic form, and reactions with C^+^
drive the chemistry. Deeper into the cloud around A_V_=~2 mag, carbon is
transformed into neutral atomic carbon and CO. At this same depth, the
abundances of radicals such as CH, CH_2_, CH_3_, CN and C_2_H peak. They
rapidly decline at larger depths due to reactions with atomic oxygen.
Only species such as CH_4_, H_2_CO, and HCN, which do not react with O,
have large abundances deep into the cloud. As a result, the CN/HCN
abundance ratio varies strongly with depth. Most observed column
densities can be reproduced to within a factor of a few, and in many
cases even better than a factor of two. Only the chemistry of
sulfur-bearing molecules does not fit the observations: if the sulfur
depletion is fixed to reproduce the observed CS column density, the
model H_2_S column density is too low and that of SO too high by an
order of magnitude. The resulting temperature structure in the
one-dimensional plane-parallel models agrees reasonably well with that
inferred from observations. Improved comparison is obtained in
two-dimensional models which take the actual geometry of the region into
account. The results of such geometrical models are presented, and are
compared with those obtained in the one-dimensional case. In general,
the agreement between one-dimensional and two-dimensional models is good
enough to justify the one-dimensional approximation in a simple cloud
like IC 63, whereas for more complex clouds the actual geometry might
become very important.
Astronomy and Astrophysics 09/1995; 302:223. · 4.59 Impact Factor
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Prensa médica argentina 10/1945; 32:2009-13.
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Astron. Ap., 302, 223 - 242 (1995).
