The following document lists the file abstract/LLITTLE_PROP_1.abs
from catalogue VI/111.
A plain copy of the file
(without headers/trailers) may be downloaded.
The observed penetration of neutral carbon deep into molecular
clouds poses problems both for standard models of PDR regions and
for interstellar chemistry. We here propose to determine how the
abundances of C+ and O vary with radius in a molecular cloud, G34.3
+0.2, for which similar CI and O information of high quality already
exists. This determination of the relative distribution of C+, C, O
and CO will help substantially towards distinguishing between
alternative models which have been proposed to account for the
carbon penetration. Such models have included one or more of the
effects of enhanced uv penetration into clouds due to clumping,
generation of internal uv photons by cosmic rays exciting molecular
hydrogen or by decay of dark matter neutrinos, oxygen depletion,
a C/O ratio >1, turbulent mixing and gravitational contraction.
Early observational studies of CI and CII indicated the
widespread distribution of both species: it was established,for
example, that the CI/CO ratio was high into depths where the
optical extinction was 10 mag or greater, while standard PDR models
predict that it should become very small for Av>4. It is difficult,
however, to establish true, rather than 'column averaged' values for
abundances deep in clouds. Recently our study of the molecular
cloud G34.3+0.2 suggested CI/CO declined from 0.4 near the perimeter
to 0.04 as far in as Av=55 (Little et al 1994). Our analysis
was aided by the spherically symmetric structure of G34.3+0.2
and the conclusion was justified by very careful radiative transfer
modelling of the angular structure and lineshapes of CI and several
CO transitions.
We propose to build on this effort by making observations across
G34.3+0.2 in the CII 158mu, OI 63 and 146 mu lines to extend our
determinations to the radial variation of C+ and O.