Molecular abundances in the Magellanic Clouds III.
Molecular abundances in the Magellanic Clouds
III. LIRS36, a star-forming region in the SMC
-
Y.-N. Chin
-
Institute of Astronomy & Astrophysics,
Academia Sinica
- P.O. Box 1-87 Nankang, 115 Taipei, Taiwan
- and
-
Radioastronomisches Institut der Universität Bonn
- Auf dem Hügel 71, D-53121 Bonn, Germany
-
C. Henkel
-
Max-Planck-Institut für Radioastronomie
- Auf dem Hügel 69, D-53121 Bonn, Germany
- and
- European Southern Observatory,
- Casilla 19001, Santiago 19, Chile
-
T.J. Millar
-
Department of Physics,
UMIST
- PO Box 88, Manchester M60 1QD, United Kingdom
-
J.B. Whiteoak
-
Australia Telescope National Facility, Radiophysics Laboratories,
- P.O. Box 76, Epping, NSW 2121, Australia
- and
-
Paul Wild Observatory,
Australia Telescope National Facility,
CSIRO
- Locked Bag 194, Narrabri NSW 2390, Australia
-
M. Marx-Zimmer
-
Radioastronomisches Institut der Universität Bonn
- Auf dem Hügel 71, D-53121 Bonn, Germany
Paper published in February 1998 by the Main Journal of
Astronomy and Astrophysics in vol. 330, pp. 901 - 909.
If you want to have a look at the complete paper, please click
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Abstract.
Detections of CO, CS, SO, C2H, HCO+, HCN, HNC,
H2CO, and C3H2 are reported from LIRS 36, a
star-forming region in the Small Magellanic Cloud.
C18O, NO, CH3OH, and most notably CN have not been
detected, while the rare isotopes 13CO and, tentatively,
C34S are seen.
This is so far the most extensive molecular multiline study of an
interstellar medium with a heavy element depletion exceeding a factor of four.
The X = N(H2)/ICO conversion
factor is about 4.8 × 1021 cm-2(K
km/s)-1, slightly larger than the local Galactic disk value.
The CO (1-0) beam averaged column density then becomes
N(H2) = 3.7 × 1021 cm-2 and the
density n(H2) = 100 cm-3.
A comparison with X-values from Rubio et al. (1993a) shows that
on small scales (R = 10 pc) X-values are more similar to
Galactic disk values than previously anticipated, favoring a neutral
interstellar medium of predominantly molecular nature in the cores.
The I(13CO)/I(C18O) line intensity ratio
indicates an underabundance of 12C18O relative to
13C16O w.r.t. Galactic clouds.
I(HCO+)/I(HCN) and I(HCN)/I(HNC) line
intensity ratios are > 1 and trace a warm (Tkin > 10K)
molecular gas exposed to a high ionizing flux.
Detections of CS J=2-1, 3-2, and 5-4 lines imply the presence of a
high density core with n(H2) = 105 -
107 cm-3.
In contrast to star-forming regions in the LMC,
the CN 1-0 line is substantially weaker than the corresponding
ground rotational transitions of HCN, HNC, and CS.
CO, CS, HCO+, and H2CO fractional abundances are a
factor about 10 smaller than corresponding values in Galactic disk clouds.
Fractional abundances of HCN, HNC, and likely CN
are even two orders of magnitude below their `normal', Galactic disk values.
The CN/CS abundance ratio is >= 1.
Based on chemical model calculations,
we suggest that this is because of the small metallicity of the SMC,
which affects the destruction of CN but not CS, and because of
the high molecular core density which also favors CN destruction.
Any suggestion or comments please
e-mail to einmann@asiaa.sinica.edu.tw.
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