Received 16th March 1998
Abstract.
In the Large Magellanic Cloud (LMC) a large number of cool
H
I clouds has been detected with temperatures much below
those found for atomic clouds in the Milky Way.
Apparently, the population of cool H
I clouds reaches
kinetic gas temperatures down to as low as 10 or 20 K.
Do these clouds play an important role in the formation of stars in the LMC?
We studied the association between the cool atomic gas and molecular gas
in the LMC by
12CO(1-0) line observations in directions of cool
H
I clouds using the 15 m
Swedish-ESO
Submillimetre Telescope (SEST).
1. Observation
We used the SEST*
to survey 12CO(1-0) line emission toward 25 lines of sight showing
cool H I (57 H I-absorption features)
with spin temperatures down to 4 K.
The sources have been selected from the H I absorption line
surveys of Dickey et al. (1994) and Marx-Zimmer et
al. (1998).
We also studied the surroundings of 13 of these lines of sight by a four point
map around the central position with 40" spacing.
The 12CO(1-0) observations were carried out in a dual beam-switching
mode (switching frequency 6 Hz) with a beam throw of 11' 37" in
azimuth and in a frequency-switching mode with a frequency offset of
15 MHz (40 km s-1).
At the frequency of 115 GHz of the J = 1-0 transition of
12CO, the SEST beamwidth is 45", corresponding to a linear
resolution of about 10 pc in the LMC.
The velocity resolution is 0.11 km s-1.
Typical integration times were 20 min on the source, which provides
an rms of about 0.06 K.
*Based on observations with the
Swedish-ESO
Submillimetre Telescope (SEST) at the
European Southern
Observatory (ESO), La Silla,
Chile
2. Results
Out of a total sample of 25 lines of sight showing cool atomic hydrogen
CO emission has been detected toward 8 lines of sight with
TMB between 0.3 K and 8 K.
Five of these lines of sight showing CO emission are in direction of
the 30 Doradus complex.
The other clouds have been detected toward the bar of the LMC, toward the far
south-west and in direction of LMC 4.
The two observed molecular clouds toward LMC 4 seem to be associated with
this supergiant shell.
Four of these clouds have not been detected in the CO survey of Cohen et al.
(1988) due to a smaller beam filling factor (8.8' resolution).
Our search for CO in the surroundings of 13 lines of sight showing cool
H I but no CO emission does not reveal CO at a projected
distance of 40" (about 10 pc) from the H I cloud
position.
There is no clear correlation of CO emission with the H I
spin temperature, Tspin, or the optical depth,
τH I, although the probability
of finding a CO-H I association increases with
τH I and with the "equivalent
width", EWH I, of the 21 cm
absorption line.
CO emission always occurs near but does not necessarily coincide with
an H I absorption feature.
The velocity differences D v = vH I - vCO, where vH I is
the H I feature closest in velocity to the CO feature, are
between 1 and 6 km s-1.
In general the value is smaller than the FWHM of the H I
feature detected.
Turbulent gas motion near 30 Doradus and LMC 4 can cause this velocity
shift of the CO gas with respect to the H I gas.
We mapped five of the detected CO clouds and found cloud sizes between about
110" × 80" (26 pc × 19 pc)
and 600" × 180"
(144 pc × 43 pc).
We find that the cool H I gas is mostly located at the rim
of the molecular clouds (Fig. 1).
Although LMC clouds seem to have lower temperatures than atomic clouds
in the Milky Way (Dickey et al. 1994; Marx-Zimmer et
al. 1998; Mebold et al. 1997), the fraction of cool H I
clouds showing CO emission is the same as in the Milky Way, where about 19% of
the H I absorption components show CO emission (Despois
and Baudry 1985).
In the Milky Way the transition from atomic to molecular hydrogen and CO,
leaving little H I, seems to take place at temperatures
below about 30 to 40 K.
The non-detection of CO towards unusually cool H I clouds
in the LMC might indicate that here the gas phase transition is determined
by a different temperature-density combination.
[Click here to see Fig. 1!]
References
- Cohen R.S., Dame M.T., Garay G., et al., 1988, ApJ 331, L95
- Despois D., Baudry A., 1985, A&A 148, 83
- Dickey J.M., Mebold U., Marx M., Amy S., Haynes R.F., Wilson W., 1994,
A&A 289, 357
- Marx-Zimmer M., Herbstmeier U., Zimmer F., Dickey J.M.,
Staveley-Smith L., Mebold U., 1998,
Proceedings of the Workshop
"The Magellanic Clouds and Other Dwarf Galaxies",
p. ???
- Mebold U., Düsterberg C., Dickey J.M., Staveley-Smith L., Kalberla P.,
1997, ApJ 490, L65
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| First version: | 10th | August, | 1998
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| Last update: | 08th | October, | 1998
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Jochen M. Braun &
Tom Richtler
(E-Mail: jbraun|richtler@astro.uni-bonn.de)