Proceedings of the
Workshop
"
The Magellanic Clouds and Other Dwarf Galaxies"
of the
Bonn/Bochum-Graduiertenkolleg
The first detection of H2 in absorption in the LMC
1Sternwarte der Universität Bonn,
Auf dem Hügel 71, D-53121 Bonn, Germany
2Astronomy Department,
University of Illinois at Urbana-Champaign,
1002 West Green Street, Urbana, IL 61801, USA
3Radioastronomisches Institut der
Universität Bonn, Auf dem Hügel 71, D-53121 Bonn,
Germany
4Kapteyn Institute & SRON,
University of Groningen
Received 16th March 1998
Abstract.
We present first results in a project using data from the
ORFEUS FUV
spectrometer in order to investigate the presence of H
2 and CO in
interstellar gas of the LMC.
We find H
2 absorption in the UV spectrum of the star LH 10:3120
in a galactic component (
v≅0 km s
-1) and
a LMC component at
v≅270 km s
-1.
The quantitative analysis of the absorption lines from the lower rotational
states (
J<5) leads to a total column density of
N = 6.6·10
18 cm
-2 for the molecular
hydrogen in the LMC component.
We herewith present the first detection of molecular hydrogen
in absorption in the LMC based on measurements with the
ORFEUS telescope.
1. Introduction
The most abundant molecule in cool interstellar clouds is H2 and
its amount can be studied through absorption lines near 1000 and
1100 Å.
Measurements with the Copernicus satellite (Spitzer et al. 1973) allowed the
first determination of H2 column densities in galactic interstellar
gas.
Thusfar, H2 absorption lines in the diffuse interstellar
medium only could be detected in our own galaxy, because of the technical
limits of the Copernicus and the absence of later observatories with the
required capabilities.
The second-most abundant molecule in cool clouds is CO,
which can be measured easily in radio emission lines.
Moreover, CO is also visible in UV absorption lines
between 1100 and 1450 Å, even if it has not been
found in spectra of LMC stars yet (de Boer et al. 1987).
Due to their dominating amount, both molecules play a very
important role in the physics and chemistry of the interstellar medium.
In the LMC, the presence of H2 is well documented
through measurements of IR emission lines (Israel & Koornneef 1988).
For the Milky Way, the crucial H2/CO ratio has been determined
to some degree of accuracy, but for the LMC a substantially different
value might be expected due to the lower metallicity of the LMC
in comparison with the Milky Way.
The ORFEUS FUV
spectrometer was launched in the SPAS III mission in November 1996.
With its high resolution spectrograph and its high sensitivity it offers
the possibility to observe H2 absorption features
in LMC star spectra for the first time.
Moreover, with the given spectral range between 900 and 1400 Å
a simultaneous detection of CO and H2 is generally possible.
2. The data
For our investigation the star 3120 in the association LH 10 (see Parker et
al. 1992) in the north western part of the Large Magellanic Cloud has been
chosen because of its brightness (spectral type O 5.5) and its moderate
extinction (E(B-V)=0.17 mag), which should produce
favourable conditions for the detection of interstellar H2
absorption lines on this line of sight.
The total exposure time for LH 10:3120 was 6000 s.
The primary data reduction for the 20 echelle orders was made by the
ORFEUS
team at the Astronomical Institute
of the University of Tübingen.
To offset the low signal to noise ratio of the analysed data subset,
a noise reduction algorithm based on a wavelet transformation has
been used to smooth the data. A section of the spectrum of LH 10:3120
with some of the identified absorption lines is given in
Fig. 1a.
Primary features of the observed star are listed in Table 1.
Table 1.
Basic properties of the target
| Name | Position | V
| Spectral
type
| E(B-V)
|
|---|
| | [h m s] | [° ' "]
| [mag] | [mag]
|
| LH 10:3120a | 4 56 46.84
| -66 24 46.6 | 12.80 | O 5.5 | 0.17
|
| a data from Parker et al. (1992)
|
3. Results
16 H2 absorption lines from the lowest 5 rotational states could be
identified in our data set. With respect to the low
count rate only stronger lines without any blends have been selected
for a first analysis.
All these lines show components at 0 km s-1 (local) and
near 270 km s-1 (LMC).
This data allows the first direct detection of molecular hydrogen in the
Large Magellanic Cloud via UV spectroscopy.
For the LMC components equivalent widths have been determined and for each
rotational state curves of growth have been plotted.
From this analysis we derive a total column density of
NJ=0,1,2,3,4 =
6.6·1018 cm-2.
Fig. 1b shows the population in
H2 of the rotational levels observed in LH 10:3120.
The column density NJ, divided by the statistical weight,
is plotted against the excitation energy EJ.
For J=1,2,3,4 we can fit an equivalent excitation temperature of
Tex = 470 K.
This high temperature can be explained by the effect of UV pumping:
the strong UV flux controls the population of the higher rotational levels
and leads to an equivalent excitation temperature for these states much
above the excitation temperature of the lowest level.
Moreover, the UV pumping depopulates the lower rotational levels and therefore
explains the rather low column densities found for J=0,1.
The lowest two rotational states lie on a fit with
Tex = 50 K leading to an upper limit for the kinetic
gas temperature for single proton-molecule collisions very similar to that
of the molecular hydrogen in the Milky Way (Spitzer & Cochran 1973).
[Click here to see Fig. 1!]
4. Conclusions and future work
As our results show, the
ORFEUS spectrum
of LH 10:3120 contains qualitative and quantitative information about the
molecular hydrogen on the line of sight to the LMC.
This is the first time that H2 has been unambiguously detected in
an ultraviolet spectrum at LMC velocity.
An example for galactic gas can be found in the publication of Snow (1977).
Remark
The data reduction was finished in February.
The conference talk was based on a peliminary data set so that the results
presented here slightly differ from our presentation in January.
A full account will be submitted to A&A Letters (de Boer et al. 1998),
together with a study of H2 absorption in the SMC
(Richter et al. 1998).
References
- de Boer K.S., Richter P., Bomans D.J., Heithausen A., Koornneef J.,
1998, A&A 338, L5
- de Boer K.S., Grewing M., Richtler T., Wamsteker W., Gry C., Panagia N.,
1987, A&A 177, L 37
- Israel F.P., Koornneef J., 1988, A&A 190, 21
- Parker J.W., Garmany C.D., Massey P., Walborn N.R., 1992, AJ 103, 1205
- Richter P., Widmann H., de Boer K.S., Appenzeller I., Barnstedt J.,
Gölz M., Grewing M., Gringel W., Kappelmann N., Krämer G.,
Werner K., 1998, A&A 338, L9
- Snow T.P. Jr., 1977, ApJ 216, 724
- Spitzer L., Cochran W.D., 1973, ApJ 186, L 23
- Spitzer L., Drake J.F., Jenkins E.B., Morton D.C., Rogerson J.B.,
York D.G., 1973, ApJ 181, L 122
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| First version: | 07th | July, | 1998
|
| Last update: | 04th | October, | 1998
|
Jochen M. Braun &
Tom Richtler
(E-Mail: jbraun|richtler@astro.uni-bonn.de)