Received 14th March 1998
Abstract.
IUE high dispersion spectra
of 18 Large Magellanic Cloud stars have been analysed with the aim of mapping
the Si
II column density in the galactic halo gas towards
the LMC.
Sample Si
II spectra in velocity space are presented
which confirm, with their general appearance, earlier results.
Low ion stage lines exhibit a component structure with strong features near
∼60 km s
-1 and ∼130 km s
-1.
For these intermediate and high velocity clouds' column densities can be
derived.
Using H
I column densities as reference the abundance of
Si
II in the galactic halo is estimated.
Maps showing the Si
II column densities in the two clouds
towards the LMC are presented.
1. Introduction
In 1981, Savage and de Boer presented a study of interstellar absorption
features toward 6 LMC stars.
They found that the low ions like Si II or
Fe II exhibit a component structure with strong
features near ∼+60 km s-1 and
∼+130 km s-1.
Column densities were given for galactic and LMC absorption structures,
but only for one star (HD 36402) column densities for the galactic
intermediate and high velocity components were given separately
(see Savage & de Boer 1981 and references therein).
Since that time many more LMC stars have been observed with the
IUE and all
the high dispersion spectra have been reprocessed by the
IUE observatories
with new reduction software.
This motivated us to investigate the halo absorption structures in as many LMC
star spectra as possible in order to map the Si II column
density in these clouds in front of the LMC.
2. Method of analysis
The radial velocity of the LMC of ∼+275 km s-1
(Westerlund 1997) produces a separation of galactic and extragalactic
interstellar absorption features which is convenient for the analysis
of distinct halo structures.
The IUE spectra have been
transformed into intensity vs. velocity plots in order to facilitate this
analysis and allow for a distinction between intermediate
(40 - 100 km s-1) and high velocity
(100 - 170 km s-1) components.
The resolution of the spectra
(λ/Δλ∼1.2·104)
is 20 - 25 km s-1 in velocity space.
Inspection of all absorption lines on a given line of sight allows to decide
whether a certain spectral structure is a true absorption or not.
The equivalent widths are measured by fitting trapezoidal profiles to the
absorption structures.
Column densities are obtained from a curve-of-growth analysis.
Whenever possible, an attempt has been made to obtain column densities for
the intermediate and high velocity structures separately.
A comparison with H I column densities leads to an
estimate of the abundances of Si II in the halo.
3. Results
Si II shows two distinct absorption structures in
the galactic halo near ∼60 km s-1 and
∼125 km s-1, which confirms earlier results of
Savage & de Boer 1981 (see Fig. 1a,
2a;
velocities are given in km s-1 LSR).
Si II column densities in the HVC tend to be higher than
those in the IVC (see Fig. 1b,
2b;
column densities are given in cm-2).
Comparison with H I column densities from McGee &
Newton 1986 and McGee et al. 1983 shows that abundances range from -4.65 to
-3.1 for the IVC and -4.56 to -2.6 for the HVC.
Assuming a solar abundance of -4.45 (de Boer et al. 1987), we note that
Si II appears to be overabundant. However, the values
are based on N (H I) from 21 cm.
Since the resolution of the Parkes telescope is rather limited
(half-power beam width ∼15 arcmin), the columns sampled in
Si II and H I most likely do not
refer to the same gas.
[Click here to see Figs. 1 and 2!]
4. Discussion
Our results are consistent with those of Savage & de Boer (1981) and McGee
et al. (1983).
Further UV observations with higher sensitivity are desirable to obtain a
denser grid of background light sources.
Thus, one could account even better for small-scale variations in column
densities.
References
- de Boer K.S., Jura M.A., Shull J.M., 1987, in 'Exploring the Universe
with the IUE Satellite', Y. Kondo (ed.), D. Reidel Publishing Company,
p. 485
- McGee R.X., Newton L.M., 1986, Proc. Astr. Soc. Austr. 6, 358
- McGee R.X., Newton L.M., Morton D.C., 1983, MNRAS 205, 1191
- Savage B.D., de Boer K.S., 1981, ApJ 243, 460
- Westerlund B.E., 1997, The Magellanic Clouds, Cambridge University Press
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| First version: | 22nd | June, | 1998
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| Last update: | 30th | September, | 1998
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Jochen M. Braun &
Tom Richtler
(E-Mail: jbraun|richtler@astro.uni-bonn.de)