| Abstract |
Observations show that the interstellar medium (ISM) contains
sub-structure on scales less than 1 pc, detected in the form of spatial
and temporal variations in column densities or optical depth. Despite
the number of detections, the nature and ubiquity of the small-scale
structure in the ISM is not yet fully understood. We use UV absorption
data mainly from the Far Ultraviolet Spectroscopic Explorer (FUSE) and
partly from the Space Telescope Imaging Spectrograph (STIS) of six Large
Magellanic Cloud (LMC) stars (Sk -67°111, LH 54-425, Sk -67°107,
Sk -67°106, Sk -67°104, and Sk -67°101) that are all located
within 5´ of each other, and analyse the physical properties of
the Galactic disc gas in front of the LMC on sub-pc scales. We analyse
absorption lines of a number of ions within the UV spectral range. Most
importantly, interstellar molecular hydrogen, neutral oxygen, and
fine-structure levels of neutral carbon have been used in order to study
changes in the density and the physical properties of the Galactic disc
gas over small angular scales. At an assumed distance of 1 kpc, the
5´ separation between Sk -67°111 and Sk -67°101 implies a
linear extent of 1.5 pc. We report on column densities of H2,
C i, N i, O i, Al ii, Si ii, P ii, S iii, Ar i, and Fe ii in our six
lines of sight, as well as C i*, C i**, Mg ii, Si iv, S ii, Mn ii, and
Ni ii for four of them. While most species do not show any significant
variation in their column densities, we find an enhancement of almost 2
dex for H2 from Sk -67°111 to Sk -67°101, accompanied
by only a small variation in the O i column density. Based on the
formation-dissociation equilibrium, we trace these variations to the
actual density variations in the molecular gas. On the smallest spatial
scale of <0.08 pc, between Sk -67°107 and LH 54-425, we find a
gas density variation of a factor of 1.8. The line of sight towards LH
54-425 does not follow the relatively smooth change seen from Sk
-67°101 to Sk -67°111, suggesting that sub-structure might exist
on a smaller spatial scale than the linear extent of our sight-lines.
The results show that we sample a mix of both neutral and ionised gas in
our six lines of sight. Towards Sk -67°101 to Sk -67°107, we
derive the temperature Texc ≃ 70 K for the inner
self-shielded part of the gas based on the rotational excitation levels
of H2, and an average density of nH ≃ 60
cm-3, typical of that for CNM. The gas towards LH 54-425 and
Sk -67°111 shows different properties, and Texc ≃
200 K. Our observations suggest that the detected H2 in these
six lines of sight (with the extent of <1.5 pc) is not necessarily
physically connected, but that we are sampling molecular cloudlets with
pathlengths <0.1-1.8 pc and possibly different densities. |