Astronomy and Astrophysics, volume 520, A26-26 (2010/9-2)
Interstellar absorptions towards the LMC: small-scale density variations in Milky Way disc gas.
NASOUDI-SHOAR S., RICHTER P., DE BOER K.S. and WAKKER B.P.
Abstract (from CDS):
Observations show that the interstellar medium (ISM) contains sub-structure on scales less than 1pc, 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, LH54-425, 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 1kpc, the 5' separation between Sk-67°111 and Sk-67°101 implies a linear extent of 1.5pc. We report on column densities of H2, CI, NI, OI, AlII, SiII, PII, SIII, ArI, and FeII in our six lines of sight, as well as CI*, CI**, MgII, SiIV, SII, MnII, and NiII for four of them. While most species do not show any significant variation in their column densities, we find an enhancement of almost 2dex for H2 from Sk-67°111 to Sk-67°101, accompanied by only a small variation in the Oi 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 <0.08pc, between Sk-67°107 and LH54-425, we find a gas density variation of a factor of 1.8. The line of sight towards LH54-425 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≃70K for the inner self-shielded part of the gas based on the rotational excitation levels of H2, and an average density of nH≃60cm–3, typical of that for CNM. The gas towards LH54-425 and Sk-67°111 shows different properties, and Texc≃200K. Our observations suggest that the detected H2 in these six lines of sight (with the extent of <1.5pc) is not necessarily physically connected, but that we are sampling molecular cloudlets with pathlengths <0.1-1.8pc and possibly different densities.