Astronomy and Astrophysics, volume 405, 1013-1023 (2003/7-3)
H2 absorption in a dense interstellar filament in the Milky Way halo.
RICHTER P., SEMBACH K.R. and HOWK J.C.
Abstract (from CDS):
We investigate interstellar absorption from molecular hydrogen (H2) and metals in an intermediate-velocity cloud (IVC) in the direction of the LMC star Sk -6880 (HD 36521), based on data from the Far Ultraviolet Spectroscopic Explorer (FUSE) satellite. H2 absorption from the Lyman and Werner bands is detected in 30 lines at radial velocities vLSR≃+50km/s in this IVC that is presumably located in the Milky Way halo. We obtain a total logarithmic H2 column density of log N(H2)=16.6 ±0.5 along with a very low Doppler parameter of b=1.5+0.8–0.2km/s. The presence of molecular material in this cloud is suprising, given the fact that the OI column density (logN(OI)=14.8±0.1) implies a very low neutral gas column density of ∼1018cm–2 (assuming a solar oxygen abundance). If the H2 column density represents its abundance in a formation-dissociation equilibrium, the data imply that the molecular gas resides in a small, dense filament at a volume density of ∼800cm–3 and a thickness of only 41 Astronomical Units (AU). The molecular filament possibly corresponds to the tiny-scale atomic structures (TSAS) in the diffuse interstellar medium observed in high-resolution optical data, HI 21 cm absorption, and in CO emission.