High velocity gas and dust evolution in Chamaeleon clouds

We report on GHRS observations which reveal conspicuous differences in the absorption spectra of two nearby stars, close to each other. The star HD102065 lies behind a cloud in Chamaeleon with unusually strong mid–IR emission, indicating a large abundance of very small dust particles. Along this line of sight, 5% of the gas (about 6 1019 cm−2) is at large velocities (up to -50 km s−1) compared to the main absorption component at vlsr ∼0 km s−1. The high velocity gas is very excited and has an unusually large silicon abundance. The other star HD96675 lies behind a cloud with standard mid–IR emission. Along this line of sight, high velocity gas is also detected, but to somewhat smaller offset velocities and with amuch lower excitation. In particular, the SiII∗ lines are not observed. From the excitation of Si+ and C+ in the direction of HD102065, we infer that the high velocity gas has a temperature higher than several 100 K and an electron density of at least 10 cm−3. These results, together with the lack of an ionizing star in the neigborhood, suggest that a large amount of kinetic energy is being deposited in this gas. The collision of an infalling cloud and a local cloud is a plausible source of energy. The peculiarity of the dust size distribution inferred from the IRAS data is likely to be related to the processes which dissipate the kinetic energy and heat the gas. A shock seems to be required to produce the excitation and ionization degrees. But the carbon ionization ratio combined with the electron density inferred from the silicon and carbon excitation implies that carbon is not in ionization equilibrium and should recombine extremely quickly. Future higher resolution observations might help solving this incoherency.