First results from the UHRF: ultra-high-resolution observations of atomic interstellar lines towards ζ Ophiuchi

Abstract

We present ultra-high-resolution observations of optical interstellar lines towards zeta Oph obtained during the commissioning phase of the new Ultra-High-Resolution Facility (UHRF) at the AAT. The UHRF has achieved its design resolving power of R approximate to 10(6), and has very clearly resolved hyperfine splitting in three of the 11 identified Na I D-line velocity components towards zeta Oph. In addition, the intrinsic line profiles of the 3s-4p Na I doublet at 3302 Angstrom have been measured for the first time. 12 velocity components are identified in the interstellar Ca II K-line absorption profile, of which seven have definite velocity counterparts in the Na I profile. Three of the Na I velocity components have b-values which correspond to a gas temperature of about 200 K, for the case of pure thermal broadening. The strongest component in the sightline (-14.8 km s(-1)) was found to have a b-value of 0.6(-0.1)(+0.2) km s(-1), corresponding to a temperature of 500 K. If a true gas kinetic temperature of 54 K is assumed to apply to this component, from the H-2 rotational excitation temperature previously measured for the zeta Oph sightline, then the observed Na I and Fe I line profiles both imply an rms turbulent velocity of 0.4 km s(-1) within the cloud, just less than the local sound speed. The observed Ca II linewidths are all significantly larger than those of the corresponding Na I components. The Ca II components are interpreted as arising from warm neutral material (intercloud medium) surrounding cooler clouds, while half of the Na I components arise from cool clouds and half arise from warm intercloud material. The measured Na I/Ca II column density ratios, and the inferred temperatures, are consistent with a model in which calcium is heavily depleted on to grains in cool clouds but is restored to the gas phase by impact desorption in the warm intercloud medium.