A Search for AU-Scale C I Structure in the Diffuse ISM

We present a multi-epoch, high resolution (R ~ 100,000) study of ultraviolet interstellar C I absorption line profiles taken by the Hubble Space Telescope. The 17 stars of this survey were chosen because each has high resolution spectra taken at least 10 years apart with the same instrument (STIS), grating (E140H), and aperture (0.2"x0.2") thus minimizing the instrumental differences in the multi-epoch comparisons. Given the proper motions and distances of these stars, typically it was possible to observe variances in their C I line profiles which correspond to structure on scales of less than 200 AU. In 16 out of the 17 sightlines no significant differences in C I line profiles between the two epochs were detected (75% of the 68 C I velocity components were measured to vary less than 20% at the 2 σ level). A measurement of ~ 5% of sightlines with variances is consistent with the fraction found recently by McEvoy et al. (2015) in their much larger survey of multi-epoch variance of Na I in optical spectra. However, the sightline toward HD210809 did show significant variance in its C I line profile. The C I absorption arising from both the J = 1 and J = 2 fine-structure states toward this star exhibits variations at an LSR velocity of -37 km/s indicative of C I structure on a scale less than 200 AU. Interestingly, the sky position of HD 210809 corresponds to the edge of an intervening H I shell discovered by Suad et al. (2012) at this same LSR velocity. This connection is consistent with the optical survey of interstellar Na I by Meyer et al. (2015) who found that nearly all of their temporally-varying sightlines involved supernova remnants, H I supershells, or stellar bow shocks.