Variable NaI Absorption toward rho Leo : Biased Neutral Formation in the Diffuse Interstellar Medium?

Abstract

We present multi-epoch KPNO Coude Feed observations of interstellar NaI and CaII absorption toward the bright star rho Leo. Comparisons of the NaI profiles observed over a period of 8 years reveal significant temporal variations in the NaI column in at least one component, implying that there is ``structure'' at scales of order of the proper motion (~12 AU). Archival HST Goddard High Resolution Spectrograph observations of the CI fine-structure excitation in the variable component suggest that the density is <20 cm-3, significantly lower than the densities inferred in past HI 21 cm and NaI studies. We suggest that the bulk of the trace neutral species are in the density peaks within an interstellar cloud. The patchy distribution of these species naturally gives rise to the large fluctuations seen on scales of 10--1,000 AU in past temporal and binary studies. This picture predicts that the scales over which fluctuations will be observed vary as a function of the ionization rate of a species.