AAS 195th Meeting, January 2000
Session 72. ISM: Neutral Gas
Display, Friday, January 14, 2000, 9:20am-6:30pm, Grand Hall

## [72.04] Small Scale Interstellar Structure towards HD~32039/40 -- Temporal \ion{Na}{1} Variations and HST/STIS Observations

J. T. Lauroesch, David M. Meyer (Northwestern University), J. C. Blades (STScI)

Observations of interstellar absorption lines toward both members of resolvable binary star systems provide a sensitive probe of small-scale structure in the diffuse ISM. Previous optical observations of \ion{Na}{1} at high spectral resolution (\deltav = 1.4~km~s-1) towards the HD 32039/40 binary system have revealed line strength variations in at least 4 velocity components, indicative of ISM structure on scales less than the projected binary separation of 4800 AU (Watson & Meyer 1996). We present new KPNO Coudé feed and HST/STIS echelle observations of multiple interstellar lines towards both stars in the HD~32039/40 system, including trace neutral species (\ion{C}{1}, \ion{Na}{1}, and \ion{Mg}{1}), generally undepleted dominant ions (\ion{S}{2} and \ion{Zn}{2}), and depleted dominant ions (\ion{Cr}{2} and \ion{Ni}{2}).

Comparisons of the \ion{Na}{1} line profiles towards HD 32040 show that the component at +9~km~s-1 varies over the three years between observations, implying that there is structure on scales of order of the proper motion (14 AU). Given the observed difference of N(\ion{Na}{1})~5\rm\times 1010 \rm cm-2, one would estimate a density of order 50,000 \rm cm-3 for the gas in this component, similar to densities estimated in past radio and optical studies. However one derives a significantly lower density in this component from the \ion{C}{1} fine structure equilibrium -- assuming a temperature of 100~K we find \rm nH\la40 \rm cm-3 -- which suggests that some other process may be giving rise to the observed \ion{Na}{1} variations in agreement with past results for \mu Cru (Lauroesch et al. 1998). The component at +18~km~s-1 shows significant differences in the columns of \ion{Zn}{2} and \ion{S}{2} between HD 32039 and HD 32040, representing the first optical/UV detection of small scale variations in a dominant ion. The measured columns suggest that N(\ion{H}{1}) varies by ~4\rm\times 1018 \rm cm-2 between the sightlines. Given the binary separation of 4800 AU, this implies that nH\ga 60{\rm cm-3} in this component. The \ion{C}{1} fine--structure equilibrium does not strongly constrain the density, and so we will explore various methods for estimating the density in this cloud.