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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.
The author(s) of this abstract have provided an email address for comments about the abstract: jtl@elvis.astro.nwu.edu