\documentstyle[paasms4,pptwocol]{article}

\newcommand{\vlsr}{$v_{\mbox{\tiny LSR}}$}
\newcommand{\cmtwo}{cm$^{-2}$}
\newcommand{\Tk}{$T_{\mbox{\rm k}}$}
\newcommand{\kms}{$\mbox{km\,s}^{-1}$}
\newcommand{\ang}{\,\AA}

\begin{document}

\title{ ULTRA HIGH RESOLUTION OBSERVATIONS OF INTERSTELLAR\\
Na~{\sc i} AND Ca~{\sc ii} K TOWARD THE HIGH GALACTIC LATITUDE STAR HD 28497}

\author{J.\ Chris Blades, M.~S.\ Sahu and Lida He\/\thanks{on leave from 
Physics Department, Rensselaer Polytechnic Institute, Troy, NY 12180}\\
\\
Space Telescope Science Institute\\
\\
3700 San Martin Drive\\
\\
Baltimore, MD 21218\\
\and
I.\ A.\ Crawford, M.\ J.\ Barlow, and F.\ Diego\\
\\
Department of Physics and Astronomy\\
\\
University College London\\
\\
Gower Street\\
\\
London, WC1E 6BT, UK}

\tobe{1 April 1997}{The Astrophysical Journal}
\recacc{21 August 1996}{8 October 1996}

\maketitle

\begin{abstract}

We present very high resolution (0.32~km~s$^{-1}$) spectra
of interstellar Na~{\sc i} D$_1$, D$_2$ and Ca~{\sc ii}~K absorption toward 
HD~28497 obtained with the Ultra-High-Resolution Facility at the
3.9m Anglo-Australian Telescope. The star is located in projection  
in a highly disturbed interstellar  region close to a number of identified 
features including the high galactic latitude molecular cloud
MBM~20, the large Orion-Eridanus shell, seen in H$_\alpha$
and H~{\sc i} 21cm maps, and a filamentary loop structure
between \vlsr\  = $-$12 and $-$4~\kms\ in the Berkeley H~{\sc i} 21cm survey
and visible on the IRAS 100~$\mu$m map.

Toward HD 28497 we detect thirteen absorption components 
in the Na~{\sc i} spectra, to a 
column density limit of $2\times10^{10}$ \cmtwo, and ten in 
Ca~{\sc ii}~K over a velocity range of $\sim70$~\kms.
Four absorption components in the Na~{\sc i} spectra 
show $\it s$-resolved hyperfine structure with $b$-values from 0.31 to 
0.40~\kms\ and column densities 
from 4.0 to $14\times10^{10}$~\cmtwo.
If we assume  the clouds represented by these 
components have no internal turbulent  velocities, their temperatures 
would range between 134 to 227~K. One of these hyperfine split (hfs) 
components, at 
\vlsr\ = $-$11.1~\kms, shows significant temporal variation in 
equivalent width compared to earlier (1977) observations, making 
this the first interstellar sightline outside the Vela supernova 
remnant to show a time-varying component. The feature may be associated 
with the filamentary loop structure seen in this region.  

There is poor correspondence between the Na~{\sc i} and Ca~{\sc ii} 
profiles: we do not detect narrow Ca~{\sc ii} profiles to the four hfs 
Na~{\sc i} components, and only three of the well-resolved components have
the same Ca~{\sc ii} and Na~{\sc i} radial velocities and consistent 
$b$-values. One of these components, at \vlsr\ = $-$30.0~\kms, has a low 
Na~{\sc i}/Ca~{\sc ii} ratio and arises in a region where turbulent motions 
dominate---properties consistent with the hypothesis that the cloud lies close 
to HD~28497. In general, however, the Na~{\sc i} and Ca~{\sc ii} occupy
different gaseous phases in the ISM.

We have compared our data with 21cm emission profiles 
obtained from the recent Leiden/Dwingeloo H~{\sc i} survey. Based on 
agreement in the velocities, the Na~{\sc i}/Ca~{\sc ii} ratio, and the kinetic 
temperatures, we conclude that the component at \vlsr\ = $-$7.5~\kms\
is associated with the front side of the large, expanding 
Orion-Eridanus shell. Unexpectedly, the molecular cloud MBM~20 is not 
detected either in our absorption spectra or in the H~{\sc i} profiles, 
indicating that HD~28497 lies away from the core of MBM~20.

Apart from the two features at $-$11 and $-$7.5~\kms, there is almost no 
agreement between the H~{\sc i} profiles  and the optical spectra. Although 
we cannot rule out the possibility that most of the H~{\sc i} lies  behind the 
star, this explanation seems unlikely because many of  the H~{\sc i} features 
have previously been attributed to foreground phenomena.  The beam sizes of 
the H~{\sc i} and the optical studies are quite different and this suggests a 
different explanation, namely that the physical sizes of  the interstellar 
structures we detect in Na~{\sc i} and Ca~{\sc ii} are not extensive enough to 
be detected in H~{\sc i}. If so, this raises questions about the usefulness in 
general of combining results obtained from H~{\sc i} 21 cm studies with results
obtained from optical (or ultraviolet) studies of the interstellar gas.

\end{abstract}