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\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}