A High-Resolution Study of \boldmath{$\eta$} Carinae's Outer Ejecta
Kerstin Weis\footnotemark
$\eta$ Carinae is a very luminous and unstable evolved star. Outflowing material ejected during the star's giant eruption in 1843 surrounds it as a nebula which consists of an inner bipolar region (the {Homunculus\/}) and the {Outer Ejecta\/}. The outer ejecta is very filamentary and shaped irregularly. Kinematic analysis, however, shows a regular bi-directional expansion despite of the complex morphology. Radial velocities in the outer ejecta reach up to 2000 kms$^{-1}$ and give rise to X-ray emission first detected by ROSAT. We will present a detailed study of the outer ejecta based on HST images, high-resolution echelle spectra for kinematic studies, images from CHANDRA/ACIS and HST-STIS spectra.
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\begin{document}
\vspace*{1cm}
\title{A High-Resolution Study of \boldmath{$\eta$} Carinae's Outer Ejecta}
 \author{Kerstin Weis\footnotemark}
\footnotetext{Feodor-Lynen-fellow, Alexander-von-Humboldt foundation}
\affil{Max-Planck-Institut f\"ur Radioastronomie, Auf dem H\"ugel 69, 53121
  Bonn, Germany and University of Minnesota, 116 Church Street SE,
  Minneapolis, MN 55455, USA }
\author{Michael F. Corcoran}
\affil{Laboratory for High Energy Astrophysics, Goddard Space Flight Center,
  Greenbelt, MD 20771, USA}
\author{Kris Davidson \& Roberta M. Humphreys}
\affil{University of Minnesota, 116 Church Street SE,
  Minneapolis, MN 55455, USA }

\begin{abstract}
$\eta$ Carinae is a very luminous and unstable evolved star.
Outflowing material ejected during the star's giant eruption 
in 1843 surrounds it as a nebula which consists of an 
inner bipolar region (the {\it Homunculus\/}) and the {\it Outer Ejecta\/}. 
The outer ejecta is very filamentary and shaped irregularly.
Kinematic analysis, however, shows a regular bi-directional 
expansion despite of the complex morphology. Radial velocities in the 
outer ejecta reach up to 2000 kms$^{-1}$ and give 
rise to X-ray emission first detected by ROSAT. We will 
present a detailed study of the outer ejecta based on HST 
images, high-resolution echelle spectra for kinematic studies, 
images from CHANDRA/ACIS and HST-STIS spectra. 
\end{abstract}

\section{Morphology, kinematics and the soft X-ray emission}

The outer part of the nebula around $\eta$ Carinae 
contains  a countless number of knots, bullets and filaments and manifests the
so-called outer ejecta (dia\-meter 60\arcsec\ or 0.67\,pc).  
A kinematic analysis using
high-resolution echelle spectra showed that the outer ejecta expand 
with velocities between $-600$\,\kms\ and  +600\,\kms\ on average
(e.g.\ Meaburn et al.\ 1996, Weis 2001a,b).
 Various filaments nevertheless show much higher 
radial velocities---e.g.\ $\sim$ 2000\,\kms (Weis 2001a,b). 
It is still unclear what triggered the outburst in 1843 and which 
mechanism not only formed 
$\eta$ Carinae's nebula but also the amazingly high expansion velocities. 
With velocities that high, X-ray emission from $\eta$ Carinae is expected and
was indeed detected (e.g.\ Cheblowski et al.\ 1984, Corcoran et al.\ 1997). With ROSAT and CHANDRA we are now able to separate
between a harder, nearly point like emission from the central source and an
extended softer emission from $\eta$ Carinae's
outer ejecta which is roughly hook shaped (e.g.\ Weis et al.\ 2001). 
Interestingly, an overlay of the optical image and the X-ray emission 
shows only a few  correlations between the 
optically emitting gas and the hot X-ray gas, see Fig. 1. 
A much stronger conformance was found in comparison with the radial 
expansion velocities. The 
expansion velocities are derived from our optical echelle spectra
(FWHM\,$\sim$\,14\,\kms) and overplotted in Fig.\,1 with negative
velocities underlined. Areas with higher X-ray emission show in general 
higher expansion velocities. The faster the gas is expanding
the more intense is the X-ray emission. The extended soft X-ray emission
from $\eta$ Carinae and especially the morphology of the X-ray nebula 
can therefore be explained by faster moving filaments which form X-ray emitting
shocks. 
The temperature of the gas (using thermal equilibrium models) 
is $\sim$ 0.65 keV (Weis et al. 2002, in prep.)  
indicating post-shock velocities of around 750\,\kms\
in very good agreement with the detected expansion velocities
of the bulk of the gas.   

 



\begin{figure}
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\parbox{4.22cm}{\vspace*{-1cm}\caption{In gray\-scale the optical (F658N) 
HST image (WF chip) is shown and in contours the 
    emission in the 0.6-1.2 keV band as detected
    with the CHANDRA ACIS S3. Numbers indicate the expansion velocities of
    certain areas, underlined num\-bers are neg\-ative velo\-cities, for 
    better
    il\-lus\-tra\-tion the font size in\-crea\-ses for higher velo\-cities. }}
\end{figure}

\begin{references}

\reference Chlebowski, T., Seward, F. D., Swank, J., Szymkowiak, A., 1984, 
\apj, 281, 665
\reference Corcoran, M. F.,  Ishibashi, K., Swank, J. H., et al. 1997, \nat,
390, 587
\reference Meaburn, J., Boumis, P., Walsh, J. R., et al. 1996, \mnras , 
282, 1313
\reference Weis, K., 2001a, in $\eta$ Carinae and other mysterious stars,
ed. T. Gull, S. Johanson, \& K. Davidson, ASP Conf. 242, 129
\reference Weis, K., 2001b, in 
Reviews in Modern Astronomy 14, Springer-Verlag, 
ed.: Schielicke, R.E, ISBN 3-9805176-4-0, 261
\reference Weis, K., Duschl, W.J., Bomans, D.J., 2001, \aap, 367, 566  

\end{references}
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