\documentstyle[paasms4]{preprint}
\input pub.sty
\newcommand{\Myr}{M$_{\odot}$~yr$^{-1}$}

\begin{document}

\title{RADIO CONTINUUM MEASUREMENTS OF SOUTHERN\\
~\\
EARLY-TYPE STARS.\\
~\\
II. A DISTANCE LIMITED SAMPLE OF WOLF-RAYET STARS}
       

\author{Claus Leitherer\\
\\
Space Telescope Science Institute,\thanks{Operated
by AURA for NASA under contract NAS5-26555} 3700 San Martin Drive, Baltimore,
       MD 21218\\
\\
       e-mail: leitherer@stsci.edu\\
\and
Jessica M.\ Chapman\\
\\
Anglo-Australian Observatory, P.O.\ Box 296, Epping,
          NSW 2121, Australia\\
\\
               and\\
\\
       Australia Telescope National Facility, P.O.\ Box 76, Epping, 
          NSW 2121, Australia\\
\\
          e-mail: jmc@aaoepp.aao.gov.au\\
\and
B\"arbel~Koribalski\\
\\
Australia Telescope National Facility, CSIRO, P.O. Box 76, Epping,
          NSW 2121, Australia\\
\\
          e-mail: bkoribal@atnf.csiro.au}

\pub{ApJ}

\recacc{30 October 1996}{22 December 1996}

\maketitle

\abstract

A distance-limited sample of southern Wolf-Rayet stars within 3~kpc of
the Sun has been observed with the Australia Telescope Compact Array
at 8.64~GHz and 4.80~GHz. Radio continuum flux densities at one or
both frequencies were obtained for 10 sources, and upper limits for
20; four sources are found to be thermal emitters on the basis of the
observed spectral index. Five sources are classified as non-thermal. One
source could not be classified.

We derive mass-loss rates for the thermal sources. After combining them with
all existing radio mass-loss rates of Wolf-Rayet stars in the northern and
southern hemisphere, we perform a comparison with mass-loss rates derived
from optical emission lines. The two methods lead to consistent results,
suggesting that the assumption of a spherically symmetric, stationary,
homogeneous stellar wind is either correct, or that deviations from this
assumption affect both methods in the same way. 

Wolf-Rayet mass-loss rates are surprisingly uniform across spectral type.
We find an average mass-loss rate of $4 \times 10^{-5}$~\Myr\ for all
types observed, except for WC9 stars, which have rates that are lower
by at least a factor of 2. An alternative explanation could be partial
recombination of helium from He$^+$ to He$^{\circ}$ in the radio region,
leading to a reduced number of free electrons, and therefore reduced 
radio flux for WC9 stars. Mass-loss rates  of  $8 \times 10^{-5}$~\Myr\ for 
late WN stars favored in recent stellar evolution models disagree 
with the observations of these subtypes.

The results of this survey suggest that $\sim40$\% of all Wolf-Rayet
stars with measured spectral index are non-thermal emitters at cm
wavelengths. This percentage is nearly twice as high as that of
non-thermal emitters among OB stars and is higher than previously
estimated for WR stars.  The nature of the non-thermal emission is
still not fully understood. Possible causes of non-thermal emission
are discussed.  In particular, we speculate that non-thermal emission
may arise from an interaction between a thermal WR wind and surrounding
material due to a shell ejected during a previous evolutionary stage,
or due to the wind of a companion star.