OB-type lose mass via stellar winds, at a rate of a few millionths of
a solar mass per year (about an earth mass per year),
Over their lifetimes (several million years) this has a
dramatic effect on their evolution. Their winds are driven
by radiation pressure via millions of metal lines, so
that the metallicity of a galaxy plays a crucial role in massive star
evolution, since it defines their internal structure, opacities
and stellar wind properties. The precise relation between metallicity and
mass-loss, a key ingredient for the reliable population synthesis of young
galaxies, is however poorly known, so we are using a variety of observations
to constrain this relationship.
As mass is `peeled' away from the surface of stars, the inner regions,
which may contain products of nuclear burning, are exposed. The
abundances of helium, carbon, nitrogen, and oxygen are sensitive
indicators of processed material. We are studying these elements,
using elaborate non-LTE model atmospheres, and high-quality spectra
including IR diagnostics.
The most direct method of deriving empirical mass-loss rates for hot stars is
through analysis of the UV resonance transitions from dominant
metal ions. However, difficulties in deriving the wind
ionization balance using currently available (trace) ions means that
mass-loss rates remain uncertain. We are using the new 2dF facility
at the Anglo-Australian Telescope
to secure optical spectra of a truly representative sample of Small
Magellanic Cloud stars, which has abundances of metals smaller than our
own Galaxy by an order of magnitude.
Archival Hubble Space Telescope FOS and GHRS data sets have been
used to collect ultraviolet evidence for large- and small-scale stellar
wind structure in extragalactic Local Group OB stars (i.e. SMC, LMC including R136,
M31, M33, and NGC6822). By comparison to previous studies of Galactic
OB stars, wind activity is principally diagnosed in individual spectrograms via
the presence of `narrow absorption components' and saturated `black' absorption
troughs in the
resonance line doublets. Their observed characteristics broadly suggest
that these stars share the same physical mechanisms for
perturbing the winds as those that act in Galactic OB stars.
Both these spectral indicators are also used to provide
reliable measures
of wind terminal velocities. These velocities are directly compared
to previously published Galactic OB star values, without reliance
on model profile fitting.
Relative to Galactic OB stars, the most discrepant terminal velocities
(and wind line profiles) are due to MAIN-SEQUENCE early O-type stars
in the SMC
( Prinja & Crowther, 1998 MNRAS 300 828
see above).
We are also attacking the mass-loss problem with the new
FUSE
telescope. Relative to IUE/HST, additional wind lines will
be observed, spanning a much wider range of species. From these data,
the degree of ionization can be determined accurately, so that
mass-loss rates can be measured with confidence. In addition, FUSE
will observe massive stars in both the Galaxy and the
Magellanic Clouds -
spanning a factor of ten in metallicity - so that the variation of
mass-loss properties with metal content will be measured,
of importance in the study of high redshift galaxies.
Our studies have been greatly enhanced by the launch of the ESA
( Infrared Space Observatory (ISO) .
We have used the Short Wavelength Spectrometer to obtain mid-IR (2.6-30
micron) spectroscopy of several WC stars.
Dessart et al. (MNRAS submitted)
and Willis et al. (MNRAS 290 371) analyse UV/optical and ISO-SWS
spectroscopy of several WC stars, including WR11, WR90, WR135 and WR146.
In all cases, abundances derived from neon lines confirmed a substantial
enrichment, by factors of 6 to 8, but these are lower than predictions
from evolutionary models imply.
We have also used spectroscopic models to observationally determine the
evolutionary paths between O and the late WN (WNL) stars in the LMC,
involving so-called Ofpe/WN9 stars. Theoretical models predict
that the most massive stars pass through an intermediate,
unstable stage corresponding to either a
Luminous Blue Variable (LBV), or a red supergiant (RSG)
phase for lower mass stars. Our studies have shown that
Ofpe/WN9 stars are probably dormant LBVs
(Crowther & Smith 1997 A&A 320, 500) .
IR studies have revealed clusters of emission line sources at the
Galactic Centre (click
HERE for
a mid-IR image) which show properties similar to Wolf-Rayet and
Of stars (e.g.
Pistol Star). We can therefore apply similar
techniques (e.g. Crowther & Smith 1996, A&A 305 541 ,
( Crowther & Bohannan 1997, A&A 320, 500
and Bohannan & Crowther, 1999, ApJ 511, 374)
to the GC sources in order to identify their nature,
currently underway using
UKIRT.
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02-Dec-99 zuserver2.star.ucl.ac.uk
