IMF FOR MASSIVE STARS
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Mary Kontizas, Ana Ines Gomez de Castro, Anita Richards

INTRODUCTION

Massive stars in stellar systems

The high mass stars are the luminous tracers of the young populations in
galaxies. They are O, B type stars and very few evolved M  type SGs.
Although they are very few massive stars in each stellar population,
the fact that they are very bright makes them easy to observe in our
galaxy and other nearby galaxies. The IMF of the bright end of the mass
scale is a crucial parameter for determining the upper limit of mass and
its connection to the environment such as metallicity.

To find the IMF we require  to know the mass of the observed stars as it
is determined from its position at the CMD (colour magnitude diagram) and
then we compare with theoretical tracks.
We need to know the absolute magnitude of each star e.i. the exact
distance of the star and absorption. The distance in extragalactic systems
is not difficult to find, since the overall distance of the parent galaxy
is known. The interstellar extinction is not always well determined.

However an efficient way of overcoming this problem, particularly for
our own Galaxy is to consider the stars in stellar associations, where
the distance and extinction is found for all stars in each
system. Young associations are also found in the Magellanic Clouds and
M31 and a lot of data are available.

Embedded massive stars

The very massive stars at the protostar phase start as small dense
ionized gas objects known as Ultra-Compact HII regions detected in the
radio domain.  The pressure is higher than the surrounding field and
the expansion starts quickly so that the region becomes a compact and
then a normal HII region. The lifetime of such a protostar is
104-10^5yr. Yet some massive protostars manage to produce bipolar
outflows & masers. The types of maser emission associated with YSOs is
a mass indicator e.g. methanol with high mass YSOs. It is found that
more evolved low-mass YSO e.g. T Tauri stars can have similar IR
colours to high-mass YSO, but T Tauris are more likely to be X-ray
emitters and even if the high-mass stars are, the colours are
distinctive.

The nature of discs and jets and in some cases stellar
activity gives some information on stellar mass and can be used as a good
indicator of close binarity/multiplicity.

The extragalactic HII regions are observed like beads along the spiral
arms of the parent galaxy. The UV spectrum of the HII regions is the
best way to discriminate the number and spectral type (e.g. mass) of
the stars. Population synthesis is often done in a self-consistent
manner with stellar evolution models for a given metallicity.

For the very massive stars (with M>10Mo) there is no pre-main sequence
phase so they begin to emit in the UV.


TARGETS

--Massive stars in M31, our own Galaxy, LMC ( Large Magellanic Cloud) and
SMC to obtain a galaxy sample with different metallicities from
very high to very poor.

--Select the young stellar associations in these galaxies.
Find the available CMDs. Extinction and distance. Production of the MFs

--Select the embedded clusters from IR surveys to have the youngest
objects in the galaxies.

--Radio masers with high mass indications.

--Extragalactic Ultra Compact HII regions.

DATA CATALOGUES - TOOLS

--Archives in the optical domain of very young star clusters and
associations in the nearby galaxies such as the MW. LMC, SMC, M31. The
superclusters in M31 have been found with point sources, which provide
the high mass end of the IMF.

--Radio archives such as ATCA, MERLIN, Puschino and Nancay (ie at
least 50% European!) and maybe the VLA will provide the necessary
spectra with very accurate positions.

--We need UV spectra of standard massive stars for the population
synthesis. This should come from IUE (since HST is too sensitive for
them, and IUE low dispersion spectrocospy is well calibrated). We
could check the IUE calibritation with the new set of standards
proposed by HST.

--Tools which use as input the UV spectra (for instance HST/STIS
long-slit) and the stellar evolution codes can be used to determine
the high mass end of the IMF and its dependence on the metallicity of
the region (or galaxy). This tool would be most useful both to
determine whether there are significant differences in the high mass
end from an area to another and also for the study of the chemical
evolution of the Universe (for high redshift galaxies the relevant
region of the spectrum moves from the UV to the optical).