Radio's role in studying massive stars
The most reliable way to estimate the rate of mass loss from massive stars
is to observe the thermal radio continuum emission from their hot stellar
winds. A program using the Australia Telescope Compact Array has detected
such emission from 11 massive stars, almost doubling the number of these
stars known at radio wavelengths.
Stars which start life more than 20 times more massive than the Sun
will develop strong stellar winds. With speeds of more than 1000 km
s-1, these winds carry off up to 90% of their star's initial mass. Such
extreme mass loss clearly influences these stars' subsequent evolution:
this is still not fully understood, but it is thought that a massive O-star
evolves through a short-lived 'luminous blue variable' (LBV) stage, during
which it ejects vast amounts of gas, to finally become a Wolf Rayet star.
In a pilot program, C. Leitherer et al. used the Australia Telescope
Compact Array to search for radio continuum emission from a sample of 12
southern stars which included Wolf Rayet stars, B-type hypergiants, Of stars
and one LBV star. They detected emission from 11 of these stars and were
able to determine accurate mass-loss rates for them. Their observations
show that, in most cases, the radio emission is thermal in nature and originates
from dense, ionised stellar winds expanding at constant velocity. The data
will be used to improve theoretical models for the evolution of massive
stars.
In a separate program, S. White et al. imaged the hot winds from
four of the best-known luminous blue variables: AG Car, HR Car, He3-519
and WRA 751, the first two in the most detail. They have been able to deduce
many of the stars' physical parameters, such as their maximum rates of mass
loss and the radio optical-depth (and hence column density structure) of
both the stars' central cores and their nebulae.
Radio continuum emission
from massive stars: C. Leitherer (STScI); J.
M. Chapman (AAO/ATNF); B.
Koribalski (ATNF). Radio properties of luminous blue variables: S. M.
White, M. R. Kundu (Uni. Maryland); R. A. Duncan (ATNF); J. M. Chapman (AAO/ATNF);
S. Drake (GSFC/NASA).
The super-massive star AG Car and its surrounding nebula of ejected
material.
Left: image made with the Australia Telescope Compact Array (combined 3-cm
and 6-cm observations)
Right: a Hubble Space Telescope image in the red (H-alpha) light of hydrogen
with two radio contours superimposed. (The straight radial lines are instrumental.)
The radio image is sensitive to only the ionised gas in the star's nebula.
The H-alpha image, however, not only shows emission from the ionised gas
but is also influenced by the presence of dust grains in the nebula: the
dust can absorb some of the nebula's instrinsic emission, as well as scattering
light from the central star. As the two images are quite similar it appears
that the dust of AG Car's nebula does not play much of a role in determining
the morphology of the Ha image.
The radio spectrum indicates that there is no non-thermal radio emission
from the nebula, i.e., the interaction of the nebula and the surrounding
medium into which it is expanding is relatively benign. However, the radio
spectrum of the star itself is non-thermal, which implies that quite strong
shocks are occurring inside the star's massive wind.
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