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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