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Session 50 - Disks and Bipolar Outflows from Hot Stars - I.
Oral session, Wednesday, June 12
If recent experience is a valid guide, very massive evolved stars typically show outward signs of axial (non-spherical) symmetry; some cases are dramatic. Large-scale bipolar structure has lately been identified around most well-studied Luminous Blue Variables (LBVs), while their country cousins the B[e] stars (not the same as Be stars) have long been thought to have equatorial disks. Radiation pressure makes these stars fundamentally different from ordinary massive stars, helping in more than one way to encourage axial and equatorial structures. Two general questions have thus arisen: Is bipolar structure practically universal among very massive evolved stars? (2) Does the crucial LBV instability occur in an essentially equatorial or axial way, to the extent that spherical theories are misleading?
The special case of \eta Carinae provides insights largely because its structure is so obvious and well-defined. Its polar lobes are dramatic enough by themselves, but the large-scale equatorial debris are even more wonderfully suggestive. HST spectroscopy and imaging are giving information about the equatorial structure over a wide range of size scales. Additional questions particularly inspired by \eta Car are (3) How seriously are mass-loss estimates modified by this type of structure? The latitudinal distribution of ejected mass is not yet known, but it is clear that any analysis of the spectrum assuming spherical symmetry would be invalid, for this or any star with similar structure. (4) Did this prominent equatorial structure result from a much smaller wind-compressed disk, or is it the result of preferentially equatorial ejection in the LBV eruption? (5) How can we tell the difference between the effects of stellar rotation and of a close companion, for such a luminous star? (Radiation pressure prevents mass exchange in a very massive close binary, so the two cases look almost alike.)
Finally, we must eventually compare the axially symmetric structures of hot evolved stars (LBV and B[e] stars) with those of the most massive cooler hypergiants, which are beginning to be observed well.
Much of this work has been supported by NASA/HST/STScI grant GO-6041.
Program listing for Wednesday