Glimpses into the Secret Life of Hot Subdwarf Stars
Steward Observatory, University of Arizona
Subdwarf O and B stars are evolved stars that hold the keys to several incompletely understood areas of stellar evolution, for example, binary evolution, mass transfer and common envelopes. Hot subdwarf stars are believed to be the helium-burning cores of normal, low mass, red giant stars that have lost nearly all of their hydrogen envelopes except for a tiny wisp of hydrogen floating on top of their helium cores. Interestingly, many hot subdwarf stars have turned out to be multi-mode pulsators, which have proved incredibly valuable for stellar pulsation studies and the rapidly expanding field of asteroseismology, allowing us to test a number of predictions from standard stellar evolutionary theory. During the last two decades, binary modeling and asteroseismology have been responsible for considerable progress in understanding the current evolutionary status of subdwarf B (sdB) stars and the evolutionary channels that produce them. It is clear that many (most?) sdB stars are the result of various interrelated, and fairly well understood, binary evolution scenarios. In contrast, the hotter subdwarf O (sdO) stars are much less well understood, partly because none of the handful of known sdO pulsators are suitable candidates for asteroseismology investigations and partly because there appear to be very few, if any, sdO counterparts to the short-period binary systems that are so common among sdB stars. A number of different theories have been advanced to explain the existence of sdO stars and account for their observed properties. Last year we conducted a survey of about 25 bright sdO field stars in an effort to identify rapid pulsators analogous to the recently discovered sdO pulsators in the Omega Cen globular cluster. Somewhat surprisingly, we didn't find a single sdO pulsator among our field sample. Instead, most unexpectedly, our precision light curves turned up two sdO stars, and subsequently an sdB star, whose luminosities were occasionally observed to drop significantly and then flicker irregularly in a manner similar to the light from accretion disks in cataclysmic variables. In particular, we suggest that these three hot subdwarfs are related to the VY Scl class of cataclysmic variables. We will discuss further evidence in favor of the hypothesis that many sdO stars, and at least a few sdB stars, are members of binary systems containing a cool component and, very possibly, an accretion disk.
Date: Thursday, 17 January 2013 Time: 11:30 Where: Université de Montréal Pavillon Roger-Gaudry, local D-460 Contact: Gilles Fontaine