Glimpses into the Secret Life of Hot Subdwarf Stars
Elizabeth Green
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 |
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