Steve Kawaler: “Some of the Asteroseismology early successes came through the work on pulsating white dwarfs in the 1970s through today”

Steve Kawaler at the XXII Canary Islands Winter School of Astrophysics.Photo: Miguel Briganti, Servicio MultiMedia (IAC)
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Steve Kawaler investigates the life and death of stars by studying their oscillations by means of Asteroseismology. A foremost specialist in compact objects, such as white dwarfs, he is a past director of the international Whole Earth Telescope network, dedicated to the study of variable astronomical objects; he is currently a leader of one of the teams that are analyzing data from the Kepler mission. With Carl J. Hansen and Virginia Trimble, he coauthored the book ‘Stellar Interiors: Physical Principles, Structure, and Evolution’. President of Division V (Variable Stars) of the International Astronomical Union (IAU) and a scientific editor with ‘The Astrophysical Journal’, this researcher at Iowa State University (United States) and member of the American Association for the Advancement of Science (AAAS) says that, as time permits, he escapes to the rhythms of the baseball diamond (mostly as a spectator these days).

As Science Editor of ‘The Astrophysical Journal’ (ApJ), what do you think of the quality of asteroseismological research being published? And the quantity? How do you think it is impacting on other fields?

One of the most gratifying things about acting as a member of the editorial panel of the ApJ is seeing the increase in the number of asteroseismology papers in that particular journal. Europeans make up a large component of the international asteroseismology community.     Perhaps because of that, many of the important papers in Asteroseismology have appeared in ‘Astronomy and Astrophysics’, and ‘Monthly Notices’ of the Royal Astronomical Society.     But we are seeing more submissions to the ApJ recently - especially with the launch of Kepler - and that signifies further development in interest from other communities (both scientific and geographic).

What can Asteroseismology tell us about the final stages of a star's life? Or about the origin of the Universe or galaxy formation?

While some claim that Asteroseismology was 'born' fairly recently, some of the early successes came through the work on pulsating white dwarfs in the 1970s through today. In the 1980s and 90s, for example, we used asteroseismic results to probe the inner compositional structure of white dwarfs, their rotation, convection, etc. These studies had a major impact on our understanding of white dwarf chemical evolution, cooling rates, neutrino emission, etc. With asteroseismically constrained white dwarf models and interior physics, these stars have become an independent tool for determining the ages of stellar systems such as globular clusters and the galaxy as a whole.

Asteroseismology can determine a star's age more accurately than any other method. How accurate is it? Could it be accurate enough to find out the age of the oldest stars and compare it with the age of the Universe, which is known? Has that been done? Do their ages agree?

Joergen Christensen-Dalsgaard, one of the other lecturers in the program, was the first to show how to use some of the basic observables for pulsating main sequence stars to determine their masses and ages. For the compact white dwarf pulsators, while Asteroseismology doesn't strongly constrain the ages of individual stars, asteroseismic models are used in calibrating the white dwarf cooling curve. This allows us to use the coolest white dwarfs in cluster and galaxy populations to place strong limits on the ages of those systems (see above). The results are in good agreement with more standard age estimates.

How are seismological readings from a star influenced by the characteristics of a planet orbiting around it?

I'm not sure there's a strong connection... though having a very stable 'asteroseismic clock' allows us to use the timing of that clock to measure slight changes in the period to detect orbital motion induced by a planetary companion. So I'd say that planet hunting by digging into asteroseismic signals is a valuable research technique. Roberto Silvotti and his team have already found one surprise - a planet orbiting a pulsating hot subdwarf, and they are looking for more such systems.

Can neutron stars and black holes pulsate? Would it be possible to observe these pulsations and obtain theoretical information about them?

A lot of theoretical work has been done to characterize neutron star oscillations.  That is a difficult problem, because not only do we need better understanding of the physical structure of stars supported by neutron degeneracy, but they are also rapidly rotating, and strongly coupled to their circumstellar environment. Models tell us that the oscillations will be extremely rapid (periods measured in tens of milliseconds) so detecting those will require some very sophisticated technology.  Fortunately, we won't have to worry about 'seeing' black hole oscillations - the event horizon will make sure of that...

XXII Canary Islands Winter School of Astrophysics

Annia Domènech

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