Project proposed by Frank Grundahl and Søren Frandsen, IFA (
Subdwarf B
stars are found on the extension of the Horizontal Branch stars, where the
extension reach all the way on the other side of the Main Sequence. These stars
are therefore less luminous than normal B stars and also have smaller sizes as
they have expelled the envelope during the last phase of Hydrogen burning.
Some of
these stars are unstable and pulsate with many frequencies corresponding to
pressure eigenmodes (p modes), which due to their small sizes and high
temperatures correspond to periods of order minutes (100 < P <
200s). It is therefore comparatively easy to get a timeseries stretching for
many periods in a rather short observing time and compute a
power spectrum that bring out a range of frequencies.
Using
these observed frequencies, one can derive properties
of the stars applying seismic techniques.
Possible
targets for the project:
Choose one of this known variables and confirm the variability by
observing a time series of around half an hour with ALFOSC using the fast
photometry mode. Next select a field in the globular cluster M 13 with a number
of sdB stars and take a time series of 3.5 hour. Check the sdB stars in the field
for variability. Position the readout strip optimally in order to get time
series for as many candidates as possible. Magnitudes for these stars are in
the range V = 17-18. So far, no pulsating sdB star has been detected in
a globular cluster.