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SPB stars in the open SMC cluster NGC 371
Pulsation in beta Cep and SPB stars are driven by the kappa mechanism which
depends critically on the metallicity. It has therefore been suggested that
beta Cep and SPB stars should be rare in the Magellanic Clouds which have lower
metallicities than the solar neighborhood. To test this prediction we have
observed the open SMC cluster NGC 371 for 12 nights in order to search for beta
Cep and SPB stars. Surprisingly, we find 29 short-period B-type variables in
the upper part of the main sequence, many of which are probably SPB stars. This
result indicates that pulsation is still driven by the kappa mechanism even in
low metallicity environments. All the identified variables have periods longer
than the fundamental radial period which means that they cannot be beta Cep
stars. Within an amplitude detection limit of 5 mmag no stars in the top of the
HR-diagram show variability with periods shorter than the fundamental radial
period. So if beta Cep stars are present in the cluster they oscillate with
amplitudes below 5 mmag, which is significantly lower than the mean amplitude
of beta Cep stars in the Galaxy. We see evidence that multimode pulsation is
more common in the upper part of the main sequence than in the lower. We have
also identified 5 eclipsing binaries and 3 periodic pulsating Be stars in the
cluster field.Comment: 8 pages, 11 figures. Accepted for publication in MNRA
Sounding stellar cycles with Kepler - II. Ground-based observations
We have monitored 20 Sun-like stars in the Kepler field-of-view for excess
flux with the FIES spectrograph on the Nordic Optical Telescope since the
launch of Kepler spacecraft in 2009. These 20 stars were selected based on
their asteroseismic properties to sample the parameter space (effective
temperature, surface gravity, activity level etc.) around the Sun. Though the
ultimate goal is to improve stellar dynamo models, we focus the present paper
on the combination of space-based and ground-based observations can be used to
test the age-rotation-activity relations.
In this paper we describe the considerations behind the selection of these 20
Sun-like stars and present an initial asteroseismic analysis, which includes
stellar age estimates. We also describe the observations from the Nordic
Optical Telescope and present mean values of measured excess fluxes. These
measurements are combined with estimates of the rotation periods obtained from
a simple analysis of the modulation in photometric observations from Kepler
caused by starspots, and asteroseismic determinations of stellar ages, to test
relations between between age, rotation and activity.Comment: Accepted for publication in MNRA
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