1,360 research outputs found
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
KIC 8410637: a 408-day period eclipsing binary containing a pulsating red giant
Detached eclipsing binaries (dEBs) are ideal targets for accurate measurement
of masses and radii of ther component stars. If at least one of the stars has
evolved off the main sequence (MS), the masses and radii give a strict
constraint on the age of the stars. Several dEBs containing a bright K giant
and a fainter MS star have been discovered by the Kepler satellite. The mass
and radius of a red giant (RG) star can also be derived from its asteroseismic
signal. The parameters determined in this way depend on stellar models and may
contain systematic errors. It is important to validate the asteroseismically
determined mass and radius with independent methods. This can be done when
stars are members of stellar clusters or members of dEBs. KIC 8410637 consists
of an RG and an MS star. The aim is to derive accurate masses and radii for
both components and provide the foundation for a strong test of the
asteroseismic method and the accuracy of the deduced mass, radius and age. We
analyse high-resolution spectra from three different spectrographs. We also
calculate a fit to the Kepler light curve and use ground-based photometry to
determine the flux ratios between the component stars in the BVRI passbands. We
measured the masses and radii of the stars in the dEB, and the classical
parameters Teff, log g and [Fe/H] from the spectra and ground-based photometry.
The RG component of KIC 8410637 is most likely in the core helium-burning red
clump phase of evolution and has an age and composition very similar to the
stars in the open cluster NGC 6819. The mass of the RG in KIC 8410637 should
therefore be similar to the mass of RGs in NGC 6819, thus lending support to
the most up-to-date version of the asteroseismic scaling relations. This is the
first direct measurement of both mass and radius for an RG to be compared with
values for RGs from asteroseismic scaling relations.Comment: Accepted 20.6.2013 for publication in Astronomy and Astrophysic
The mass and age of the first SONG target: the red giant 46 LMi
Context. The Stellar Observation Network Group (SONG) is an initiative to build a worldwide network of 1m telescopes with high-precision radial-velocity spectrographs. Here we analyse the first radial-velocity time series of a red-giant star measured by the SONG
telescope at Tenerife. The asteroseismic results demonstrate a major increase in the achievable precision of the parameters for red-giant
stars obtainable from ground-based observations. Reliable tests of the validity of these results are needed, however, before the accuracy
of the parameters can be trusted.
Aims. We analyse the first SONG time series for the star 46 LMi, which has a precise parallax and an angular diameter measured from interferometry, and therefore a good determination of the stellar radius. We use asteroseismic scaling relations to obtain an accurate mass, and modelling to determine the age.
Methods. A 55-day time series of high-resolution, high S/N spectra were obtained with the first SONG telescope. We derive the asteroseismic parameters by analysing the power spectrum. To give a best guess on the large separation of modes in the power spectrum, we have applied a new method which uses the scaling of Kepler red-giant stars to 46 LMi.
Results. Several methods have been applied: classical estimates, seismic methods using the observed time series, and model calculations to derive the fundamental parameters of 46 LMi. Parameters determined using the different methods are consistent within the uncertainties. We find the following values for the mass M (scaling), radius R (classical), age (modelling), and surface gravity (combining mass and radius): M = 1.09 ± 0.04 M⊙, R = 7.95 ± 0.11 R⊙ age t = 8.2 ± 1.9 Gy, and log g = 2.674 ± 0.013.
Conclusions. The exciting possibilities for ground-based asteroseismology of solar-like oscillations with a fully robotic network have been illustrated with the results obtained from just a single site of the SONG network. The window function is still a severe problem which will be solved when there are more nodes in the network
A search for solar-like oscillations in K giants in the globular cluster M4
To expand the range in the colour-magnitude diagram where asteroseismology
can be applied, we organized a photometry campaign to find evidence for
solar-like oscillations in giant stars in the globular cluster M4. The aim was
to detect the comb-like p-mode structure characteristic for solar-like
oscillations in the amplitude spectra. The two dozen main target stars are in
the region of the bump stars and have luminosities in the range 50-140 Lsun. We
collected 6160 CCD frames and light curves for about 14000 stars were
extracted. We obtain high quality light curves for the K giants, but no clear
oscillation signal is detected. High precision differential photometry is
possible even in very crowded regions like the core of M4. Solar-like
oscillations are probably present in K giants, but the amplitudes are lower
than classical scaling laws predict.Comment: 14 pages, 16 figures, accepted for publication in A&
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