605 research outputs found
WFIRST Ultra-Precise Astrometry II: Asteroseismology
WFIRST microlensing observations will return high-precision parallaxes,
sigma(pi) < 0.3 microarcsec, for the roughly 1 million stars with H<14 in its
2.8 deg^2 field toward the Galactic bulge. Combined with its 40,000 epochs of
high precision photometry (~0.7 mmag at H_vega=14 and ~0.1 mmag at H=8), this
will yield a wealth of asteroseismic data of giant stars, primarily in the
Galactic bulge but including a substantial fraction of disk stars at all
Galactocentric radii interior to the Sun. For brighter stars, the astrometric
data will yield an external check on the radii derived from the two
asteroseismic parameters, and nu_max, while for the fainter ones, it
will enable a mass measurement from the single measurable asteroseismic
parameter nu_max. Simulations based on Kepler data indicate that WFIRST will be
capable of detecting oscillations in stars from slightly less luminous than the
red clump to the tip of the red giant branch, yielding roughly 1 million
detections.Comment: 13 pages, 6 figures, submitted to JKA
The relation between and for solar-like oscillations
Establishing relations between global stellar parameters and asteroseismic
quantities can help improve our understanding of stellar astrophysics and
facilitate the interpretation of observations. We present an observed relation
between the large frequency separation, , and the frequency of
maximum power, . We find that is proportional to
, allowing prediction of to about 15 per cent
given . Our result is further supported by established scaling
relations for and and by extended stellar model
calculations, which confirm that can be estimated using this
relation for basically any star showing solar-like oscillations in the
investigated range (0.5<M/Msol<4.0).Comment: 5 pages, 8 figures, Letter accepted by MNRA
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&
Asteroseismology of red giants from the first four months of Kepler data: Global oscillation parameters for 800 stars
We have studied solar-like oscillations in ~800 red-giant stars using Kepler
long-cadence photometry. The sample includes stars ranging in evolution from
the lower part of the red-giant branch to the Helium main sequence. We
investigate the relation between the large frequency separation (Delta nu) and
the frequency of maximum power (nu_max) and show that it is different for red
giants than for main-sequence stars, which is consistent with evolutionary
models and scaling relations. The distributions of nu_max and Delta nu are in
qualitative agreement with a simple stellar population model of the Kepler
field, including the first evidence for a secondary clump population
characterized by M ~> 2 M_sun and nu_max ~ 40-110 muHz. We measured the small
frequency separations delta nu_02 and delta nu_01 in over 400 stars and delta
nu_03 in over 40. We present C-D diagrams for l=1, 2 and 3 and show that the
frequency separation ratios delta nu_02/Delta nu and delta nu_01/Delta nu have
opposite trends as a function of Delta nu. The data show a narrowing of the l=1
ridge towards lower nu_max, in agreement with models predicting more efficient
mode trapping in stars with higher luminosity. We investigate the offset
epsilon in the asymptotic relation and find a clear correlation with Delta nu,
demonstrating that it is related to fundamental stellar parameters. Finally, we
present the first amplitude-nu_max relation for Kepler red giants. We observe a
lack of low-amplitude stars for nu_max ~> 110 muHz and find that, for a given
nu_max between 40-110 muHz, stars with lower Delta nu (and consequently higher
mass) tend to show lower amplitudes than stars with higher Delta nu.Comment: 12 pages, 12 figures, accepted for publication in Ap
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