323 research outputs found
Differential Rotation in Convective Envelopes: Constraints from Eclipsing Binaries
Over time, tides synchronize the rotation periods of stars in a binary system
to the orbital period. However, if the star exhibits differential rotation then
only a portion of it can rotate at the orbital period, so the rotation period
at the surface may not match the orbital period. The difference between the
rotation and orbital periods can therefore be used to infer the extent of the
differential rotation. We use a simple parameterization of differential
rotation in stars with convective envelopes in circular orbits to predict the
difference between the surface rotation period and the orbital period.
Comparing this parameterization to observed eclipsing binary systems, we find
that in the surface convection zones of stars in short-period binaries there is
very little radial differential rotation, with . This holds even for longer orbital periods, though it is harder to say
which systems are synchronized at long periods, and larger differential
rotation is degenerate with asynchronous rotation.Comment: 19 pages, published in MNRAS. Corrected typos and cases where
solar/anti-solar were swappe
Differential Rotation in Convective Envelopes: Constraints from Eclipsing Binaries
Over time, tides synchronize the rotation periods of stars in a binary system to the orbital period. However, if the star exhibits differential rotation, then only a portion of it can rotate at the orbital period, so the rotation period at the surface may not match the orbital period. The difference between the rotation and orbital periods can therefore be used to infer the extent of the differential rotation. We use a simple parametrization of differential rotation in stars with convective envelopes in circular orbits to predict the difference between the surface rotation period and the orbital period. Comparing this parametrization to observed eclipsing binary systems, we find that in the surface convection zones of stars in short-period binaries there is very little radial differential rotation, with |râ_rlnâΩ| < 0.02. This holds even for longer orbital periods, though it is harder to say which systems are synchronized at long periods, and larger differential rotation is degenerate with asynchronous rotation
Rapid Rotation of Low-Mass Red Giants Using APOKASC: A Measure of Interaction Rates on the Post-main-sequence
We investigate the occurrence rate of rapidly rotating (10 km
s), low-mass giant stars in the APOGEE-Kepler (APOKASC) fields with
asteroseismic mass and surface gravity measurements. Such stars are likely
merger products and their frequency places interesting constraints on stellar
population models. We also identify anomalous rotators, i.e. stars with 5 km
s10 km s that are rotating significantly faster
than both angular momentum evolution predictions and the measured rates of
similar stars. Our data set contains fewer rapid rotators than one would expect
given measurements of the Galactic field star population, which likely
indicates that asteroseismic detections are less common in rapidly rotating red
giants. The number of low-mass moderate (5-10 km s) rotators in our
sample gives a lower limit of 7% for the rate at which low-mass stars interact
on the upper red giant branch because single stars in this mass range are
expected to rotate slowly. Finally, we classify the likely origin of the rapid
or anomalous rotation where possible. KIC 10293335 is identified as a merger
product and KIC 6501237 is a possible binary system of two oscillating red
giants.Comment: 39 pages, 8 figures, 4 tables. Accepted for publication in the
Astrophysical Journal. For a brief video discussing key results from this
paper see http://youtu.be/ym_0nV7_YqI . The full table 1 is available at
http://www.astronomy.ohio-state.edu/~tayar/tab1_full.tx
Cannibals in the thick disk II -- Radial-velocity monitoring of the young alpha-rich stars
We report the results from new observations from a long-term radial velocity
monitoring campaign complemented with high resolution spectroscopy, as well as
new astrometry and seismology of a sample of 41 red giants from the third
version of APOKASC, which includes young alpha rich (YAR) stars. The aim is to
better characterize the YAR stars in terms of binarity fraction, mass,
abundance trends and kinematic properties. The radial velocities of HERMES,
APOGEE and Gaia were combined to determine the binary fraction among YAR stars.
In combination with their mass estimate, their evolutionary status, chemical
composition and kinematic properties, it allows to better constrain the nature
of these objects. We find that the frequency of binaries among over-massive
stars is not significantly different than that of the other stars in our
sample, but that the most massive YAR stars are indeed single, which has been
predicted by population synthesis models. Studying their [C/N], [C/Fe] and
[N/Fe] trends with mass, many over-massive stars do not follow the APOKASC
stars, favouring the scenario that most of them are product of mass transfer.
Our sample further includes two under-massive stars, with sufficiently low
masses so that these stars could not have reached the red giant phase without
significant mass loss. Both over-massive and under-massive stars might show
some anomalous APOGEE abundances such as N, Na, P, K and Cr, although higher
resolution optical spectroscopy might be needed to confirm these findings.
Considering the significant fraction of stars that are formed in pairs and the
variety of ways that make mass transfer possible, the diversity in properties
in terms of binarity and chemistry of the over-massive and under-massive stars
studied here implies that it is not safe to directly relate the mass of the YAR
stars with age and that most of these objects are likely not young.Comment: Submitted to A&A, comments welcome
Response to Comment on "A Non-Interacting Low-Mass Black Hole -- Giant Star Binary System"
van den Heuvel & Tauris argue that if the red giant star in the system 2MASS
J05215658+4359220 has a mass of 1 solar mass (M), then its unseen
companion could be a binary composed of two 0.9 M stars, making a
triple system. We contend that the existing data are most consistent with a
giant of mass M, implying a black hole companion of
M.Comment: 5 page
Kepler red-clump stars in the field and in open clusters: Constraints on core mixing
Convective mixing in helium-core-burning (HeCB) stars is one of the outstanding issues in stellar modelling. The precise asteroseismic measurements of gravity-mode period spacing (&dela;Ï1) have opened the door to detailed studies of the near-core structure of such stars, which had not been possible before. Here, we provide stringent tests of various core-mixing scenarios against the largely unbiased population of red-clump stars belonging to the old-open clusters monitored by Kepler, and by coupling the updated precise inference on &dela;Ï1 in thousands of field stars with spectroscopic constraints. We find that models with moderate overshooting successfully reproduce the range observed of &dela;Ï1 in clusters. In particular, we show that there is no evidence for the need to extend the size of the adiabatically stratified core, at least at the beginning of the HeCB phase. This conclusion is based primarily on ensemble studies of &dela;Ï1 as a function of mass and metallicity. While &dela;Ï1 shows no appreciable dependence on the mass, we have found a clear dependence of &dela;Ï1 on metallicity, which is also supported by predictions from models
Target Selection for the SDSS-IV APOGEE-2 Survey
APOGEE-2 is a high-resolution, near-infrared spectroscopic survey observing
roughly 300,000 stars across the entire sky. It is the successor to APOGEE and
is part of the Sloan Digital Sky Survey IV (SDSS-IV). APOGEE-2 is expanding
upon APOGEE's goals of addressing critical questions of stellar astrophysics,
stellar populations, and Galactic chemodynamical evolution using (1) an
enhanced set of target types and (2) a second spectrograph at Las Campanas
Observatory in Chile. APOGEE-2 is targeting red giant branch (RGB) and red
clump (RC) stars, RR Lyrae, low-mass dwarf stars, young stellar objects, and
numerous other Milky Way and Local Group sources across the entire sky from
both hemispheres. In this paper, we describe the APOGEE-2 observational design,
target selection catalogs and algorithms, and the targeting-related
documentation included in the SDSS data releases.Comment: 19 pages, 6 figures. Accepted to A
Poster CS20.5 - Weakened magnetic braking supported by asteroseismic rotation
Studies using asteroseismic ages and rotation rates from star-spot rotation have indicated that standard age-rotation relations may break down roughly half-way through the main sequence lifetime, a phenomenon referred to as weakened magnetic braking. While rotation rates from spots can be difficult to determine for older, less active stars, rotational splitting of asteroseismic oscillation frequencies can provide rotation rates for both active and quiescent stars, and so can confirm whether this effect really takes place on the main sequence. In this talk, Iâll show how we obtained asteroseismic rotation rates of 91 main sequence stars showing high signal-to-noise modes of oscillation. Using these new rotation rates, along with effective temperatures, metallicities and seismic masses and ages, we built a hierarchical Bayesian mixture model that showed that our new ensemble more closely agreed with weakened magnetic braking, over a standard rotational evolution scenario
Establishing the accuracy of asteroseismic mass and radius estimates of giant stars III. KIC4054905, an eclipsing binary with two 10 Gyr thick disk RGB stars
Eclipsing binary stars with an oscillating giant component allow accurate
stellar parameters to be derived and asteroseismic methods to be tested and
calibrated. To this aim, suitable systems need to be firstly identified and
secondly measured precisely and accurately. KIC 4054905 is one such system,
which has been identified, but with measurements of a relatively low precision
and with some confusion regarding its parameters and evolutionary state. Our
aim is to provide a detailed and precise characterisation of the system and to
test asteroseismic scaling relations. Dynamical and asteroseismic parameters of
KIC4054905 were determined from Kepler photometry and multi-epoch
high-resolution spectra from FIES at the Nordic Optical Telescope. KIC 4054905
was found to belong to the thick disk and consist of two lower red giant branch
(RGB) components with nearly identical masses of 0.95 and an age of
Gyr. The most evolved star displays solar-like oscillations, which
suggest that the star belongs to the RGB, supported also by the radius, which
is significantly smaller than the red clump phase for this mass and
metallicity. Masses and radii from corrected asteroseismic scaling relations
can be brought into full agreement with the dynamical values if the RGB phase
is assumed, but a best scaling method could not be identified. We measured
dynamical masses and radii with a precision better than 1.0%. We firmly
establish the evolutionary nature of the system to be that of two early RGB
stars with an age close to 10 Gyr, unlike previous findings. The metallicity
and Galactic velocity suggest that the system belongs to the thick disk of the
Milky Way. We investigate the agreement between dynamical and asteroseismic
parameters for KIC 4054905. Consistent solutions exist, but the need to analyse
more systems continues in order to establish the accuracy of asteroseismic
methods.Comment: Accepted for publication in Astronomy & Astrophysics, 11 pages, 3
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