427 research outputs found
A Class of Eccentric Binaries with Dynamic Tidal Distortions Discovered with Kepler
We have discovered a class of eccentric binary systems within the Kepler data
archive that have dynamic tidal distortions and tidally-induced pulsations.
Each has a uniquely shaped light curve that is characterized by periodic
brightening or variability at time scales of 4-20 days, frequently accompanied
by shorter period oscillations. We can explain the dominant features of the
entire class with orbitally-varying tidal forces that occur in close, eccentric
binary systems. The large variety of light curve shapes arises from viewing
systems at different angles. This hypothesis is supported by spectroscopic
radial velocity measurements for five systems, each showing evidence of being
in an eccentric binary system. Prior to the discovery of these 17 new systems,
only four stars, where KOI-54 is the best example, were known to have evidence
of these dynamic tides and tidally-induced oscillations. We perform preliminary
fits to the light curves and radial velocity data, present the overall
properties of this class and discuss the work required to accurately model
these systems.Comment: 13 pages, submitted to Ap
Evolutionary influences on the structure of red-giant acoustic oscillation spectra from 600d of Kepler observations
Context: The Kepler space mission is reaching continuous observing times long
enough to start studying the fine structure of the observed p-mode spectra.
Aims: In this paper, we aim to study the signature of stellar evolution on the
radial and p-dominated l=2 modes in an ensemble of red giants that show
solar-type oscillations. Results: We find that the phase shift of the central
radial mode (eps_c) is significantly different for red giants at a given large
frequency separation (Dnu_c) but which burn only H in a shell (RGB) than those
that have already ignited core He burning. Even though not directly probing the
stellar core the pair of local seismic observables (Dnu_c, eps_c) can be used
as an evolutionary stage discriminator that turned out to be as reliable as the
period spacing of the mixed dipole modes. We find a tight correlation between
eps_c and Dnu_c for RGB stars and no indication that eps_c depends on other
properties of these stars. It appears that the difference in eps_c between the
two populations becomes if we use an average of several radial orders, instead
of a local, i.e. only around the central radial mode, Dnu to determine the
phase shift. This indicates that the information on the evolutionary stage is
encoded locally, in the shape of the radial mode sequence. This shape turns out
to be approximately symmetric around the central radial mode for RGB stars but
asymmetric for core He burning stars. We computed radial modes for a sequence
of RG models and find them to qualitatively confirm our findings. We also find
that, at least in our models, the local Dnu is an at least as good and mostly
better proxy for both the asymptotic spacing and the large separation scaled
from the model density than the average Dnu. Finally, we investigate the
signature of the evolutionary stage on the small frequency separation and
quantify the mass dependency of this seismic parameter.Comment: 12 pages, 9 figures, accepted for publication in A&
Timing by Stellar Pulsations as an Exoplanet Discovery Method
The stable oscillations of pulsating stars can serve as accurate timepieces,
which may be monitored for the influence of exoplanets. An external companion
gravitationally tugs the host star, causing periodic changes in pulsation
arrival times. This method is most sensitive to detecting substellar companions
around the hottest pulsating stars, especially compact remnants like white
dwarfs and hot subdwarfs, as well as delta Scuti variables (A stars). However,
it is applicable to any pulsating star with sufficiently stable oscillations.
Care must be taken to ensure that the changes in pulsation arrival times are
not caused by intrinsic stellar variability; an external, light-travel-time
effect from an exoplanet identically affects all pulsation modes. With more
long-baseline photometric campaigns coming online, this method is yielding new
detections of substellar companions.Comment: 9 pages, 2 figures: Invited review to appear in 'Handbook of
Exoplanets,' Springer Reference Works, edited by Hans J. Deeg and Juan
Antonio Belmont
The K2 Mission: Characterization and Early results
The K2 mission will make use of the Kepler spacecraft and its assets to
expand upon Kepler's groundbreaking discoveries in the fields of exoplanets and
astrophysics through new and exciting observations. K2 will use an innovative
way of operating the spacecraft to observe target fields along the ecliptic for
the next 2-3 years. Early science commissioning observations have shown an
estimated photometric precision near 400 ppm in a single 30 minute observation,
and a 6-hour photometric precision of 80 ppm (both at V=12). The K2 mission
offers long-term, simultaneous optical observation of thousands of objects at a
precision far better than is achievable from ground-based telescopes. Ecliptic
fields will be observed for approximately 75-days enabling a unique exoplanet
survey which fills the gaps in duration and sensitivity between the Kepler and
TESS missions, and offers pre-launch exoplanet target identification for JWST
transit spectroscopy. Astrophysics observations with K2 will include studies of
young open clusters, bright stars, galaxies, supernovae, and asteroseismology.Comment: 25 pages, 11 figures, Accepted to PAS
Measuring The Evolutionary Rate Of Cooling Of ZZ Ceti
We have finally measured the evolutionary rate of cooling of the pulsating hydrogen atmosphere (DA) white dwarf ZZ Ceti (Ross 548), as reflected by the drift rate of the 213.13260694 s period. Using 41 yr of time-series photometry from 1970 November to 2012 January, we determine the rate of change of this period with time to be dP/dt = (5.2 +/- 1.4) x 10(-15) s s(-1) employing the O - C method and (5.45 +/- 0.79) x 10(-15) s s(-1) using a direct nonlinear least squares fit to the entire lightcurve. We adopt the dP/dt obtained from the nonlinear least squares program as our final determination, but augment the corresponding uncertainty to a more realistic value, ultimately arriving at the measurement of dP/dt = (5.5 +/- 1.0) x 10(-15) s s(-1). After correcting for proper motion, the evolutionary rate of cooling of ZZ Ceti is computed to be (3.3 +/- 1.1) x 10(-15) s s(-1). This value is consistent within uncertainties with the measurement of (4.19 +/- 0.73) x 10(-15) s s(-1) for another similar pulsating DA white dwarf, G 117-B15A. Measuring the cooling rate of ZZ Ceti helps us refine our stellar structure and evolutionary models, as cooling depends mainly on the core composition and stellar mass. Calibrating white dwarf cooling curves with this measurement will reduce the theoretical uncertainties involved in white dwarf cosmochronometry. Should the 213.13 s period be trapped in the hydrogen envelope, then our determination of its drift rate compared to the expected evolutionary rate suggests an additional source of stellar cooling. Attributing the excess cooling to the emission of axions imposes a constraint on the mass of the hypothetical axion particle.NSF AST-1008734, AST-0909107Norman Hackerman Advanced Research Program 003658-0252-2009Astronom
- …