Engineering topics

Bedding, T R

Publications and source records attributed to Bedding, T R.

8 records

DOE OSTI2012

FUNDAMENTAL PROPERTIES OF STARS USING ASTEROSEISMOLOGY FROM KEPLER AND CoRoT AND INTERFEROMETRY FROM THE CHARA ARRAY

We present results of a long-baseline interferometry campaign using the PAVO beam combiner at the CHARA Array to measure the angular sizes of five main-sequence stars, one subgiant and four red giant stars for which solar-like oscillations have been detected by either Kepler or CoRoT. By combining interferometric angular diameters, Hipparcos parallaxes, asteroseismic densities, bolometric fluxes, and high-resolution spectroscopy, we derive a full set of near-model-independent fundamental properties for the sample. We first use these properties to test asteroseismic scaling relations for the frequency of maximum power ({nu}{sub max}) and the large frequency separation ({Delta}{nu}). We find excellent agreement within the observational uncertainties, and empirically show that simple estimates of asteroseismic radii for main-sequence stars are accurate to {approx}< 4%. We furthermore find good agreement of our measured effective temperatures with spectroscopic and photometric estimates with mean deviations for stars between T {sub eff} = 4600-6200 K of -22 {+-} 32 K (with a scatter of 97 K) and -58 {+-} 31 K (with a scatter of 93 K), respectively. Finally, we present a first comparison with evolutionary models, and find differences between observed and theoretical properties for the metal-rich main-sequence star HD 173701. We conclude that the constraints presented in this study will have strong potential for testing stellar model physics, in particular when combined with detailed modeling of individual oscillation frequencies.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2012

A UNIFORM ASTEROSEISMIC ANALYSIS OF 22 SOLAR-TYPE STARS OBSERVED BY KEPLER

Asteroseismology with the Kepler space telescope is providing not only an improved characterization of exoplanets and their host stars, but also a new window on stellar structure and evolution for the large sample of solar-type stars in the field. We perform a uniform analysis of 22 of the brightest asteroseismic targets with the highest signal-to-noise ratio observed for 1 month each during the first year of the mission, and we quantify the precision and relative accuracy of asteroseismic determinations of the stellar radius, mass, and age that are possible using various methods. We present the properties of each star in the sample derived from an automated analysis of the individual oscillation frequencies and other observational constraints using the Asteroseismic Modeling Portal (AMP), and we compare them to the results of model-grid-based methods that fit the global oscillation properties. We find that fitting the individual frequencies typically yields asteroseismic radii and masses to {approx}1% precision, and ages to {approx}2.5% precision (respectively, 2, 5, and 8 times better than fitting the global oscillation properties). The absolute level of agreement between the results from different approaches is also encouraging, with model-grid-based methods yielding slightly smaller estimates of the radius and mass and slightly older values for the stellar age relative to AMP, which computes a large number of dedicated models for each star. The sample of targets for which this type of analysis is possible will grow as longer data sets are obtained during the remainder of the mission.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2012

MASSES OF SUBGIANT STARS FROM ASTEROSEISMOLOGY USING THE COUPLING STRENGTHS OF MIXED MODES

For a few decades now, asteroseismology, the study of stellar oscillations, has enabled us to probe the interiors of stars with great precision. It allows stringent tests of stellar models and can provide accurate radii, masses, and ages for individual stars. Of particular interest are the mixed modes that occur in subgiant solar-like stars since they can place very strong constraints on stellar ages. Here, we measure the characteristics of the mixed modes, particularly the coupling strength, using a grid of stellar models for stars with masses between 0.9 and 1.5 M{sub Sun }. We show that the coupling strength of the l = 1 mixed modes is predominantly a function of stellar mass and appears to be independent of metallicity. This should allow an accurate mass evaluation, further increasing the usefulness of mixed modes in subgiants as asteroseismic tools.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2011

TESTING SCALING RELATIONS FOR SOLAR-LIKE OSCILLATIONS FROM THE MAIN SEQUENCE TO RED GIANTS USING KEPLER DATA

We have analyzed solar-like oscillations in {approx}1700 stars observed by the Kepler Mission, spanning from the main sequence to the red clump. Using evolutionary models, we test asteroseismic scaling relations for the frequency of maximum power ({nu}{sub max}), the large frequency separation ({Delta}{nu}), and oscillation amplitudes. We show that the difference of the {Delta}{nu}-{nu}{sub max} relation for unevolved and evolved stars can be explained by different distributions in effective temperature and stellar mass, in agreement with what is expected from scaling relations. For oscillation amplitudes, we show that neither (L/M){sup s} scaling nor the revised scaling relation by Kjeldsen and Bedding is accurate for red-giant stars, and demonstrate that a revised scaling relation with a separate luminosity-mass dependence can be used to calculate amplitudes from the main sequence to red giants to a precision of {approx}25%. The residuals show an offset particularly for unevolved stars, suggesting that an additional physical dependency is necessary to fully reproduce the observed amplitudes. We investigate correlations between amplitudes and stellar activity, and find evidence that the effect of amplitude suppression is most pronounced for subgiant stars. Finally, we test the location of the cool edge of the instability strip in the Hertzsprung-Russell diagram using solar-like oscillations and find the detections in the hottest stars compatible with a domain of hybrid stochastically excited and opacity driven pulsation.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2011

MODELING KEPLER OBSERVATIONS OF SOLAR-LIKE OSCILLATIONS IN THE RED GIANT STAR HD 186355

We have analyzed oscillations of the red giant star HD 186355 observed by the NASA Kepler satellite. The data consist of the first five quarters of science operations of Kepler, which cover about 13 months. The high-precision time-series data allow us to accurately extract the oscillation frequencies from the power spectrum. We find that the frequency of the maximum oscillation power, {nu}{sub max}, and the mean large frequency separation, {Delta}{nu}, are around 106 and 9.4 {mu}Hz, respectively. A regular pattern of radial and non-radial oscillation modes is identified by stacking the power spectra in an echelle diagram. We use the scaling relations of {Delta}{nu} and {nu}{sub max} to estimate the preliminary asteroseismic mass, which is confirmed with the modeling result (M = 1.45 {+-} 0.05 M{sub Sun }) using the Yale Rotating stellar Evolution Code (YREC7). In addition, we constrain the effective temperature, luminosity, and radius from comparisons between observational constraints and models. A number of mixed l = 1 modes are also detected and taken into account in our model comparisons. We find a mean observational period spacing for these mixed modes of about 58 s, suggesting that this red giant branch star is in the shell hydrogen-burning phase.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2011

GRANULATION IN RED GIANTS: OBSERVATIONS BY THE KEPLER MISSION AND THREE-DIMENSIONAL CONVECTION SIMULATIONS

The granulation pattern that we observe on the surface of the Sun is due to hot plasma rising to the photosphere where it cools down and descends back into the interior at the edges of granules. This is the visible manifestation of convection taking place in the outer part of the solar convection zone. Because red giants have deeper convection zones than the Sun, we cannot a priori assume that their granulation is a scaled version of solar granulation. Until now, neither observations nor one-dimensional analytical convection models could put constraints on granulation in red giants. With asteroseismology, this study can now be performed. We analyze {approx}1000 red giants that have been observed by Kepler during 13 months. We fit the power spectra with Harvey-like profiles to retrieve the characteristics of the granulation (timescale {tau}{sub gran} and power P{sub gran}). We search for a correlation between these parameters and the global acoustic-mode parameter (the position of maximum power, {nu}{sub max}) as well as with stellar parameters (mass, radius, surface gravity (log g), and effective temperature (T{sub eff})). We show that {tau}{sub eff}{proportional_to}{nu}{sup -0.89}{sub max} and P{sub gran}{proportional_to}{nu}{sup -1.90}{sub max}, which is consistent with the theoretical predictions. We find that the granulation timescales of stars that belong to the red clump have similar values while the timescales of stars in the red giant branch are spread in a wider range. Finally, we show that realistic three-dimensional simulations of the surface convection in stars, spanning the (T{sub eff}, log g) range of our sample of red giants, match the Kepler observations well in terms of trends.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2011

CONSTRUCTING A ONE-SOLAR-MASS EVOLUTIONARY SEQUENCE USING ASTEROSEISMIC DATA FROM KEPLER

Asteroseismology of solar-type stars has entered a new era of large surveys with the success of the NASA Kepler mission, which is providing exquisite data on oscillations of stars across the Hertzsprung-Russell diagram. From the time-series photometry, the two seismic parameters that can be most readily extracted are the large frequency separation ({Delta}{nu}) and the frequency of maximum oscillation power ({nu}{sub max}). After the survey phase, these quantities are available for hundreds of solar-type stars. By scaling from solar values, we use these two asteroseismic observables to identify for the first time an evolutionary sequence of 1 M{sub sun} field stars, without the need for further information from stellar models. Comparison of our determinations with the few available spectroscopic results shows an excellent level of agreement. We discuss the potential of the method for differential analysis throughout the main-sequence evolution and the possibility of detecting twins of very well-known stars.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2011

VERIFICATION OF THE KEPLER INPUT CATALOG FROM ASTEROSEISMOLOGY OF SOLAR-TYPE STARS

We calculate precise stellar radii and surface gravities from the asteroseismic analysis of over 500 solar-type pulsating stars observed by the Kepler space telescope. These physical stellar properties are compared with those given in the Kepler Input Catalog (KIC), determined from ground-based multi-color photometry. For the stars in our sample, we find general agreement but we detect an average overestimation bias of 0.23 dex in the KIC determination of log (g) for stars with log (g){sub KIC} > 4.0 dex, and a resultant underestimation bias of up to 50% in the KIC radii estimates for stars with R{sub KIC} < 2 R{sub sun}. Part of the difference may arise from selection bias in the asteroseismic sample; nevertheless, this result implies there may be fewer stars characterized in the KIC with R {approx} 1 R{sub sun} than is suggested by the physical properties in the KIC. Furthermore, if the radius estimates are taken from the KIC for these affected stars and then used to calculate the size of transiting planets, a similar underestimation bias may be applied to the planetary radii.

79 ASTRONOMY AND ASTROPHYSICS↗