It has been observed that hot Jupiters located within 0.08 AU of their host stars commonly display radii in excess of those expected based on models. A number of theoretical explanations for this ...phenomenon have been suggested, but the ability of any one mechanism to account for the full range of observations remains to be rigorously proven. I identify an additional heating mechanism, arising from the interaction of the interplanetary magnetic field and the planetary magnetosphere, and show that this is capable of providing enough energy to explain the observed planetary radii. Such a model predicts that the degree of heating should be dependent on the stellar magnetic field, for which stellar activity serves as a proxy. Accordingly, I examine populations of hot Jupiters from the Kepler database and confirm that stellar activity (determined using Kepler CDPP levels) is correlated with the presence of planetary radii inflated beyond the basal level of R = 0.87 R sub(J) identified by previous researchers. I propose that the primary mechanism for transferring energy from the magnetosphere to the planetary interior is Joule heating arising from global electric circuits analogous to those seen in solar system objects.
Aims. We present the most precise light curve ever obtained of a detached eclipsing binary star and use it investigate the inclusion of non- linear limb darkening laws in light curve models of ...eclipsing binaries. This light curve, of the bright eclipsing system \beta Aurigae, was obtained using the star tracker aboard the WIRE satellite and contains 30 000 datapoints with a point-to-point scatter of 0.3 mmag. Methods. We analyse the WIRE light curve using a version of the EBOP code modified to include non-linear limb darkening laws and to directly incorporate observed times of minimum light and spectroscopic light ratios into the photometric solution as individual observations. We also analyse the dataset with the Wilson-Devinney code to ensure that the two models give consistent results. Results. EBOP is able to provide an excellent fit to the high-precision WIRE data. Whilst the fractional radii of the stars are only defined to a precision of 5% by this light curve, including an accurate published spectroscopic light ratio improves this dramatically to 0.5%. Using non-linear limb darkening improves the quality of the fit significantly compared to the linear law and causes the measured radii to increase by 0.4%. It is possible to derive all of the limb darkening coefficients from the light curve, although they are strongly correlated with each other. The fitted coefficients agree with theoretical predictions to within their fairly large error estimates. We were able to obtain a reasonably good fit to the data using the Wilson-Devinney code, but only using the highest available integration accuracy and by iterating for a long time. Bolometric albedos of 0.6 were found, which are appropriate to convective rather than radiative envelopes. Conclusions. The radii and masses of the components of \beta are R_{\rm A} = 2.762\pm0.017{R}_{\odot}, R_{\rm B} = 2.568\pm0.017{R}_{\odot}, M_{\rm A} = 2.376\pm0.027{M}_{\odot} and M_{\rm B} = 2.291\pm0.027{M}_{\odot}, where A and B denote the primary and secondary star, respectively. Theoretical stellar evolutionary models can match these parameters for a solar metal abundance and an age of 450-500 Myr. The Hipparcos trigonometric parallax and an interferometrically-derived orbital parallax give distances to \beta which are in excellent agreement with each other and with distances derived using surface brightness relations and several sets of empirical and theoretical bolometric corrections.
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 ~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 Delta *tgran and power P gran). We search for a correlation between these parameters and the global acoustic-mode parameter (the position of maximum power, Delta *nmax) as well as with stellar parameters (mass, radius, surface gravity (log g), and effective temperature (T eff)). We show that Delta *teff Delta *n--0.89 max and P gran Delta *n--1.90 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 eff, log g) range of our sample of red giants, match the Kepler observations well in terms of trends.
We have analyzed solar-like oscillations in ~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 ( Delta *nmax), the large frequency separation ( Delta *D Delta *n), and oscillation amplitudes. We show that the difference of the Delta *D Delta *n- Delta *nmax 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) s scaling nor the revised scaling relation by Kjeldsen & 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 ~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.
Abstract
We present the first results from the Transiting Exoplanet Survey Satellite (TESS) on the rotational and pulsational variability of magnetic chemically peculiar A-type stars. We analyse TESS ...2-min cadence data from sectors 1 and 2 on a sample of 83 stars. Five new rapidly oscillating Ap (roAp) stars are announced. One of these pulsates with periods around 4.7 min, making it the shortest period roAp star known to date. Four out of the five new roAp stars are multiperiodic. Three of these and the singly periodic one show the presence of rotational mode splitting. Individual frequencies are provided in all cases. In addition, seven previously known roAp stars are analysed. Additional modes of oscillation are found in some stars, while in others we are able to distinguish the true pulsations from possible aliases present in the ground-based data. We find that the pulsation amplitude in the TESS filter is typically a factor of 6 smaller than that in the B filter, which is usually used for ground-based observations. For four roAp stars we set constraints on the inclination angle and magnetic obliquity, through the application of the oblique pulsator model. We also confirm the absence of roAp-type pulsations down to amplitude limits of 6 and 13 $\mu$mag, respectively, in two of the best characterized non-oscillating Ap (noAp) stars. We announce 27 new rotational variables along with their rotation periods, and provide different rotation periods for seven other stars. Finally, we discuss how these results challenge state-of-the-art pulsation models for roAp stars.
Mass estimates of K giants are generally very uncertain. Traditionally, stellar masses of single field stars are determined by comparing their location in the Hertzsprung-Russell diagram with stellar ...evolutionary models. Applying an additional method to determine the mass is therefore of significant interest for understanding stellar evolution. We present the time series analysis of 11 K giants recently observed with the WIRE satellite. With this comprehensive sample, we report the first confirmation that the characteristic acoustic frequency, upsilon sub(max), can be predicted for K giants by scaling from the solar acoustic cutoff frequency. We are further able to utilize our measurements of upsilon sub(max) to determine an asteroseismic mass for each star with a lower uncertainty compared to the traditional method, for most stars in our sample. This indicates good prospects for the application of our method on the vast amounts of data that will soon come from the COROT and Kepler space missions.
We have monitored 20 Sun-like stars in the Kepler field-of-view for excess flux with the Fibre-fed Echelle Spectrograph on the Nordic Optical Telescope since the launch of Kepler spacecraft in 2009. ...These 20 stars were selected based on their asteroseismic properties to sample the parameter space (effective temperature, surface gravity, activity level, etc.) around the Sun. Though the ultimate goal is to improve stellar dynamo models, we focus the present paper on the combination of space-based and ground-based observations that can be used to test the age-rotation-activity relations. In this paper we describe the considerations behind the selection of these 20 Sun-like stars and present an initial asteroseismic analysis, which includes stellar age estimates. We also describe the observations from the Nordic Optical Telescope and present mean values of measured excess fluxes. These measurements are combined with estimates of the rotation periods obtained from a simple analysis of the modulation in photometric observations from Kepler caused by starspots, and asteroseismic determinations of stellar ages, to test relations between age, rotation and activity.
Abstract
The NASA Transiting Exoplanet Survey Satellite (TESS) is observing tens of millions of stars with time spans ranging from ∼27 days to about 1 yr of continuous observations. This vast amount ...of data contains a wealth of information for variability, exoplanet, and stellar astrophysics studies but requires a number of processing steps before it can be fully utilized. In order to efficiently process all the TESS data and make it available to the wider scientific community, the TESS Data for Asteroseismology working group, as part of the TESS Asteroseismic Science Consortium, has created an automated open-source processing pipeline to produce light curves corrected for systematics from the short- and long-cadence raw photometry data and to classify these according to stellar variability type. We will process all stars down to a TESS magnitude of 15. This paper is the next in a series detailing how the pipeline works. Here, we present our methodology for the automatic variability classification of TESS photometry using an ensemble of supervised learners that are combined into a metaclassifier. We successfully validate our method using a carefully constructed labeled sample of Kepler Q9 light curves with a 27.4 days time span mimicking single-sector TESS observations, on which we obtain an overall accuracy of 94.9%. We demonstrate that our methodology can successfully classify stars outside of our labeled sample by applying it to all ∼167,000 stars observed in Q9 of the Kepler space mission.
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 *D Delta *n) and the frequency of maximum power ( Delta *nmax) 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 Delta *nmax and Delta *D Delta *n 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 and Delta *nmax 40-110 Delta *mHz. We measured the small frequency separations Delta *d Delta *n02 and Delta *d Delta *n01 in over 400 stars and Delta *d Delta *n03 in over 40. We present C-D diagrams for l = 1, 2, and 3 and show that the frequency separation ratios Delta *d Delta *n02/ Delta *D Delta *n and Delta *d Delta *n01/ Delta *D Delta *n have opposite trends as a function of Delta *D Delta *n. The data show a narrowing of the l = 1 ridge toward lower Delta *nmax, in agreement with models predicting more efficient mode trapping in stars with higher luminosity. We investigate the offset in the asymptotic relation and find a clear correlation with Delta *D Delta *n, demonstrating that it is related to fundamental stellar parameters. Finally, we present the first amplitude- Delta *nmax relation for Kepler red giants. We observe a lack of low-amplitude stars for Delta *nmax 110 Delta *mHz and find that, for a given Delta *nmax between 40 and 110 Delta *mHz, stars with lower Delta *D Delta *n (and consequently higher mass) tend to show lower amplitudes than stars with higher Delta *D Delta *n.
Context.
Chemically peculiar (CP) stars with a measurable magnetic field comprise the group of mCP stars. The pulsating members define the subgroup of rapidly oscillating Ap (roAp) stars, of which
α
...Cir is the brightest member. Hence,
α
Cir allows the application of challenging techniques, such as interferometry, very high temporal and spectral resolution photometry, and spectroscopy in a wide wavelength range, that have the potential to provide unique information about the structure and evolution of a star.
Aims.
Based on new photometry from BRITE-Constellation, obtained with blue and red filters, and on photometry from
WIRE
,
SMEI
, and
TESS
we attempt to determine the surface spot structure of
α
Cir and investigate pulsation frequencies.
Methods.
We used photometric surface imaging and frequency analyses and Bayesian techniques in order to quantitatively compare the probability of different models.
Results.
BRITE-Constellation photometry obtained from 2014 to 2016 is put in the context of space photometry obtained by
WIRE
,
SMEI
, and
TESS
. This provides improvements in the determination of the rotation period and surface features (three spots detected and a fourth one indicated). The main pulsation frequencies indicate two consecutive radial modes and one intermediate dipolar mode. Advantages and problems of the applied Bayesian technique are discussed.
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