arXiv subjects

W. J. Chaplin

Publications and source records attributed to W. J. Chaplin.

At least 91 records · Page 5Linked to original sources

arXiv2014

Amplitude variability in satellite photometry of the non-radially pulsating O9.5V star zeta Oph

We report a time-series analysis of satellite photometry of the non-radially pulsating Oe star zeta Oph, principally using data from SMEI obtained 2003--2008, but augmented with MOST and WIRE results. Amplitudes of the strongest photometric signals, at 5.18, 2.96, and 2.67/d, each vary independently over the 6-year monitoring period (from ca. 30 to <2 mmag at 5.18/d), on timescales of hundreds of days. Signals at 7.19/d and 5.18/d have persisted (or recurred) for around two decades. Supplementary spectroscopic observations show an H-alpha emission episode in 2006; this coincided with small increases in amplitudes of the three strongest photometric signals.

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arXiv2014

'Rapid fire' spectroscopy of Kepler solar-like oscillators

The NASA Kepler mission has been continuously monitoring the same field of the sky since the successful launch in March 2009, providing high-quality stellar lightcurves that are excellent data for asteroseismology, far superior to any other observations available at the present. In order to make a meaningful analysis and interpretation of the asteroseismic data, accurate fundamental parameters for the observed stars are needed. The currently available parameters are quite uncertain as illustrated by e.g. Thygesen et al. (A&A 543, A160, 2012), who found deviations as extreme as 2.0 dex in [Fe/H] and log g, compared to catalogue values. Thus, additional follow-up observations for these targets are needed in order to put firm limits on the parameter space investigated by the asteroseismic modellers. Here, we propose a metod for deriving accurate metallicities of main sequence and subgiant solar-like oscillators from medium resolution spectra with a moderate S/N. The method takes advantage of the additional constraints on the fundamental parameters, available from asteroseismology and multi-color photometry. The approach enables us to reduce the analysis overhead significantly when doing spectral synthesis, which in turn will increases the efficiency of follow-up observations.

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arXiv2014

Prospects for detecting asteroseismic binaries in Kepler data

Asteroseismology may in principle be used to detect unresolved stellar binary systems comprised of solar-type stars and/or red giants. This novel method relies on the detection of the presence of two solar-like oscillation spectra in the frequency spectrum of a single lightcurve. Here, we make predictions of the numbers of systems that may be detectable in data already collected by the NASA Kepler Mission. Our predictions, which are based upon TRILEGAL and BiSEPS simulations of the Kepler field of view, indicate that as many as 200 or more ``asteroseismic binaries'' may be detectable in this manner. Most of these binaries should be comprised of two He-core-burning red giants. Owing largely to the limited numbers of targets with the requisite short-cadence Kepler data, we expect only a small number of detected binaries containing solar-type stars. The predicted yield of detections is sensitive to the assumed initial mass ratio distribution of the binary components and therefore represents a sensitive calibration of the much debated initial mass ratio distribution near mass ratio unity.

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arXiv2014

Limits on surface gravities of Kepler planet-candidate host stars from non-detection of solar-like oscillations

We present a novel method for estimating lower-limit surface gravities log g of Kepler targets whose data do not allow the detection of solar-like oscillations. The method is tested using an ensemble of solar-type stars observed in the context of the Kepler Asteroseismic Science Consortium. We then proceed to estimate lower-limit log g for a cohort of Kepler solar-type planet-candidate host stars with no detected oscillations. Limits on fundamental stellar properties, as provided by this work, are likely to be useful in the characterization of the corresponding candidate planetary systems. Furthermore, an important byproduct of the current work is the confirmation that amplitudes of solar-like oscillations are suppressed in stars with increased levels of surface magnetic activity.

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arXiv2014

Seismic constraints on the radial dependence of the internal rotation profiles of six Kepler subgiants and young red giants

Context : We still do not know which mechanisms are responsible for the transport of angular momentum inside stars. The recent detection of mixed modes that contain the signature of rotation in the spectra of Kepler subgiants and red giants gives us the opportunity to make progress on this issue. Aims: Our aim is to probe the radial dependance of the rotation profiles for a sample of Kepler targets. For this purpose, subgiants and early red giants are particularly interesting targets because their rotational splittings are more sensitive to the rotation outside the deeper core than is the case for their more evolved counterparts. Methods: We first extract the rotational splittings and frequencies of the modes for six young Kepler red giants. We then perform a seismic modeling of these stars using the evolutionary codes CESAM2k and ASTEC. By using the observed splittings and the rotational kernels of the optimal models, we perform inversions of the internal rotation profiles of the six stars. Results: We obtain estimates of the mean rotation rate in the core and in the convective envelope of these stars. We show that the rotation contrast between the core and the envelope increases during the subgiant branch. Our results also suggest that the core of subgiants spins up with time, contrary to the RGB stars whose core has been shown to spin down. For two of the stars, we show that a discontinuous rotation profile with a deep discontinuity reproduces the observed splittings significantly better than a smooth rotation profile. Interestingly, the depths that are found most probable for the discontinuities roughly coincide with the location of the H-burning shell, which separates the layers that contract from those that expand. These results will bring observational constraints to the scenarios of angular momentum transport in stars.

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arXiv2013

Measurement of acoustic glitches in solar-type stars from oscillation frequencies observed by Kepler

For the very best and brightest asteroseismic solar-type targets observed by Kepler, the frequency precision is sufficient to determine the acoustic depths of the surface convective layer and the helium ionization zone. Such sharp features inside the acoustic cavity of the star, which we call acoustic glitches, create small oscillatory deviations from the uniform spacing of frequencies in a sequence of oscillation modes with the same spherical harmonic degree. We use these oscillatory signals to determine the acoustic locations of such features in 19 solar-type stars observed by the Kepler mission. Four independent groups of researchers utilized the oscillation frequencies themselves, the second differences of the frequencies and the ratio of the small and large separation to locate the base of the convection zone and the second helium ionization zone. Despite the significantly different methods of analysis, good agreement was found between the results of these four groups, barring a few cases. These results also agree reasonably well with the locations of these layers in representative models of the stars. These results firmly establish the presence of the oscillatory signals in the asteroseismic data and the viability of several techniques to determine the location of acoustic glitches inside stars.

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arXiv2013

Study of KIC 8561221 observed by Kepler: an early red giant showing depressed dipolar modes

The continuous high-precision photometric observations provided by the CoRoT and Kepler space missions have allowed us to better understand the structure and dynamics of red giants using asteroseismic techniques. A small fraction of these stars shows dipole modes with unexpectedly low amplitudes. The reduction in amplitude is more pronounced for stars with higher frequency of maximum power. In this work we want to characterize KIC 8561221 in order to confirm that it is currently the least evolved star among this peculiar subset and to discuss several hypotheses that could help explain the reduction of the dipole mode amplitudes. We used Kepler short- and long-cadence data combined with spectroscopic observations to infer the stellar structure and dynamics of KIC 8561221. We then discussed different scenarios that could contribute to the reduction of the dipole amplitudes such as a fast rotating interior or the effect of a magnetic field on the properties of the modes. We also performed a detailed study of the inertia and damping of the modes. We have been able to characterize 37 oscillations modes, in particular, a few dipole modes above nu_max that exhibit nearly normal amplitudes. We have inferred a surface rotation period of around 91 days and uncovered the existence of a variation in the surface magnetic activity during the last 4 years. As expected, the internal regions of the star probed by the l = 2 and 3 modes spin 4 to 8 times faster than the surface. With our grid of standard models we are able to properly fit the observed frequencies. Our model calculation of mode inertia and damping give no explanation for the depressed dipole modes. A fast rotating core is also ruled out as a possible explanation. Finally, we do not have any observational evidence of the presence of a strong deep magnetic field inside the star.

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arXiv2013

Asteroseismic fundamental properties of solar-type stars observed by the NASA Kepler Mission

We use asteroseismic data obtained by the NASA Kepler Mission to estimate the fundamental properties of more than 500 main-sequence and sub-giant stars. Data obtained during the first 10 months of Kepler science operations were used for this work, when these solar-type targets were observed for one month each in a survey mode. Stellar properties have been estimated using two global asteroseismic parameters and complementary photometric and spectroscopic data. Homogeneous sets of effective temperatures were available for the entire ensemble from complementary photometry; spectroscopic estimates of T_eff and [Fe/H] were available from a homogeneous analysis of ground-based data on a subset of 87 stars. [Abbreviated version... see paper for full abstract.]

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arXiv2013

Calibrations of Atmospheric Parameters Obtained from the First Year of SDSS-III APOGEE Observations

The SDSS-III Apache Point Observatory Galactic Evolution Experiment (APOGEE) is a three year survey that is collecting 100,000 high-resolution spectra in the near-IR across multiple Galactic populations. To derive stellar parameters and chemical compositions from this massive data set, the APOGEE Stellar Parameters and Chemical Abundances Pipeline (ASPCAP) has been developed. Here, we describe empirical calibrations of stellar parameters presented in the first SDSS-III APOGEE data release (DR10). These calibrations were enabled by observations of 559 stars in 20 globular and open clusters. The cluster observations were supplemented by observations of stars in NASA's Kepler field that have well determined surface gravities from asteroseismic analysis. We discuss the accuracy and precision of the derived stellar parameters, considering especially effective temperature, surface gravity, and metallicity; we also briefly discuss the derived results for the abundances of the alpha-elements, carbon, and nitrogen. Overall, we find that ASPCAP achieves reasonably accurate results for temperature and metallicity, but suffers from systematic errors in surface gravity. We derive calibration relations that bring the raw ASPCAP results into better agreement with independently determined stellar parameters. The internal scatter of ASPCAP parameters within clusters suggests that, metallicities are measured with a precision better than 0.1 dex, effective temperatures better than 150 K, and surface gravities better than 0.2 dex. The understanding provided by the clusters and Kepler giants on the current accuracy and precision will be invaluable for future improvements of the pipeline.

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arXiv2013

Asteroseismic analysis of the CoRoT target HD 169392

The satellite CoRoT (Convection, Rotation, and planetary Transits) has provided high-quality data for almost six years. We show here the asteroseismic analysis and modeling of HD169392A, which belongs to a binary system weakly gravitationally bound as the distance between the two components is of 4250 AU. The main component, HD169392A, is a G0IV star with a magnitude of 7.50 while the second component is a G0V-G2IV star with a magnitude of 8.98. This analysis focuses on the main component, as the secondary one is too faint to measure any seismic parameters. A complete modeling has been possible thanks to the complementary spectroscopic observations from HARPS, providing Teff=5985+/-60K, log g=3.96+/-0.07, and [Fe/H]=- 0.04+/-0.10.

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arXiv2013

Tests of the asymptotic large frequency separation of acoustic oscillations in solar-type and red giant stars

Asteroseismology, i.e. the study of the internal structures of stars via their global oscillations, is a valuable tool to obtain stellar parameters such as mass, radius, surface gravity and mean density. These parameters can be obtained using certain scaling relations which are based on an asymptotic approximation. Usually the observed oscillation parameters are assumed to follow these scaling relations. Recently, it has been questioned whether this is a valid approach, i.e., whether the order of the observed oscillation modes are high enough to be approximated with an asymptotic theory. In this work we use stellar models to investigate whether the differences between observable oscillation parameters and their asymptotic estimates are indeed significant. We compute the asymptotic values directly from the stellar models and derive the observable values from adiabatic pulsation calculations of the same models. We find that the extent to which the atmosphere is included in the models is a key parameter. Considering a larger extension of the atmosphere beyond the photosphere reduces the difference between the asymptotic and observable values of the large frequency separation. Therefore, we conclude that the currently suggested discrepancies in the scaling relations might have been overestimated. Hence, based on the results presented here we believe that the suggestions of Mosser et al. (2013) should not be followed without careful consideration.

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arXiv2013

Sounding stellar cycles with Kepler - II. Ground-based observations

We have monitored 20 Sun-like stars in the Kepler field-of-view for excess flux with the FIES 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 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 between age, rotation and activity.

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arXiv2013

Asteroseismic surface gravity for evolved stars

Context: Asteroseismic surface gravity values can be of importance in determining spectroscopic stellar parameters. The independent log(g) value from asteroseismology can be used as a fixed value in the spectroscopic analysis to reduce uncertainties due to the fact that log(g) and effective temperature can not be determined independently from spectra. Since 2012, a combined analysis of seismically and spectroscopically derived stellar properties is ongoing for a large survey with SDSS/APOGEE and Kepler. Therefore, knowledge of any potential biases and uncertainties in asteroseismic log(g) values is now becoming important. Aims: The seismic parameter needed to derive log(g) is the frequency of maximum oscillation power (nu_max). Here, we investigate the influence of nu_max derived with different methods on the derived log(g) values. The large frequency separation between modes of the same degree and consecutive radial orders (Dnu) is often used as an additional constraint for the determination of log(g). Additionally, we checked the influence of small corrections applied to Dnu on the derived values of log(g). Methods We use methods extensively described in the literature to determine nu_max and Dnu together with seismic scaling relations and grid-based modeling to derive log(g). Results: We find that different approaches to derive oscillation parameters give results for log(g) with small, but different, biases for red-clump and red-giant-branch stars. These biases are well within the quoted uncertainties of ~0.01 dex (cgs). Corrections suggested in the literature to the Dnu scaling relation have no significant effect on log(g). However somewhat unexpectedly, method specific solar reference values induce biases of the order of the uncertainties, which is not the case when canonical solar reference values are used.

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arXiv2013

A large sample of calibration stars for Gaia: log g from Kepler and CoRoT

Asteroseismic data can be used to determine surface gravities with precisions of < 0.05 dex by using the global seismic quantities Deltanu and nu_max along with Teff and [Fe/H]. Surface gravity is also one of the four stellar properties to be derived by automatic analyses for 1 billion stars from Gaia data (workpackage GSP_Phot). We explore seismic data from MS F, G, K stars (solar-like stars) observed by Kepler as a potential calibration source for methods that Gaia will use for object characterisation (log g). We calculate log g for bright nearby stars for which radii and masses are known, and using their global seismic quantities in a grid-based method, we determine an asteroseismic log g to within 0.01 dex of the direct calculation, thus validating the accuracy of our method. We find that errors in Teff and mainly [Fe/H] can cause systematic errors of 0.02 dex. We then apply our method to a list of 40 stars to deliver precise values of surface gravity, i.e. sigma < 0.02 dex, and we find agreement with recent literature values. Finally, we explore the precision we expect in a sample of 400+ Kepler stars which have their global seismic quantities measured. We find a mean uncertainty (precision) on the order of <0.02 dex in log g over the full explored range 3.8 < log g < 4.6, with the mean value varying only with stellar magnitude (0.01 - 0.02 dex). We study sources of systematic errors in log g and find possible biases on the order of 0.04 dex, independent of log g and magnitude, which accounts for errors in the Teff and [Fe/H] measurements, as well as from using a different grid-based method. We conclude that Kepler stars provide a wealth of reliable information that can help to calibrate methods that Gaia will use, in particular, for source characterisation with GSP_Phot where excellent precision (small uncertainties) and accuracy in log g is obtained from seismic data.

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arXiv2013

Stellar ages and convective cores in field main-sequence stars: first asteroseismic application to two Kepler targets

Using asteroseismic data and stellar evolution models we make the first detection of a convective core in a Kepler field main-sequence star, putting a stringent constraint on the total size of the mixed zone and showing that extra mixing beyond the formal convective boundary exists. In a slightly less massive target the presence of a convective core cannot be conclusively discarded, and thus its remaining main-sequence life time is uncertain. Our results reveal that best-fit models found solely by matching individual frequencies of oscillations corrected for surface effects do not always properly reproduce frequency combinations. Moreover, slightly different criteria to define what the best-fit model is can lead to solutions with similar global properties but very different interior structures. We argue that the use of frequency ratios is a more reliable way to obtain accurate stellar parameters, and show that our analysis in field main-sequence stars can yield an overall precision of 1.5%, 4%, and 10% in radius, mass and age, respectively. We compare our results with those obtained from global oscillation properties, and discuss the possible sources of uncertainties in asteroseismic stellar modeling where further studies are still needed.

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arXiv2013

Asteroseismic determination of obliquities of the exoplanet systems Kepler-50 and Kepler-65

Results on the obliquity of exoplanet host stars -- the angle between the stellar spin axis and the planetary orbital axis -- provide important diagnostic information for theories describing planetary formation. Here we present the first application of asteroseismology to the problem of stellar obliquity determination in systems with transiting planets and Sun-like host stars. We consider two systems observed by the NASA Kepler Mission which have multiple transiting small (super-Earth sized) planets: the previously reported Kepler-50 and a new system, Kepler-65, whose planets we validate in this paper. Both stars show rich spectra of solar-like oscillations. From the asteroseismic analysis we find that each host has its rotation axis nearly perpendicular to the line of sight with the sines of the angles constrained at the 1-sigma level to lie above 0.97 and 0.91, respectively. We use statistical arguments to show that coplanar orbits are favoured in both systems, and that the orientations of the planetary orbits and the stellar rotation axis are correlated.

astro-ph.EP↗

arXiv2013

The Sun-as-a-star observations: GOLF & VIRGO on SoHO, and BiSON network

Sun-as-a-star observations are very important for the study of the conditions within the Sun and in particular for the deep interior where higher degree modes do not penetrate. They are also of significance in this era of dramatic advances in stellar asteroseismology as they are comparable to those measured in other stars by asteroseismic missions such as CoRoT, Kepler, and MOST. More than 17 years of continuous measurements of SoHO and more than 30 years of BiSON observations provide very long data sets of uninterrupted helioseismic observations. In this work, we discuss the present status of all these facilities that continue to provide state- of-the-art measurements and invaluable data to improve our knowledge of the deepest layers of the Sun and its structural changes during the activity cycle.

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arXiv2013

Differential population studies using asteroseismology: solar-like oscillating giants in CoRoT fields LRc01 and LRa01

Solar-like oscillating giants observed by the space-borne satellites CoRoT and Kepler can be used as key tracers of stellar populations in the Milky Way. When combined with additional photometric/spectroscopic constraints, the pulsation spectra of solar-like oscillating giant stars not only reveal their radii, and hence distances, but also provide well-constrained estimates of their masses, which can be used as proxies for the ages of these evolved stars. In this contribution we provide supplementary material to the comparison we presented in Miglio et al. (2013) between populations of giants observed by CoRoT in the fields designated LRc01 and LRa01.

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