Authors. E. Plachy, L. Molnár, R. Szabó, K. Kolenberg, E. Bányai

Journal. Communications from the Konkoly Observatory, Vol. 105

Abstract. In a few years the Kepler and TESS missions will provide ultra- precise photometry for thousands of RR Lyrae and hundreds of Cepheid stars. In the extended Kepler mission all targets are proposed in the Guest Observer (GO) Program, while the TESS space telescope will work with full frame images and a ~15-16th mag brightness limit with the possibility of short cadence measurements for a limited number of pre-selected objects. This paper highlights some details of the enormous and important work of the target selection process made by the members of Working Group 7 (WG#7) of the Kepler and TESS Asteroseismic Science Consortium.

Links. NASA ADS, arXiv

Authors. Ottó Hanyecz, Róbert Szabó

Journal. Communications from the Konkoly Observatory, Vol. 105

Abstract. It is interesting to ask what fraction of the total available RR Lyrae (RRL) sample that falls in the Kepler and K2 Fields of View (FoV) is known or discovered. In order to answer this question we compared the known RRL sam- ple in the Kepler and K2 fields with synthetic Galactic models. The Catalina Sky Survey RRL sample was used to calibrate our method. We found that a large number of faint RRL stars is missing from Kepler and K2 fields.

Links. NASA ADS, arXiv

Authors. F. Perez Hernandez, R. A. Garcia, E. Corsaro, S. A. Triana, J. De Ridder

Journal. Astronomy & Astrophysics

Abstract. Frequencies of acoustic and mixed modes in red giant stars are now determined with high precision thanks to the long continuous observations provided by the NASA Kepler mission. Here we consider the eigenfrequencies of nineteen low-luminosity red giant stars selected by Corsaro et al. (2015) for a detailed peak-bagging analysis. Our objective is to obtain stellar parameters by using individual mode frequencies and spectroscopic information. We use a forward modelling technique based on a minimization procedure combining the frequencies of the p modes, the period spacing of the dipolar modes, and the spectroscopic data. Consistent results between the forward modelling technique and values derived from the seismic scaling relations are found but the errors derived using the former technique are lower. The average error for log g is 0.002 dex, compared to 0.011 dex from the frequency of maximum power and 0.10 dex from the spectroscopic analysis. Relative errors in the masses and radii are on average 2 and 0.5 per cent respectively, compared to 3 and 2 per cent derived from the scaling relations. No reliable determination of the initial helium abundances and the mixing length parameters could be made. Finally, for our grid of models with a given input physics, we found that low-mass stars require higher values of the overshooting parameter.

Links. A&A, NASA ADS, arXiv

Authors. B. Löptien, A. C. Birch, T. L. Duvall Jr., L. Gizon, J. Schou

Journal. Astronomy & Astrophysics

Abstract. Context. Local correlation tracking of granulation (LCT) is an important method for measuring horizontal flows in the photosphere. This method exhibits a systematic error that looks like a flow converging towards disk center, also known as the shrinking-Sun effect.
Aims. We aim at studying the nature of the shrinking-Sun effect for continuum intensity data and at deriving a simple model that can explain its origin.
Methods. We derived LCT flow maps by running the local correlation tracking code FLCT on tracked and remapped continuum intensity maps provided by the Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory. We also computed flow maps from synthetic continuum images generated from STAGGER code simulations of solar surface convection. We investigated the origin of the shrinking-Sun effect by generating an average granule from synthetic data from the simulations.
Results. The LCT flow maps derived from HMI and from the simulations exhibit a shrinking-Sun effect of comparable magnitude. The origin of this effect is related to the apparent asymmetry of granulation originating from radiative transfer effects when observing with a viewing angle inclined from vertical. This causes, in combination with the expansion of the granules, an apparent motion towards disk center.

Links. A&A, NASA ADS, arXiv

Authors. Vincent G. A. Böning, Markus Roth, Wolfgang Zima, Aaron C. Birch, Laurent Gizon

Journal. The Astrophysical Journal

Abstract. We extend an existing Born approximation method for calculating the linear sensitivity of helioseismic travel times to flows from Cartesian to spherical geometry. This development is necessary for using the Born approximation for inferring large-scale flows in the deep solar interior. In a first sanity check, we compare two f−mode kernels from our spherical method and from an existing Cartesian method. The horizontal and total integrals agree to within 0.3 %. As a second consistency test, we consider a uniformly rotating Sun and a travel distance of 42 degrees. The analytical travel-time difference agrees with the forward-modelled travel-time difference to within 2 %. In addition, we evaluate the impact of different choices of filter functions on the kernels for a meridional travel distance of 42 degrees. For all filters, the sensitivity is found to be distributed over a large fraction of the convection zone. We show that the kernels depend on the filter function employed in the data analysis process. If modes of higher harmonic degree (90≲l≲170) are permitted, a noisy pattern of a spatial scale corresponding to l≈260 appears near the surface. When mainly low-degree modes are used (l≲70), the sensitivity is concentrated in the deepest regions and it visually resembles a ray-path-like structure. Among the different low-degree filters used, we find the kernel for phase-speed filtered measurements to be best localized in depth.

Links. ApJ, NASA ADS, arXiv

Authors. Ehsan Moravveji, Richard H. D. Townsend, Conny Aerts, Stephane Mathis

Journal. The Astrophysical Journal

Abstract. KIC 7760680 is so far the richest slowly pulsating B star, by exhibiting 36 consecutive dipole (ℓ=1) gravity (g-) modes. The monotonically decreasing period spacing of the series, in addition to the local dips in the pattern confirm that KIC 7760680 is a moderate rotator, with clear mode trapping in chemically inhomogeneous layers. We employ the traditional approximation of rotation to incorporate rotational effects on g-mode frequencies. Our detailed forward asteroseismic modelling of this g-mode series reveals that KIC 7760680 is a moderately rotating B star with mass ∼3.25 M⊙. By simultaneously matching the slope of the period spacing, and the number of modes in the observed frequency range, we deduce that the equatorial rotation frequency of KIC 7760680 is 0.4805 day−1, which is 26\% of its Roche break up frequency. The relative deviation of the model frequencies and those observed is less than one percent. We succeed to tightly constrain the exponentially-decaying convective core overshooting parameter to fov≈0.024±0.001. This means that convective core overshooting can coexist with moderate rotation. Moreover, models with exponentially-decaying overshoot from the core outperform those with the classical step-function overshoot. The best value for extra diffusive mixing in the radiatively stable envelope is confined to logDext≈0.75±0.25 (with Dext in cm2 sec−1), which is notably smaller than theoretical predictions.

Links. ApJ, NASA ADS, arXiv

Authors. D. R. Reese, W. J. Chaplin, G. R. Davies, A. Miglio, H. M. Antia, W. H. Ball, S. Basu, G. Buldgen, J. Christensen-Dalsgaard, H. R. Coelho, S. Hekker, G. Houdek, Y. Lebreton, A. Mazumdar, T. S. Metcalfe, V. Silva Aguirre, D. Stello, K. Verma

Journal. Astronomy & Astrophysics

Abstract. Context: Detailed oscillation spectra comprising individual frequencies for numerous solar-type stars and red giants are or will become available. These data can lead to a precise characterisation of stars.
Aims: Our goal is to test and compare different methods for obtaining stellar properties from oscillation frequencies and spectroscopic constraints, in order to evaluate their accuracy and the reliability of the error bars.
Methods: In the context of the SpaceInn network, we carried out a hare-and-hounds exercise in which one group produced “observed” oscillation spectra for 10 artificial solar-type stars, and various groups characterised these stars using either forward modelling or acoustic glitch signatures.
Results: Results based on the forward modelling approach were accurate to 1.5 % (radius), 3.9 % (mass), 23 % (age), 1.5 % (surface gravity), and 1.8 % (mean density). For the two 1 Msun stellar targets, the accuracy on the age is better than 10 % thereby satisfying PLATO 2.0 requirements. The average accuracies for the acoustic radii of the base of the convection zone, the He II ionisation, and the Gamma_1 peak were 17 %, 2.4 %, and 1.9 %, respectively. Glitch fitting analysis seemed to be affected by aliasing problems for some of the targets.
Conclusions: Forward modelling is the most accurate approach, but needs to be complemented by model-independent results from, e.g., glitch analysis. Furthermore, global optimisation algorithms provide more robust error bars.

Links. A&A, NASA ADS, arXiv

Authors. Francesco Borsa, Monica Rainer, Ennio Poretti

Journal. Astronomy & Astrophysics

Abstract. Transmission spectroscopy can be used to constrain the properties of exoplanetary atmospheres. During a transit, the light blocked from the atmosphere of the planet leaves an imprint in the light coming from the star. This has been shown for many exoplanets with different techniques, with both photometry and spectroscopy. We aim at testing chromatic line-profile tomography as a new tool to investigate exoplanetary atmospheres. The signal imprinted on the cross-correlation function (CCF) by a planet transiting its star is dependent on the planet-to-star radius ratio. We want to verify if the precision reachable on the CCF obtained from a subset of the spectral orders of the HARPS spectrograph is enough to discriminate the radius of a planet at different wavelengths. Methods. We analyze HARPS archival data of three transits of HD 189733b. We divide the HARPS spectral range in 7 broadbands, calculating for each band the ratio between the area of the out-of-transit CCF and the area of the signal imprinted by the planet on it during the full part of the transit. We take into account the effect of the limb darkening using the theoretical coefficients of a linear law. Averaging the results of three different transits allows us to obtain a good quality broadband transmission spectrum of HD 189733b, with a precision greater than that of the chromatic RM effect. Results. We proved chromatic line-profile tomography to be an interesting way to reveal broadband transmission spectra of exoplanets: our analysis of the atmosphere of HD 189733b is in agreement with other ground- and space-based observations. The independent analysis of different transits puts in evidence the probability that stellar activity plays a role in the extracted transmission spectrum. Care has thus to be taken when claiming for Rayleigh scattering in the atmosphere of exoplanets orbiting active stars using only one transit.

Links. A&A, NASA ADS, arXiv