Authors. Zhao Guo, Douglas R. Gies, Rachel A. Matson, Antonio García Hernández

Journal. The Astrophysical Journal

Abstract. KIC 9851944 is a short period (P=2.16 days) eclipsing binary in the {\it Kepler} field of view. By combining the analysis of {\it Kepler} photometry and phase resolved spectra from Kitt Peak National Observatory and Lowell Observatory, we determine the atmospheric and physical parameters of both stars. The two components have very different radii (2.27R⊙, 3.19R⊙) but close masses (1.76M⊙, 1.79M⊙) and effective temperatures (7026K, 6902K), indicating different evolutionary stages. The hotter primary is still on the main sequence (MS), while the cooler and larger secondary star has evolved to post-MS, burning hydrogen in a shell. A comparison with coeval evolutionary models shows that it requires solar metallicity and a higher mass ratio to fit the radii and temperatures of both stars simultaneously. Both components show δ Scuti type pulsations which we interpret as p-modes and p and g mixed modes. After a close examination of the evolution of δ Scuti pulsational frequencies, we make a comparison of the observed frequencies with those calculated from MESA/GYRE.

Links. ApJ, NASA ADS, arXiv

Authors. A. Miglio, W. J. Chaplin, K. Brogaard, M. N. Lund, B. Mosser, G. R. Davies, R. Handberg, A. P. Milone, A. F. Marino, D. Bossini, Y. P. Elsworth, F. Grundahl, T. Arentoft, L. R. Bedin, T. L. Campante, J. Jessen-Hansen, C. D. Jones, J. S. Kuszlewicz, L. Malavolta, V. Nascimbeni, E. L. Sandquist

Journal. Monthly Notices of the Royal Astronomical Society

Abstract. Asteroseismic constraints on K giants make it possible to infer radii, masses and ages of tens of thousands of field stars. Tests against independent estimates of these properties are however scarce, especially in the metal-poor regime. Here, we report the detection of solar-like oscillations in 8 stars belonging to the red-giant branch and red-horizontal branch of the globular cluster M4. The detections were made in photometric observations from the K2 Mission during its Campaign 2. Making use of independent constraints on the distance, we estimate masses of the 8 stars by utilising different combinations of seismic and non-seismic inputs. When introducing a correction to the Delta nu scaling relation as suggested by stellar models, for RGB stars we find excellent agreement with the expected masses from isochrone fitting, and with a distance modulus derived using independent methods. The offset with respect to independent masses is lower, or comparable with, the uncertainties on the average RGB mass (4-10%, depending on the combination of constraints used). Our results lend confidence to asteroseismic masses in the metal poor regime. We note that a larger sample will be needed to allow more stringent tests to be made of systematic uncertainties in all the observables (both seismic and non-seismic), and to explore the properties of RHB stars, and of different populations in the cluster.

Links. MNRAS, NASA ADS, arXiv

Authors. Kaori Nagashima, Takashi Sekii, Laurent Gizon, Aaron C. Birch

Journal. Astronomy & Astrophysics

Abstract. Context: The cross-covariance of solar oscillations observed at pairs of points on the solar surface is a fundamental ingredient in time-distance helioseismology. Wave travel times are extracted from the cross-covariance function and are used to infer the physical conditions in the solar interior. Aims: Understanding the statistics of the two-point cross-covariance function is a necessary step towards optimizing the measurement of travel times. Methods: By modeling stochastic solar oscillations, we evaluate the variance of the cross-covariance function as function of time-lag and distance between the two points. Results: We show that the variance of the cross-covariance is independent of both time-lag and distance in the far field, i.e., when they are large compared to the coherence scales of the solar oscillations. Conclusions: The constant noise level for the cross-covariance means that the signal-to-noise ratio for the cross-covariance is proportional to the amplitude of the expectation value of the cross-covariance. This observation is important for planning data analysis efforts.

Links. A&A, NASA ADS, arXiv

Authors. A. R. G. Santos, M. S. Cunha, P. P. Avelino, W. J. Chaplin, T. L. Campante

Journal. Monthly Notices of the Royal Astronomical Society

Abstract. Activity-related variations in the solar oscillation properties have been known for 30 years. However, the relative importance of the different contributions to the observed variations is not yet fully understood. Our goal is to estimate the relative contribution from sunspots to the observed activity-related variations in the frequencies of the acoustic modes. We use a variational principle to relate the phase differences induced by sunspots on the acoustic waves to the corresponding changes in the frequencies of the global acoustic oscillations. From the sunspot properties (area and latitude as a function of time), we are able to estimate the spot-induced frequency shifts. These are then combined with a smooth frequency shift component, associated with long-term solar-cycle variations, and the results compared with the frequency shifts derived from the Global Oscillation Network Group (GONG) data. The result of this comparison is consistent with a sunspot contribution to the observed frequency shifts of roughly 30 per cent, with the remaining 70 per cent resulting mostly from a global, non-stochastic variation, possibly related to the changes in the overall magnetic field. Moreover, analysis of the residuals obtained after the subtraction of the model frequency shifts from the observations indicates the presence of a 1.5-yr periodicity in the data in phase with the quasi-biennial variations reported in the literature.

Links. MNRAS, NASA ADS, arXiv

Authors. Brian Kirk, Kyle Conroy, Andrej Prša, Michael Abdul-Masih, Angela Kochoska, Gal Matijevič, Kelly Hambleton, Thomas Barclay, Steven Bloemen, Tabetha Boyajian, Laurance R. Doyle, B.J. Fulton, Abe Johannes Hoekstra, Kian Jek, Stephen R. Kane, Veselin Kostov, David Latham, Tsevi Mazeh, Jerome A. Orosz, Joshua Pepper, Billy Quarles, Darin Ragozzine, Avi Shporer, John Southworth, Keivan Stassun, Susan E. Thompson, William F. Welsh, Eric Agol, Aliz Derekas, Jonathan Devor, Debra Fischer, Gregory Green, Jeff Gropp, Tom Jacobs, Cole Johnston, Daryll Matthew LaCourse, Kristian Saetre, Hans Schwengeler, Jacek Toczyski, Griffin Werner, Matthew Garrett, Joanna Gore, Arturo O. Martinez, Isaac Spitzer, Justin Stevick, Pantelis C. Thomadis, Eliot Halley Vrijmoet, Mitchell Yenawine, Natalie Batalha, William Borucki

Journal. The Astrophysical Journal

Abstract. The primary Kepler Mission provided nearly continuous monitoring of ∼200,000 objects with unprecedented photometric precision. We present the final catalog of eclipsing binary systems within the 105 deg² Kepler field of view. This release incorporates the full extent of the data from the primary mission (Q0-Q17 Data Release). As a result, new systems have been added, additional false positives have been removed, ephemerides and principal parameters have been recomputed, classifications have been revised to rely on analytical models, and eclipse timing variations have been computed for each system. We identify several classes of systems including those that exhibit tertiary eclipse events, systems that show clear evidence of additional bodies, heartbeat systems, systems with changing eclipse depths, and systems exhibiting only one eclipse event over the duration of the mission. We have updated the period and galactic latitude distribution diagrams and included a catalog completeness evaluation. The total number of identified eclipsing and ellipsoidal binary systems in the Kepler field of view has increased to 2878, 1.3% of all observed Kepler targets. An online version of this catalog with downloadable content and visualization tools is maintained at http://keplerEBs.villanova.edu.

Links. ApJ, NASA ADS, arXiv

Authors. S. Deheuvels, I. Brandão, V. Silva Aguirre, J. Ballot, E. Michel, M. S. Cunha, Y. Lebreton, T. Appourchaux

Journal. Astronomy & Astrophysics

Abstract. Our poor understanding of the boundaries of convective cores generates large uncertainties on the extent of these cores and thus on stellar ages. Our aim is to use asteroseismology to consistently measure the extent of convective cores in a sample of main-sequence stars whose masses lie around the mass-limit for having a convective core. We first test and validate a seismic diagnostic that was proposed to probe in a model-dependent way the extent of convective cores using the so-called r010ratios, which are built with l=0 and l=1 modes. We apply this procedure to 24 low-mass stars chosen among Kepler targets to optimize the efficiency of this diagnostic. For this purpose, we compute grids of stellar models with both the CESAM2k and MESA evolution codes, where the extensions of convective cores are modeled either by an instantaneous mixing or as a diffusion process. Among the selected targets, we are able to unambiguously detect convective cores in eight stars and we obtain seismic measurements of the extent of the mixed core in these targets with a good agreement between the CESAM2k and MESA codes. By performing optimizations using the Levenberg-Marquardt algorithm, we then obtain estimates of the amount of extra-mixing beyond the core that is required in CESAM2k to reproduce seismic observations for these eight stars and we show that this can be used to propose a calibration of this quantity. This calibration depends on the prescription chosen for the extra-mixing, but we find that it should be valid also for the code MESA, provided the same prescription is used. This study constitutes a first step towards the calibration of the extension of convective cores in low-mass stars, which will help reduce the uncertainties on the ages of these stars.

Links. A&A, NASA ADS, arXiv

Authors. Zoltán Kolláth

Journal. Communications from the Konkoly Observatory, Vol. 105

Abstract. Space-based photometric telescopes stirred up stellar astrophysics in the last decade, and RR Lyrae stars have not been an exception from that either. The long, quasi-continuous, high-precision data from MOST, CoRoT and Kepler revealed a wealth of new insights about this well-known variable class. One of the most surprising mysteries turned out to be the apparent omnipresence of a common additional mode in all RRd and RRc stars. Moreover, fundamental-mode stars seem to populate two distinct classes, one of which is characterized by the presence of additional modes and/or modulation, and another limited to strict single-mode pulsation. The presence of additional modes and multiple modulations in RRab stars allowed us to construct Petersen diagrams for these parameters: while the pulsation modes show clear structures according to period ratios, there seems to be no relation between the modulation periods themselves.

Links. NASA ADS, arXiv

Authors. Zoltán Kolláth

Journal. Communications from the Konkoly Observatory, Vol. 105

Abstract. Hydrodynamic models of RR Lyrae pulsation display a very rich behaviour. Contrary to earlier expectations, high order resonances play a crucial role in the nonlinear dynamics representing the interacting modes. Chaotic attractors can be found at different time scales: both in the pulsation itself and in the amplitude equations shaping the possible modulation of the oscillations. Although there is no one-to-one connection between the nonlinear features found in the numerical models and the observed behaviour, the richness of the found phenomena suggests that the interaction of modes should be taken seriously in the study of the still unsolved puzzle of Blazhko effect. One of the main lessons of this complex system is that we should rethink the simple interpretation of the observed effect of resonances.

Links. NASA ADS, arXiv

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