Authors.Tkachenko, A. ; Aerts, C. ; Pavlovski, K. ; Degroote, P. ; Pápics, P. I. ; Moravveji, E. ; Lehmann, H. ; Kolbas, V. ; Clémer, K.

Journal.Monthly Notices of the Royal Astronomical Society, Volume 442, Issue 1, p.616-628

Abstract.High-mass binary stars are known to show an unexplained discrepancy between the dynamical masses of the individual components and those predicted by models. In this work, we study Sigma Scorpii, a double-lined spectroscopic binary system consisting of two B-type stars residing in an eccentric orbit. The more massive primary component is a beta Cep-type pulsating variable star. Our analysis is based on a time series of some 1000 high-resolution spectra collected with the CORALIE spectrograph in 2006, 2007, and 2008. We use two different approaches to determine the orbital parameters of the star; the spectral disentangling technique is used to separate the spectral contributions of the individual components in the composite spectra. The non-LTE-based spectrum analysis of the disentangled spectra reveals two stars of similar spectral type and atmospheric chemical composition. Combined with the orbital inclination angle estimate found in the literature, our orbital elements allow a mass estimate of 14.7 ± 4.5 and 9.5 ± 2.9 M&sun; for the primary and secondary component, respectively. The primary component is found to pulsate in three independent modes, of which two are identified as fundamental and second overtone radial modes, while the third is an l = 1 non-radial mode. Seismic modelling of the pulsating component refines stellar parameters to 13.5^{+0.5}_{-1.4} and 8.7^{+0.6}_{-1.2} M&sun;, and delivers radii of 8.95^{+0.43}_{-0.66} and 3.90^{+0.58}_{-0.36} R&sun; for the primary and secondary, respectively. The age of the system is estimated to be ˜12 Myr.

Links. NASA ADS,arXiv

Authors.Schmid, V. S. ; Themeßl, N. ; Breger, M. ; Degroote, P. ; Aerts, C. ; Beck, P. G. ; Tkachenko, A. ; Van Reeth, T. ; Bloemen, S. ; Debosscher, J. ; Castanheira, B. G. ; McArthur, B. E. ; Pápics, P. I. ; Fritz, V. ; Falcon, R. E.

Journal. Astronomy & Astrophysics, Volume 570, October 2014

Abstract.More than 40 years of ground-based photometric observations of the delta Sct star 4CVn revealed 18 independent oscillation frequencies, including radial as well as non-radial p-modes of low spherical degree l<=2. From 2008 to 2011, more than 2000 spectra were obtained at the 2.1-m Otto-Struve telescope at the McDonald Observatory. We present the analysis of the line-profile variations, based on the Fourier-parameter fit method, detected in the absorption lines of 4CVn, which carry clear signatures of the pulsations. From a non-sinusoidal, periodic variation of the radial velocities, we discovered that 4CVn is an eccentric binary system, with an orbital period Porb = 124.44 +/- 0.03 d and an eccentricity e = 0.311 +/- 0.003. We firmly detect 20 oscillation frequencies, 9 of which are previously unseen in photometric data, and attempt mode identification for the two dominant modes, f1 = 7.3764 c/d and f2 = 5.8496 c/d, and determine the prograde or retrograde nature of 7 of the modes. The projected rotational velocity of the star, vsini ~ 106.7 km/s, translates to a rotation rate of veq/vcrit >= 33%. This relatively high rotation rate hampers unique mode identification, since higher-order effects of rotation are not included in the current methodology. We conclude that, in order to achieve unambiguous mode identification for 4CVn, a complete description of rotation and the use of blended lines have to be included in mode-identification techniques.

Links. A&A, NASA ADS,arXiv

Authors.Zwintz, K. ; Ryabchikova, T. ; Lenz, P. ; Pamyatnykh, A. A. ; Fossati, L. ; Sitnova, T. ; Breger, M. ; Poretti, E. ; Rainer, M. ; Hareter, M. ; Mantegazza, L.

Journal.Astronomy & Astrophysics, Volume 567, id.A4, 7 pp.

Abstract.Context. HD 144277 was previously discovered by Microvariability and Oscillations of Stars (MOST) space photometry to be a young and hot δ Scuti star showing regular groups of pulsation frequencies. The first asteroseismic models required lower than solar metallicity to fit the observed frequency range based on a purely photometric analysis.
Aims: The aim of the present paper is to determine, by means of high-resolution spectroscopy, fundamental stellar parameters required for the asteroseismic model of HD 144277, and subsequently, to refine it.
Methods: High-resolution, high signal-to-noise spectroscopic data obtained with the HARPS spectrograph were used to determine the fundamental parameters and chemical abundances of HD 144277. These values were put into context alongside the results from asteroseismic models.
Results: The effective temperature, Teff, of HD 144277 was determined as 8640 +300-100 K, log g is 4.14 ± 0.15 and the projected rotational velocity, υsini, is 62.0 ± 2.0 km s-1. As the υsini value is significantly larger than previously assumed, we refined the first asteroseimic model accordingly. The overall metallicity Z was determined to be 0.011 where the light elements He, C, O, Na, and S show solar chemical composition, but the heavier elements are significantly underabundant. In addition, the radius of HD 144277 was determined to be 1.55 ± 0.65 R☉ from spectral energy distribution fitting, based on photometric data taken from the literature.
Conclusions: From the spectroscopic observations, we could confirm our previous assumption from asteroseismic models that HD 144277 has less than solar metallicity. The fundamental parameters derived from asteroseismology, Teff, log g, L/L☉ and R/R☉ agree within one sigma to the values found from spectroscopic analysis. As the υsini value is significantly higher than assumed in the first analysis, near-degeneracies and rotational mode coupling were taken into account in the new models. These suggest that HD 144277 has an equatorial rotational velocity of about 80 km s-1 and is seen equator-on. The observed frequencies are identified as prograde modes.This work is based on ground-based observations made with the 3.6 m telescope at La Silla Observatory under the ESO Large Programme LP185.D-0056.

Links. NASA ADS,arXiv

Authors.Salmon, S. J. A. J. ; Montalbàn, J. ; Reese, D. R. ; Dupret, M. -A. ; Eggenberger, P.

Journal.Astronomy & Astrophysics, Volume 569, id.A18, 8 pp.

Abstract.The recent variability survey of the NGC 3766 cluster revealed a considerable number of periodic variable stars in a region of the H-R diagram where no pulsation is expected. This region lies between the instability strips of the delta Scuti and SPB stars. Moreover the periods of the new phenomenon, P~0.1-0.7 d, do not allow to associate it a priori to either of these two types of pulsations. Stars in the NGC 3766 cluster are known as fast rotators with rotational velocities typically larger than half of their critical velocity. Rotation can affect both the geometrical properties and period domain of pulsations. It also alters the apparent stellar luminosity through gravity darkening, effect seldom taken considered in theoretical studies of the rotation-pulsation interaction. We explore if both of these effects are able to deliver a consistent interpretation for the observed properties of the “new variables” in NGC 3766: explaining their presence outside the known instability strips and their variability periods. We carry out an instability analysis of SPB models within the framework of the Traditional Approximation of Rotation and study the visibility of modes according to the angle of view and rotation. We also check how gravity darkening affects the effective temperature and luminosity of stellar models for different angles of view and rotation velocities. At the red (cold) border of the instability strip, prograde sectoral modes are preferentially excited and their visibilities are maximum when seen equator-on. Furthermore low-mass SPB models seen equator-on can appear in the gap between non-rotating SPB and delta Scuti stars due to gravity darkening. In that case, periods of these most visible modes are shifted to the 0.2-0.5 d range due to the effects of the Coriolis force. We hence suggest that the new variable stars observed in NGC 3766 are actually fast rotating SPB pulsators.

Links. NASA ADS,arXiv

Authors.Molnar, L. ; Plachy, E. ; Szabo, R.

Journal.Information Bulletin on Variable Stars, 6108, 1.

Abstract.With the step-and-stare approach of the K2 mission, Kepler will be able to observe a large number of Cepheid an RR Lyrae stars. In this paper we summarize the target selection efforts, the scientific goals that can be achieved by Kepler, and the first impressions based on the K2 two-wheel engineering test data.

Links. NASA ADS,arXiv

Authors.Alvan, L. ; Brun, A. S. ; Mathis, S.

Journal.Astronomy & Astrophysics, Volume 565, id.A42, 26 pp.

Abstract.Context. Internal gravity waves (IGWs) are studied for their impact on the angular momentum transport in stellar radiation zones and the information they provide about the structure and dynamics of deep stellar interiors. We present the first 3D nonlinear numerical simulations of IGWs excitation and propagation in a solar-like star.
Aims: The aim is to study the behavior of waves in a realistic 3D nonlinear time-dependent model of the Sun and to characterize their properties.
Methods: We compare our results with theoretical and 1D predictions. It allows us to point out the complementarity between theory and simulation and to highlight the convenience, but also the limits, of the asymptotic and linear theories.
Results: We show that a rich spectrum of IGWs is excited by the convection, representing about 0.4% of the total solar luminosity. We study the spatial and temporal properties of this spectrum, the effect of thermal damping, and nonlinear interactions between waves. We give quantitative results for the modes’ frequencies, evolution with time and rotational splitting, and we discuss the amplitude of IGWs considering different regimes of parameters.
Conclusions: This work points out the importance of high-performance simulation for its complementarity with observation and theory. It opens a large field of investigation concerning IGWs propagating nonlinearly in 3D spherical structures. The extension of this work to other types of stars, with different masses, structures, and rotation rates will lead to a deeper and more accurate comprehension of IGWs in stars.

Links. NASA ADS,arXiv

Authors.Morel, T. ; Miglio, A. ; Lagarde, N. ; Montalbán, J. ; Rainer, M. ; Poretti, E. ; Eggenberger, P. ; Hekker, S. ; Kallinger, T. ; Mosser, B. ; Valentini, M. ; Carrier, F. ; Hareter, M. ; Mantegazza, L.

Journal.Astronomy & Astrophysics, Volume 564, id.A119, 20 pp.

Abstract.A precise characterisation of the red giants in the seismology fields of the CoRoT satellite is a prerequisite for further in-depth seismic modelling. High-resolution FEROS and HARPS spectra were obtained as part of the ground-based follow-up campaigns for 19 targets holding great asteroseismic potential. These data are used to accurately estimate their fundamental parameters and the abundances of 16 chemical species in a self-consistent manner. Some powerful probes of mixing are investigated (the Li and CNO abundances, as well as the carbon isotopic ratio in a few cases). The information provided by the spectroscopic and seismic data is combined to provide more accurate physical parameters and abundances. The stars in our sample follow the general abundance trends as a function of the metallicity observed in stars of the Galactic disk. After an allowance is made for the chemical evolution of the interstellar medium, the observational signature of internal mixing phenomena is revealed through the detection at the stellar surface of the products of the CN cycle. A contamination by NeNa-cycled material in the most massive stars is also discussed. With the asteroseismic constraints, these data will pave the way for a detailed theoretical investigation of the physical processes responsible for the transport of chemical elements in evolved, low- and intermediate-mass stars.Based on observations collected at La Silla Observatory, ESO (Chile) with the FEROS and HARPS spectrograph at the 2.2 and 3.6-m telescopes under programs LP178.D-0361, LP182.D-0356, and LP185.D-0056.Appendix A is available in electronic form at http://www.aanda.orgTables A.2 to A.6 are available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/564/A119

Links. NASA ADS,arXiv

Authors.Metcalfe, T. S. ; Creevey, O. L. ; Dogan, G. ; Mathur, S. ; Xu, H. ; Bedding, T. R. ; Chaplin, W. J. ; Christensen-Dalsgaard, J. ; Karoff, C. ; Trampedach, R. ; Benomar, O. ; Brown, B. P. ; Buzasi, D. L. ; Campante, T. L. ; Celik, Z. ; Cunha, M. S. ; Davies, G. R. ; Deheuvels, S. ; Derekas, A. ; Di Mauro, M. P. ; Garcia, R. A. ; Guzik, J. A. ; Howe, R. ; MacGregor, K. B. ; Mazumdar, A. ; Montalban, J. ; Monteiro, M. J. P. F. G. ; Salabert, D. ; Serenelli, A. ; Stello, D. ; Steslicki, M. ; Suran, M. D. ; Yildiz, M. ; Aksoy, C. ; Elsworth, Y. ; Gruberbauer, M. ; Guenther, D. B. ; Lebreton, Y. ; Molaverdikhani, K. ; Pricopi, D. ; Simoniello, R. ; White, T. R.

Journal.The Astrophysical Journal Supplement, Volume 214, Issue 2, article id. 27, 13 pp.

Abstract.Recently the number of main-sequence and subgiant stars exhibiting solar-like oscillations that are resolved into individual mode frequencies has increased dramatically. While only a few such data sets were available for detailed modeling just a decade ago, the Kepler mission has produced suitable observations for hundreds of new targets. This rapid expansion in observational capacity has been accompanied by a shift in analysis and modeling strategies to yield uniform sets of derived stellar properties more quickly and easily. We use previously published asteroseismic and spectroscopic data sets to provide a uniform analysis of 42 solar-type Kepler targets from the Asteroseismic Modeling Portal (AMP). We find that fitting the individual frequencies typically doubles the precision of the asteroseismic radius, mass and age compared to grid-based modeling of the global oscillation properties, and improves the precision of the radius and mass by about a factor of three over empirical scaling relations. We demonstrate the utility of the derived properties with several applications.

Links. NASA ADS,arXiv

Authors.Brandão, I. M. ; Cunha, M. S. ; Christensen-Dalsgaard, J.

Journal.Monthly Notices of the Royal Astronomical Society, Volume 438, Issue 2, p.1751-1761

Abstract.Particular diagnostic tools may isolate the signature left on the oscillation frequencies by the presence of a small convective core. Their frequency derivative is expected to provide information about convective core’s properties and stellar age. The main goal of this work is to study the potential of the diagnostic tools with regard to the inference of stellar age and stellar core’s properties. For that, we computed diagnostic tools and their frequency derivatives from the oscillation frequencies of main-sequence models with masses between 1.0 and 1.6 M☉ and with different physics. We considered the dependence of the diagnostic tools on stellar age and on the size of the relative discontinuity in the squared sound speed at the edge of the convectively unstable region. We find that the absolute value of the frequency derivatives of the diagnostic tools increases as the star evolves on the main sequence. The fraction of stellar main-sequence evolution for models with masses >1.2 M☉ may be estimated from the frequency derivatives of two of the diagnostic tools. For lower mass models, constraints on the convective core’s overshoot can potentially be derived based on the analysis of the same derivatives. For at least 35 per cent of our sample of stellar models, the frequency derivative of the diagnostic tools takes its maximum absolute value on the frequency range where observed oscillations may be expected.

Links. NASA ADS,arXiv

Authors.Nagashima, Kaori ; Löptien, Björn ; Gizon, Laurent ; Birch, Aaron C. ; Cameron, Robert ; Couvidat, Sebastien ; Danilovic, Sanja ; Fleck, Bernhard ; Stein, Robert

Journal.Solar Physics, Volume 289, Issue 9, pp.3457-3481

Abstract.The Solar Dynamics Observatory/Helioseismic and Magnetic Imager (SDO/HMI) filtergrams, taken at six wavelengths around the Fe I 6173.3 {\AA} line, contain information about the line-of-sight velocity over a range of heights in the solar atmosphere. Multi-height velocity inferences from these observations can be exploited to study wave motions and energy transport in the atmosphere. Using realistic convection simulation datasets provided by the STAGGER and MURaM codes, we generate synthetic filtergrams and explore several methods for estimating Dopplergrams. We investigate at which height each synthetic Dopplergram correlates most strongly with the vertical velocity in the model atmospheres. On the basis of the investigation, we propose two Dopplergrams other than the standard HMI-algorithm Dopplergram produced from HMI filtergrams: a line-center Dopplergram and an average-wing Dopplergram. These two Dopplergrams correlate most strongly with vertical velocities at the heights of 30 – 40 km above (line-center) and 30 – 40 km below (average-wing) the effective height of the HMI-algorithm Dopplergram. Therefore, we can obtain velocity information from two layers separated by about a half of a scale height in the atmosphere, at best. The phase shifts between these multi-height Dopplergrams from observational data as well as those from the simulated data are also consistent with the height-difference estimates in the frequency range above the photospheric acoustic cutoff frequency.

Links. NASA ADS,arXiv, Springer