All posts by Filipe Pereira

Kepler’s first view of O-star variability: K2 data of five O stars in Campaign 0 as a proof of concept for O-star asteroseismology

Authors. B. Buysschaert, C. Aerts, S. Bloemen, J. Debosscher, C. Neiner, M. Briquet, J. Vos, P. Papics, R. Manick, V. Schmid, H. Van Winkel, A. Tkachenko

Journal. Monthly Notices of the Royal Astronomical Society

Abstract. We present high-precision photometric light curves of five O-type stars observed with the refurbished Kepler satellite during its Campaign 0. For one of the stars, we also assembled high-resolution ground-based spectroscopy with the HERMES spectrograph attached to the 1.2 m Mercator telescope. The stars EPIC 202060097 (O9.5V) and EPIC 202060098 (O7V) exhibit monoperiodic variability due to rotational modulation with an amplitude of 5.6 and 9.3 mmag and a rotation period of 2.63 and 5.03 d, respectively. EPIC 202060091 (O9V) and EPIC 202060093 (O9V:pe) reveal variability at low frequency but the cause is unclear. EPIC 202060092 (O9V:p) is discovered to be a spectroscopic binary with at least one multiperiodic β Cep-type pulsator whose detected mode frequencies occur in the range [0.11, 6.99] d-1 and have amplitudes between 0.8 and 2.0 mmag. Its pulsation spectrum is shown to be fully compatible with the ones predicted by core-hydrogen burning O-star models. Despite the short duration of some 33 d and the limited data quality with a precision near 100 μmag of these first K2 data, the diversity of possible causes for O-star variability already revealed from campaigns of similar duration by the MOST and CoRoT satellites is confirmed with Kepler. We provide an overview of O-star space photometry and give arguments why future K2 monitoring during Campaigns 11 and 13 at short cadence, accompanied by time-resolved high-precision high-resolution spectroscopy, opens up the possibility of in-depth O-star seismology.


Horizontal Flows in Active Regions from Ring-diagram and Local Correlation Tracking Methods

Authors. Jain, Kiran; Tripathy, S. C.; Ravindra, B.; Komm, R.; Hill, F.

Journal.  The Astrophysical Journal

Abstract. Continuous high-cadence and high-spatial resolution Dopplergrams allow us to study sub-surface dynamics that may be further extended to explore precursors of visible solar activity on the surface. Since the p-mode power is absorbed in the regions of high magnetic field, the inferences in these regions are often presumed to have large uncertainties. In this paper, using the Dopplergrams from space-borne Helioseismic Magnetic Imager (HMI), we compare horizontal flows in a shear layer below the surface and the photospheric layer in and around active regions. The photospheric flows are calculated using local correlation tracking (LCT) method while the ring-diagram (RD) technique of helioseismology is used to infer flows in the sub-photospheric shear layer. We find a strong positive correlation between flows from both methods near the surface. This implies that despite the absorption of acoustic power in the regions of strong magnetic field, the flows inferred from the helioseismology are comparable to those from the surface measurements. However, the magnitudes are significantly different; the flows from the LCT method are smaller by a factor of 2 than the helioseismic measurements. Also, the median difference between direction of corresponding vectors is 49 degree.

Links. ApJNASA ADS, arXiv

Determining the evolutionary stage of HD163899 on the basis of its oscillation spectrum

Authors. Ostrowski, Jakub; Daszyńska-Daszkiewicz, Jadwiga; Cugier, Henryk

Journal.  Astronomy in Focus, Volume 1

Abstract. We present the new interpretation of the oscillation spectrum of HD 163899 based on the new determinations of the effective temperature, mass-luminosity ratio and rotational velocity. These new parameters strongly prefer the more massive models than previously considered. Now it is also possible that the star could be in the main sequence stage. Using the oscillation spectrum as a gauge, we intend to establish which stage of evolution corresponds better to HD 163899.

Links. NASA ADS, arXiv

Internal rapid rotation and its implications for stellar structure and pulsations

Authors. Reese, Daniel R.

Journal.  EPJ Web of Conferences

Abstract. Massive and intermediate mass stars play a crucial role in astrophysics. Indeed, massive stars are the main producers of heavy elements, explode in supernovae at the end of their short lifetimes, and may be the progenitors of gamma ray bursts. Intermediate mass stars, although not destined to explode in supernovae, display similar phenomena, are much more numerous, and have some of the richest pulsation spectra.

A key to understanding these stars is understanding the effects of rapid rotation on their structure and evolution. These effects include centrifugal deformation and gravity darkening which can be observed immediately, and long terms effects such as rotational mixing due to shear turbulence, which prolong stellar lifetime, modify chemical yields, and impact the stellar remnant at the end of their lifetime. In order to understand these effects, a number of models have been and are being developed over the past few years. These models lead to increasingly sophisticated predictions which need to be tested through observations. A particularly promising source of constraints is seismic observations as these may potentially lead to detailed information on their internal structure. However, before extracting such information, a number of theoretical and observational hurdles need to be overcome, not least of which is mode identification. The present proceedings describe recent progress in modelling these stars and show how an improved understanding of their pulsations, namely frequency patterns, mode visibilities, line profile variations, and mode excitation, may help with deciphering seismic observations.


The space photometry revolution and our understanding of RR Lyrae stars

Authors. Szabó, R.; Benkő, M. J.; Paparó, M.; Chapellier, E.; Poretti, E.; Baglin, A.; Weiss, W. W.; Kolenberg, K.; Guggenberger, E.; Le Borgne, J.-F.

Journal.  EPJ Web of Conferences

Abstract. The study of RR Lyrae stars has recently been invigorated thanks to the long, uninterrupted, ultra-precise time series data provided by the Kepler and CoRoT space telescopes. We give a brief overview of the new observational findings concentrating on the connection between period doubling and the Blazhko modulation, and the omnipresence of additional periodicities in all RR Lyrae subtypes, except for non-modulated RRab stars. Recent theoretical results demonstrate that if more than two modes are present in a nonlinear dynamical system such as a high-amplitude RR Lyrae star, the outcome is often an extremely intricate dynamical state. Thus, based on these discoveries, an underlying picture of complex dynamical interactions between modes is emerging which sheds new light on the century-old Blazhko-phenomenon, as well. New directions of theoretical efforts, like multidimensional hydrodynamical simulations, future space photometric missions and detailed spectroscopic investigations will pave the way towards a more complete understanding of the atmospheric and pulsation dynamics of these enigmatic touchstone objects.


Connection between the period and the amplitude of the Blazhko effect

Authors. Benkő, József M.; Szabó, Róbert

Journal.  EPJ Web of Conferences

Abstract. We found a possible relationship between the modulation period and the amplitude of the Blazhko RR Lyrae stars: long modulation period generally implies high modulation amplitude while the short modulation period results in small amplitude. Although this effect is much more a tendency than a strict rule, it can be detected easily in the space-born time series data produced by Kepler and CoRoT. Good quality ground-based data show this relation, too. This phenomenon could give us constraints for the physics of the Blazhko effect.


HD183648: a Kepler eclipsing binary with anomalous ellipsoidal variations and a pulsating component

Authors. Derekas, A.; Borkovits, T.; Fuller, J.; Szabó, Gy. M.; Pavlovski, K.; Csák, B.; Dózsa, Á.; Kovács, J.; Szabó, R.; Hambleton, K. M.; Kinemuchi, K.; Kolbas, V.; Kurtz, D. W.; Maloney, F.; Prša, A.; Southworth, J.; Sztakovics, J.; Bíró, I. B.; Jankovics, I.

Journal.  EPJ Web of Conferences

Abstract. KIC 8560861 (HD 183648) is a marginally eccentric (e = 0.05) eclipsing binary with an orbital period of Porb = 31.973 d, exhibiting mmag amplitude pulsations on time scales of a few days. We present the results of the complex analysis of high and medium-resolution spectroscopic data and Kepler Q0 – Q16 long cadence photometry.


Classical variables in the era of space photometric missions

Authors. Molnár, L.; Plachy, E.; Szabó, R.; Benkő, J. M.

Journal.  EPJ Web of Conferences

Abstract. The space photometric missions like CoRoT and Kepler transformed our view of pulsating stars, including the well-known RR Lyrae and Cepheid classes. The K2, TESS and PLATO missions will expand these investigations to larger sample sizes and to specific stellar populations.


Test of a new method for seismic indices and granulation parameters extraction

Authors. Peralta, R. A.; Samadi, R.; Michel, E.

Journal.  EPJ Web of Conferences

Abstract. In the framework of the data base project SSI (Stellar Seismic Indices), we have developed and tested a new method aiming at optimizing the simultaneous measurement of both the seismic indices characterizing the oscillations (Δν, νmax) and the indices characterizing the granulation signature. Here, we describe this method which is intended to take advantage of the MLE (maximum likelihood estimate) algorithm combined with the parametrized representation of the red giants pulsation spectrum following the Universal Pattern. We report its performances tested on Monte Carlo simulations.