Bayesian Asteroseismology of 23 Solar-Like Kepler Targets

We study 23 previously published Kepler targets to perform a consistent grid-based Bayesian asteroseismic analysis and compare our results to those obtained via the Asteroseismic Modelling Portal (AMP). We find differences in the derived stellar parameters of many targets and their uncertainties. While some of these differences can be attributed to systematic effects between stellar evolutionary models, we show that the different methodologies deliver incompatible uncertainties for some parameters. Using non-adiabatic models and our capability to measure surface effects, we also investigate the dependency of these surface effects on the stellar parameters. Our results suggest a dependence of the magnitude of the surface effect on the mixing length parameter which also, but only minimally, affects the determination of stellar parameters. While some stars in our sample show no surface effect at all, the most significant surface effects are found for stars that are close to the Sun's position in the HR diagram.Comment: 14 pages, 9 figures, accepted for publication in MNRA

Bayesian asteroseismology

xvi, 172 leaves : ill. (some col.) ; 29 cm.Includes abstract.Includes bibliographical references.This thesis presents a new probabilistic method for the asteroseismic analysis of stellar structure and evolution with the goal of providing a universal tool to improve our knowledge of stellar modelling. This new method implements the advantages of Bayesian analysis, such as the treatment of systematic errors and nuisance parameters, the modular structure of Bayesian analysis, and the correct normalization of all probabilities. First, a general introduction to asteroseismology is provided, followed by an comprehensive guide to Bayesian analysis. The derivation of the new method then follows, and its subsequent application to current problems in asteroseismology is also presented. An in-depth analysis of the Sun is performed in order to investigate long standing problems with the solar chemical composition. This also reveals the presence of systematic problems in the modelling of the Sun, potentially requiring new developments in solar modelling. Finally, the new method is also applied to 23 stars that were observed with the Kepler satellite, in order to perform a comparative investigation with respect to published results from other teams, and to study systematic errors in the stellar models

Asteroseismic modelling of the roAp star gamma Equulei based on observations obtained by the MOST satellite

roAp (rapidly oscillating Ap) Sterne sind pulsierende Sterne mit starken organisierten Magnetfeldern, die zur Gruppe der (im Allgemeinen nicht pulsierenden) Ap Sterne gehören. Erst wenige Objekte dieser Art wurden bisher entdeckt. Mit ihren chemisch–pekuliaren Sternatmosphären, ihren heterogenen Eigenschaften, was Rotation, Magnetfeldstärke und Pulsationscharakteristik betrifft, sowie ihrer geringen Anzahl, stellen roAp Sterne ein interessantes aber sehr komplexes Thema in der stellaren Astronomie dar. Eine äußerst vielversprechende Möglichkeit, die Vielzahl an physikalischen Effekten aufzulösen, die in diesen Objekten eine Rolle spielen, ist die Astero- seismologie. Sie untersucht, was Pulsationen über den Sternaufbau verraten. Diese Magisterarbeit berichtet vom Versuch, bei einem roAp Stern erstmalig lediglich mittels der Pulsation auf die Fundamentalparameter und den Aufbau des Sterns rückzuschließen. Mit Hilfe des MOST–Asteroseismologiesatelliten wurden Helligkeitsschwankungen des roAp Sterns γ Equulei (γ Equ) über 19 Tage lang gemessen, wobei insgesamt beinahe ca. 50000 Datenpunkte gewonnen werden konnten. Diese Daten wurden dann mit den aktuellsten Pulsationsmodellen dieses Sterntyps verglichen. Der MOST-Datensatz ermöglicht durch seine außergewöhnliche Qualität zum ersten Mal eine eindeutige Auflösung von γ Equ’s Frequenzen, und liefert somit die Grundbedingung für eine derartige Untersuchung.roAp (rapidly oscillating Ap) stars are pulsating stars with strong, organized magnetic fields. They belong to the larger group of (in general non-pulsating) Ap stars. Only a few objects of this class are known up to now. Due to the many convoluted effects, like their peculiar atmospheres, there heterogeneity concerning rotation, magnetic field strength, and pulsation characteristics, they are interesting but difficult stars to study. One of the most promising tools to improve our understanding of this class of stars is asteroseismology, which tries to infer details of stellar structure simply from a star’s pulsation. This thesis reports on the first attempt to use asteroseismology for a roAp star as way to derive the fundamental stellar parameters from pulsation modes alone. With the help of the MOST–satellite, which was explicitly designed for asterseismological purposes, the varying brightness of the roAp star γ Equulei (γ Equ) has been monitored for over 19 days. All in all, about 50000 measurements were obtained. These were then compared to the latest generation of roAp pulsation models. Due to its impressive quality, the MOST–data set fulfils the requirements for such a study by being the first to unambiguously resolve γ Equ’s frequencies

MOST photometry of the RRd Lyrae variable AQ Leo: Two radial modes, 32 combination frequencies, and beyond

Highly precise and nearly uninterrupted optical photometry of the RR Lyrae star AQ Leo was obtained with the MOST (Microvariability & Oscillations of STars) satellite over 34.4 days in February-March 2005. AQ Leo was the first known double-mode RR Lyrae pulsator (RRd star). Three decades after its discovery, MOST observations have revealed that AQ Leo oscillates with at least 42 frequencies, of which 32 are linear combinations (up to the sixth order) of the radial fundamental mode and its first overtone. Evidence for period changes of these modes is found in the data. The other intrinsic frequencies may represent an additional nonradial pulsation mode and its harmonics (plus linear combinations) which warrant theoretical modeling. The unprecedented number of frequencies detected with amplitudes down to millimag precision also presents an opportunity to test nonlinear theories of mode growth and saturation in RR Lyrae pulsators.Comment: accepted for publication in MNRAS; revision v2 : broken references have been fixe

A remarkable long-term light curve, and deep, low-state spectroscopy: Swift & XMM-Newton monitoring of the NLS1 galaxy Mkn 335

The Narrow-line Seyfert 1 galaxy (NLS1) Mkn 335 is remarkable because it has repeatedly shown deep, long X-ray low-states which show pronounced spectral structure. It has become one of the prototype AGN in deep minimum X-ray states. Here we report on the continuation of our ongoing monitoring campaign with Swift and the examination of the low state X-ray spectra based on a 200 ks triggered observation with XMM in June 2009. Swift has continuously monitored Mkn 335 since May 2007 typically on a monthly basis. This is one of the longest simultaneous UV/X-ray light curves so far obtained for an active galactic nucleus (AGN). Mkn 335 has shown strong X-ray variability even on time scales of hours. In the UV, it turns out to be one of the most variable among NLS1s. Long-term Swift monitoring allow us to examine correlations between the UV, X-rays and X-ray hardness ratios. We find no significant correlation or lag between the UV and X-ray variability; however, we do find distinct trends in the behavior of the hardness ratio variability. The hardness ratio and count rate are correlated in the low-flux state, but no correlation is seen in the high-state. The X-ray low-state spectra of the 2007 and 2009 XMM observations display significant spectral variability. We fit the X-ray spectra with a suite of phenomenological models in order to characterize the data. The broad band CCD spectrum can be fitted equally well with partial absorption and blurred reflection models. These more complicated models are explored in further detail in upcoming work.Comment: 23 pages, 8 figures, 4 Tables, ApJ Suppl. accepte

Observations of intensity fluctuations attributed to granulation and faculae on Sun-like stars from the Kepler mission

Publisher's version/PDFSun-like stars show intensity fluctuations on a number of timescales due to various physical phenomena on their surfaces. These phenomena can convincingly be studied in the frequency spectra of these stars—while the strongest signatures usually originate from spots, granulation, and p-mode oscillations, it has also been suggested that the frequency spectrum of the Sun contains a signature of faculae. We have analyzed three stars observed for 13 months in short cadence (58.84 s sampling) by the Kepler mission. The frequency spectra of all three stars, as for the Sun, contain signatures that we can attribute to granulation, faculae, and p-mode oscillations. The temporal variability of the signatures attributed to granulation, faculae, and p-mode oscillations was analyzed and the analysis indicates a periodic variability in the granulation and faculae signatures—comparable to what is seen in the Sun

Solar-like oscillations in low-luminosity red giants: first results from Kepler

Publisher's version/PDFWe have measured solar-like oscillations in red giants using time-series photometry from the first 34 days of science operations of the Kepler Mission. The light curves, obtained with 30 minute sampling, reveal clear oscillations in a large sample of G and K giants, extending in luminosity from the red clump down to the bottom of the giant branch. We confirm a strong correlation between the large separation of the oscillations ([Delta]ν) and the frequency of maximum power (ν[subscript max]). We focus on a sample of 50 low-luminosity stars (ν[subscript max] [is greater than] 100 [micro]Hz, L [less than or similar to] 30 L[subscript circled dot] ) having high signal-to-noise ratios and showing the unambiguous signature of solar-like oscillations. These are H-shell-burning stars, whose oscillations should be valuable for testing models of stellar evolution and for constraining the star formation rate in the local disk. We use a new technique to compare stars on a single echelle diagram by scaling their frequencies and find well-defined ridges corresponding to radial and non-radial oscillations, including clear evidence for modes with angular degree l = 3. Measuring the small separation between l = 0 and l = 2 allows us to plot the so-called C-D diagram of [delta]ν[subscript 02] versus [Delta]ν. The small separation [delta]ν[subscript 01] of l = 1 from the midpoint of adjacent l = 0 modes is negative, contrary to the Sun and solar-type stars. The ridge for l = 1 is notably broadened, which we attribute to mixed modes, confirming theoretical predictions for low-luminosity giants. Overall, the results demonstrate the tremendous potential of Kepler data for asteroseismology of red giants

Solving the mode identification problem in asteroseismology of F stars observed with Kepler

Asteroseismology of F-type stars has been hindered by an ambiguity in identification of their oscillation modes. The regular mode pattern that makes this task trivial in cooler stars is masked by increased linewidths. The absolute mode frequencies, encapsulated in the asteroseismic variable epsilon, can help solve this impasse because the values of epsilon implied by the two possible mode identifications are distinct. We find that the correct epsilon can be deduced from the effective temperature and the linewidths and we apply these methods to a sample of solar-like oscillators observed with Kepler.Comment: 7 pages, 4 figures, 1 table, accepted for publication in The Astrophysical Journal Letter

Oscillation mode linewidths of main-sequence and subgiant stars observed by Kepler

Publisher's version/PDFContext. Solar-like oscillations have been observed by Kepler and CoRoT in several solar-type stars. Aims. We study the variations in the stellar p-mode linewidth as a function of effective temperature. Methods. We study a time series of nine months of Kepler data. We analyse the power spectra of 42 cool main-sequence stars and subgiants using both maximum likelihood estimators and Bayesian estimators to recover individual mode characteristics such as frequencies, linewidths, and mode heights. Results. We report on the mode linewidth at both maximum power and maximum mode height for these 42 stars as a function of effective temperature. Conclusions. We show that the mode linewidth at either maximum mode height or maximum amplitude follows a scaling relation with effective temperature, which is a combination of a power law and a lower bound. The typical power-law index is about 13 for the linewidth derived from the maximum mode height, and about 16 for the linewidth derived from the maximum amplitude, while the lower bound is about 0.3 [mu]Hz and 0.7 [mu]Hz, respectively. We stress that this scaling relation is only valid for cool main-sequence stars and subgiants, and does not have any predictive power outside the temperature range of these stars