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

Solar-type oscillations on the giant branch

Gegen Ende ihres Lebens dehnen sich sonnenähnliche Sterne erheblich aus und werden zu Roten Riesen. Dabei zeigen sie, so wie auch die Sonne selbst, stark gedämpfte Oszillationen, die durch die turbulenten konvektiven Strömungen in ihren äußeren Hüllen angeregt werden. Diese Schwingungen ermöglichen die seismologische Erkundung der inneren Sternstruktur und erlauben unter anderem die Bestimmung des Sternalters aufgrund der Ausdehnung des Kerns. Bisher war es nicht geklärt, ob man bei Roten Riesen, ähnlich wie bei sonnenähnlichen Sternen, radiale und nicht-radiale Schwingungen beobachtet, deren Lebenszeiten signifikant länger sind als bei sonnenähnlichen Sternen, oder nur radiale Oszillationen mit Lebenszeiten vergleichbar zu den in sonnenähnlichen Sternen. Letzteres würde die aus den beobachtbaren Schwingungen ableitbaren Informationen erheblich einschränken. Zu Beginn meiner Arbeit gab es noch keine allgemein anerkannte Theorie welche die zeitliche Entwicklung von Konvektion berücksichtigt und damit die Anregung und Dämpfung von sonnenähnlicher Pulsation erklärt. Weiters beschränkten sich die Beobachtungen hauptsächlich auf den Nachweis von sonnenähnlicher Pulsation. Aber das Interesse und das Potenzial zur Untersuchung der Struktur dieser Sterne waren groß, sodass die kanadische Weltraummission MOST und das europäische Satellitenprojekt CoRoT Programme zur Beobachtung von sonnenähnlich pulsierenden Roten Riesen entwickelten. Ein Teil dieser Arbeit beschäftigt sich mit dem Nachweis von radialen und nicht-radialen Schwingungen in der 28 Tage langen MOST Präzisionsphotometrie des G9.5 Riesen ε Oph. Die Oszillationsfrequenzen wurden unter der Annahme von relativ stabilen Schwingungen, d.h. unter der Annahme von langen Lebenszeiten, extrahiert. Deren Signifikanz wurde bezüglich des lokalen Hintergrundrauschens bewertet, welches durch die stellare Oberflächengranulation verursacht wird und mit Hilfe eines einfachen Potenzgesetzes berechnet werden kann. Die beobachteten Frequenzen wurden mit Modellfrequenzen aus einem umfangreichen Gitter von Sternmodellen verglichen, um Modelle zu identifizieren, deren Eigenfrequenzen möglichst gut mit den beobachteten übereinstimmen. Das am besten passende Modell erklärt 18 der 21 beobachteten Frequenzen als radiale und nicht-radiale Oszillationen und liegt innerhalb der ±1σ Fehlergrenzen von ε Oph’s Position im H-R Diagram und dessen interferometrisch bestimmten Radius. Aufgrund des relativ kurzen Datensatzes waren die Lebenszeiten der Schwingungen nicht direkt messbar. Die Streuung der beobachteten Frequenzen um die Modellfrequenzen deutet jedoch auf eine durchschnittliche Lebenszeit von 10 bis 20 Tagen hin. Diese Interpretation ist aber kontroversiell. So behaupten etwa Barban et.al. (2007), im gleichen Datensatz von ε Oph nur radiale Schwingungen mit sehr kurzen Lebenszeiten nachweisen zu können. Deren Resultat widerspricht somit meinem und stellt generell das asteroseismologische Potential von Roten Riesen in Frage. Diese Unklarheit konnte mit Hilfe der ersten 150 Tage langen Beobachtungskampagne von CoRoT beseitigt werden. Mehr als 300 Sterne wurden gefunden, die ein für Rote Riesen typisches Granulations- und Pulsationsverhalten zeigen. Mit Hilfe einer halbautomatischen Prozedur konnten die pulsierenden Roten Riesen unter den ca. 11000 beobachteten Sternen identifiziert werden. Exemplarisch für die große Zahl von neu entdeckten pulsierenden Roten Riesen habe ich zwei Sterne genauer untersucht. In einem ersten Schritt wurden die Fourierspektren mit Hilfe eines Potenzgesetzes bezüglich des stellaren Hintergrundsignals korrigiert. Die residualen Fourierspektren zeigen ein klares Muster aus radialen und nicht-radialen Oszillationen, deren Frequenzen mit Hilfe von Lorentzprofilen ermittelt wurden. Eine erste Abschätzung über die stellaren Massen und Radien ließ sich aus den globalen Pulsationsparametern ableiten. Im Fall der beiden CoRoT Sterne wurden Modelle gefunden, die alle 13 bzw. 12 extrahierten Frequenzen innerhalb der Beobachtungsfehler als radiale und nicht-radiale Schwingungen erklären. Weiters deuten die schmalen Profile der beobachteten Pulsationsmoden und die relative Stabilität des Signals in einer Zeit-Frequenzanalyse darauf hin, dass die Lebenszeiten der Schwingungen bei etwa 20 bis 50 Tagen liegen. Als wichtigste Resultate meiner Arbeit sehe ich: 1. Die Entscheidung der Kontroverse um die Existenz von nicht-radialen Pulsationsmoden in Roten Riesen. 2. Die Lebenszeiten dieser Schwingungen sind erheblich länger als bei sonnenähnlichen Sternen. 3. Die beobachteten Oszillationen lassen sich durch Frequenzen von Modellen Roter Riesen eindeutig erklären. Dies unterstreicht das große asteroseismologische Potenzial dieser Sterne und liefert einen Beitrag zum besseren Verständnis der späten Stadien in der Sternentwicklung.Towards the end of their lives, stars like the Sun greatly expand and become red giants. Like the Sun, they show strongly damped oscillations stochastically excited by the turbulent convective motions in their outer envelopes. These oscillation frequencies provide great potential for seismic probing of the internal structure of red-giant stars, and allow to determine, e.g., the stellar age. It has been unknown whether red giants exhibit radial and nonradial oscillations as it is known for sun-like stars but with significantly longer lifetimes, or radial modes only with lifetimes comparable to those of sun-like stars. In the second case this would seriously limit the asteroseismically deducible information. At the beginning of this study, no commonly accepted theory taking into account the temporal evolution of convection was available which is necessary to explain the driving and damping of solar-type oscillations. Furthermore, the observations were still in their infancy. But the interest and potential to investigate the structure of red giants was high, and both the Canadian space mission MOST and the European satellite CoRoT developed programs to observe pulsating red giants. Here I report on the detection of both radial and nonradial oscillations in the 28 days long high-precision MOST photometry of the G9.5 giant ε Oph. I have extracted the mode frequencies assuming the signal to be relatively stable, i.e. assuming the lifetimes to be long. Their significance was evaluated with respect to a simple power law model fit representing the local background noise due to intrinsic surface granulation. The extracted frequencies were then compared to those of an extensive grid of stellar models in order to search for models whose oscillation spectra best matches the observed frequencies. The best fit model explains 18 of the 21 observed frequencies as radial and nonradial p modes. It is located within ±1σ of ε Oph’s position in the H-R diagram and its interferometrically determined radius. The lifetimes of the observed oscillations are not directly accessible due to the relatively short data set. But the small scatter of the frequencies about the model predicted frequencies indicates that the average lifetime could be as long as 10 to 20 days. This interpretation is quite controversial. For example, Barban et. al. (2007) claimed to find short living radial modes only in the same data set of ε Oph. Consequently, their findings strongly contradicts my result and questions the asteroseismic potential of red giants in general. This ambiguity could be solved by the first 150 days long-run observations of CoRoT. More than 300 stars have been identified showing a granulation and pulsation signal in a frequency and amplitude range typical for solar-type pulsation in red giants. A semi-automatic method was used to identify the red-giant candidates among the about 11000 exofield targets. Exemplary for the large number of CoRoT red-giant pulsators I have analyzed two stars in detail. In a first step, their power spectra are corrected for the intrinsic background signal using power law model fits. The residual power spectra show a clear pattern of radial and nonradial modes and Lorentzian profile fits are used to extract the frequencies of 12 and 13 p modes, respectively. First estimates for the stellar mass and radii are determined from global pulsation parameter. In case of the two CoRoT stars, I found red-giant models whose oscillation spectra match all observed frequencies as radial and nonradial modes with an angular degree of up to and including 3. Furthermore, the narrow profiles of the observed modes and the relative stability of these modes in a time-frequency analysis indicates that the mode lifetimes are of the order of 20 to 50 days. As the main result of this thesis I conclude: 1. To resolve the controversy about the existence of nonradial modes in red giants. 2. Their lifetimes are significantly longer than those in sun-like stars. 3. The observable oscillations are consistent with theoretical eigenspectra of red-giant models. This finally approves the high asteroseismic potential of red-giant stars and will contribute to a better understanding of the late stages of stellar evolution

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

Asteroseismology of massive stars with the TESS mission: the runaway Beta Cep pulsator PHL 346 = HN Aqr

We report an analysis of the first known Beta Cep pulsator observed by the TESS mission, the runaway star PHL 346 = HN Aqr. The star, previously known as a singly-periodic pulsator, has at least 34 oscillation modes excited, 12 of those in the g-mode domain and 22 p modes. Analysis of archival data implies that the amplitude and frequency of the dominant mode and the stellar radial velocity were variable over time. A binary nature would be inconsistent with the inferred ejection velocity from the Galactic disc of 420 km/s, which is too large to be survivable by a runaway binary system. A kinematic analysis of the star results in an age constraint (23 +- 1 Myr) that can be imposed on asteroseismic modelling and that can be used to remove degeneracies in the modelling process. Our attempts to match the excitation of the observed frequency spectrum resulted in pulsation models that were too young. Hence, asteroseismic studies of runaway pulsators can become vital not only in tracing the evolutionary history of such objects, but to understand the interior structure of massive stars in general. TESS is now opening up these stars for detailed asteroseismic investigation.Comment: accepted for ApJ

Investigating the properties of granulation in the red giants observed by Kepler

More than 1000 red giants have been observed by NASA/Kepler mission during a nearly continuous period of ~ 13 months. The resulting high-frequency resolution (< 0.03 muHz) allows us to study the granulation parameters of these stars. The granulation pattern results from the convection motions leading to upward flows of hot plasma and downward flows of cooler plasma. We fitted Harvey-like functions to the power spectra, to retrieve the timescale and amplitude of granulation. We show that there is an anti-correlation between both of these parameters and the position of maximum power of acoustic modes, while we also find a correlation with the radius, which agrees with the theory. We finally compare our results with 3D models of the convection.Comment: 4 pages, 1 figure. To appear in the ASP proceedings of "The 61st Fujihara seminar: Progress in solar/stellar physics with helio- and asteroseismology", 13th-17th March 2011, Hakone, Japa

The K2-HERMES Survey: Age and Metallicity of the Thick Disc

Asteroseismology is a promising tool to study Galactic structure and evolution because it can probe the ages of stars. Earlier attempts comparing seismic data from the {\it Kepler} satellite with predictions from Galaxy models found that the models predicted more low-mass stars compared to the observed distribution of masses. It was unclear if the mismatch was due to inaccuracies in the Galactic models, or the unknown aspects of the selection function of the stars. Using new data from the K2 mission, which has a well-defined selection function, we find that an old metal-poor thick disc, as used in previous Galactic models, is incompatible with the asteroseismic information. We show that spectroscopic measurements of [Fe/H] and [$\alpha$/Fe] elemental abundances from the GALAH survey indicate a mean metallicity of $\log (Z/Z_{\odot})=-0.16$ for the thick disc. Here $Z$ is the effective solar-scaled metallicity, which is a function of [Fe/H] and [$\alpha$/Fe]. With the revised disc metallicities, for the first time, the theoretically predicted distribution of seismic masses show excellent agreement with the observed distribution of masses. This provides an indirect verification of the asteroseismic mass scaling relation is good to within five percent. Using an importance-sampling framework that takes the selection function into account, we fit a population synthesis model of the Galaxy to the observed seismic and spectroscopic data. Assuming the asteroseismic scaling relations are correct, we estimate the mean age of the thick disc to be about 10 Gyr, in agreement with the traditional idea of an old $\alpha$-enhanced thick disc.Comment: 21 pages, submitted to MNRA

An asteroseismic membership study of the red giants in three open clusters observed by Kepler: NGC6791, NGC6819, and NGC6811

Studying star clusters offers significant advances in stellar astrophysics due to the combined power of having many stars with essentially the same distance, age, and initial composition. This makes clusters excellent test benches for verification of stellar evolution theory. To fully exploit this potential, it is vital that the star sample is uncontaminated by stars that are not members of the cluster. Techniques for determining cluster membership therefore play a key role in the investigation of clusters. We present results on three clusters in the Kepler field of view based on a newly established technique that uses asteroseismology to identify fore- or background stars in the field, which demonstrates advantages over classical methods such as kinematic and photometry measurements. Four previously identified seismic non-members in NGC6819 are confirmed in this study, and three additional non-members are found -- two in NGC6819 and one in NGC6791. We further highlight which stars are, or might be, affected by blending, which needs to be taken into account when analysing these Kepler data.Comment: 12 pages, 9 figures, 5 tables, accepted by Ap

Asteroseismology of the open clusters NGC 6791, NGC 6811, and NGC 6819 from nineteen months of Kepler photometry

We studied solar-like oscillations in 115 red giants in the three open clusters NGC 6791, NGC 6811, and NGC 6819, based on photometric data covering more than 19 months with NASA's Kepler space telescope. We present the asteroseismic diagrams of the asymptotic parameters \delta\nu_02, \delta\nu_01 and \epsilon, which show clear correlation with fundamental stellar parameters such as mass and radius. When the stellar populations from the clusters are compared, we see evidence for a difference in mass of the red giant branch stars, and possibly a difference in structure of the red clump stars, from our measurements of the small separations \delta\nu_02 and \delta\nu_01. Ensemble \'{e}chelle diagrams and upper limits to the linewidths of l = 0 modes as a function of \Delta\nu of the clusters NGC 6791 and NGC 6819 are also shown, together with the correlation between the l = 0 ridge width and the T_eff of the stars. Lastly, we distinguish between red giant branch and red clump stars through the measurement of the period spacing of mixed dipole modes in 53 stars among all the three clusters to verify the stellar classification from the color-magnitude diagram. These seismic results also allow us to identify a number of special cases, including evolved blue stragglers and binaries, as well as stars in late He-core burning phases, which can be potentially interesting targets for detailed theoretical modeling.Comment: 30 pages, 15 figures, 1 table, accepted to Ap

Gravity modes as a way to distinguish between hydrogen- and helium-burning red giant stars

Red giants are evolved stars that have exhausted the supply of hydrogen in their cores and instead burn hydrogen in a surrounding shell. Once a red giant is sufficiently evolved, the helium in the core also undergoes fusion. Outstanding issues in our understanding of red giants include uncertainties in the amount of mass lost at the surface before helium ignition and the amount of internal mixing from rotation and other processes. Progress is hampered by our inability to distinguish between red giants burning helium in the core and those still only burning hydrogen in a shell. Asteroseismology offers a way forward, being a powerful tool for probing the internal structures of stars using their natural oscillation frequencies. Here we report observations of gravity-mode period spacings in red giants that permit a distinction between evolutionary stages to be made. We use high-precision photometry obtained with the Kepler spacecraft over more than a year to measure oscillations in several hundred red giants. We find many stars whose dipole modes show sequences with approximately regular period spacings. These stars fall into two clear groups, allowing us to distinguish unambiguously between hydrogen-shell-burning stars (period spacing mostly about 50 seconds) and those that are also burning helium (period spacing about 100 to 300 seconds).Comment: to appear as a Letter to Natur