David B. Guenther

Evolutionary influences on the structure of red-giant acoustic oscillation spectra from 600d of Kepler observations

Context. The Kepler space mission is reaching continuous observing times long enough to also start studying the fine structure of the observed pressure-mode spectra. Aims. In this paper, we aim to study the signature of stellar evolut ion on the radial and pressure-dominated l = 2 modes in an ensemble of red giants that show solar-type oscillations. Methods. We use established methods to automatically identify the mode degree of l = 0 and 2 modes and measure the large (�νc) and small (δν02) frequency separation around the central radial mode. We then determine the phase shiftǫc of the central radial mode, i.e. the linear offset in the asymptotic fit to the acoustic modes. Furthermore w e measure the individual frequencies of radial modes and investigate their average curvature. Results. We find that ǫc is significantly di fferent for red giants at a given �νc but which burn only H in a shell (RGB) than those that have already ignited core He burning. Even though not directly probing the stellar core the pair of local seismic observa bles (�νc,ǫc) can be used as an evolutionary stage discriminator that turn ed out to be as reliable as the period spacing of the mixed dipole modes. We find a tight correlation between ǫc and �νc for RGB stars and unlike less evolved stars we find no indicati on thatǫc depends on other properties of the star. It appears that the difference inǫc between the two populations becomes smaller and eventually indistinguishable if we use an average of several radial orders, instead of a loc al, i.e. only around the central radial mode, large separati on to determine the phase shift. This indicates that the information on the e volutionary stage is encoded locally, more precisely in the shape of the radial mode sequence. This shape turns out to be approximately symmetric around the central radial mode for RGB stars but asymmetric for core He burning stars. We computed radial mode frequencies for a sequence of red-giant models and find them to qualitat ively confirm our findings. We also find that, at least in our models, t he local �ν is an at least as good and mostly better proxy for both the asymptotic spacing and the large separation scaled from the model density than the average �ν. Finally, we investigate the signature of the evolutionary stage onδν02 and quantify the mass dependency of this seismic parameter.

Photometric variability of the T Tauri star TW Hya on time-scales of hours to years

MOST (Microvariability & Oscillations of STars) and ASAS (All Sky Automated Survey) observations have been used to characterize photometric variability of TW Hya on time scales from a fraction of a day to 7.5 weeks and from a few days to 8 years, respectively. The two data sets have very different uncertainties and temporal coverage properties and cannot be directly combined, nevertheless, they suggests a global variability spectrum with “flicker noise” properties, i.e. with amplitudes a / 1/ p f, over > 4 decades in frequency, in the range f = 0.0003 to 10 cycles per day (c/d). A 3.7 d period is clearly present in the continuous 11 day, 0.07 d time resolution, observations by MOST in 2007. Brightness extrema coincide with zero-velocity crossings in periodic (3.56 d) radial velocity variability detected in contemporaneous spectroscopic observations of Setiawan et al. (2008) and interpreted as caused by a planet. The 3.56/3.7 d periodicity was entirely absent in the second, four times longer MOST run in 2008, casting doubt on the planetary explanation. Instead, a spectrum of unstable single periods within the range of 2 – 9 days was observed; the tendency of the periods to progressively shorten was well traced using the wavelet analysis. The evolving periodicities and the overall flicker-noise characteristics of the TW Hya variability suggest a combination of several mechanisms, with the dominant ones probably related to the accretion processes from the disk around the star.

On the detection of Lorentzian profiles in a power spectrum: a Bayesian approach using ignorance priors

Aims. Deriving accurate frequencies, amplitudes, and mode lifetimes from stochastically driven pulsation is challenging, more so, if one demands that realistic error estimates be given for all model fitting parameters. As has been shown by other authors, the traditional method of fitting Lorentzian profiles to the power spectrum of time-resolved photometric or spectroscopic data via the maximum likelihood estimation (MLE) procedure delivers good approximations for these quantities. We, however, show that a conservative Bayesian approach allows one to treat the detection of modes with minimal assumptions (i.e., about the existence and identity of the modes). Methods. We derive a conservative Bayesian treatment for the probability of Lorentzian profiles being present in a power spectrum and describe an efficient implementation that evaluates the probability density distribution of parameters by using a Markov-chain Monte Carlo (MCMC) technique. Results. Potentially superior to “best-fit” procedure like MLE, which only provides formal uncertainties, our method samples and approximates the actual probability distributions for all parameters involved. Moreover, it avoids shortcomings that make the MLE treatment susceptible to the built-in assumptions of a model that is fitted to the data. This is especially relevant when analyzing solartype pulsation in stars other than the Sun where the observations are of lower quality and can be over-interpreted. As an example, we

Nonradial p-modes in the G9.5 giant [epsilon] Ophiuchi? Pulsation model fits to MOST photometry

The G9.5 giant ∈ Oph shows evidence of radial p-mode pulsations in both radial velocity and luminosity. We re-examine the observed frequencies in the photometry and radial velocities and find a best model fit to 18 of the 21 most significant photometric frequencies. The observed frequencies are matched to both radial and nonradial modes in the best model fit. The small scatter of the frequencies about the model predicted frequencies indicate that the average lifetimes of the modes could be as long as 10-20 d. The best fit model itself, constrained only by the observed frequencies, lies within ±1σ of e Ophs position in the HR-diagram and the interferometrically determined radius.

The nature of p-modes and granulation in HD 49933 observed by CoRoT

Context. Recent observations of HD 49933 by the space-photometric mission CoRoT provide photometric evidence of solar type oscillations in a star other than our Sun. The first published reduction, analysis, and interpretation of the CoRoT data yielded a spectrum of p-modes with l = 0, 1, and 2. Aims. We present our own analysis of the CoRoT data in an attempt to compare the detected pulsation modes with eigenfrequencies of models that are consistent with the observed luminosity and surface temperature. Methods. We used the Gruberbauer et al. frequency set derived based on a more conservative Bayesian analysis with ignorance priors and fit models from a dense grid of model spectra. We also introduce a Bayesian approach to searching and quantifying the best model fits to the observed oscillation spectra. Results. We identify 26 frequencies as radial and dipolar modes. Our best fitting model has solar composition and coincides within the error box with the spectroscopically determined position of HD 49933 in the H-R diagram. We also show that lower-than-solar Z models have a lower probability of matching the observations than the solar metallicity models. To quantify the effect of the deficiencies in modeling the stellar surface layers in our analysis, we compare adiabatic and nonadiabatic model fits and find that the latter reproduces the observed frequencies better.

Solar-like oscillations and activity in Procyon: A comparison of the 2007 MOST and ground-based radial velocity campaigns

We compare the simultaneous 2007 space-based MOST photometry and ground-based radial velocity (RV) observations of the F5 star Procyon. We identify slow variations in the MOST data that are similar to those reported in the RV time series and confirm by comparison with the Sun that these variations are likely the signature of stellar activity. The MOST power spectrum yields clear evidence for individual oscillation frequencies that match those found in the RV data by Bedding et al. We identify the same ridges due to modes of different spherical degree in both data sets, but are not able to confirm a definite ridge identification using the MOST data. We measure the luminosity amplitude per radial mode Al=0,phot = 9.1 ± 0.5 ppm. Combined with the estimate for the RV data by Arentoft et al., this gives a mean amplitude ratio of Al=0,phot/Al=0,RV = 0.24 ± 0.02 ppm cm −1 s, considerably higher than expected from scaling relations but in reasonable agreement with theoretical models by Houdek. We also compare the amplitude ratio as a function of frequency and find that the maximum of the oscillation envelope is shifted to higher frequencies in photometry than in velocity.

MOST Photometry and DDO Spectroscopy of the Eclipsing (White Dwarf + Red Dwarf) Binary V471 Tau*

The Hyades K2 V + WD system 471 Tau is a prototype postYcommon envelope system and a likely cataclysmic binary progenitor. We present 10 days of nearly continuous optical photometry by the MOST (Microvariability and Oscillations of Stars) satellite and partly simultaneous optical spectroscopy from DDO (David Dunlap Observatory) of the binary. The photometric data indicate that the spot coverage of the K dwarf component was less than observed inthepast,suggestingthatwemonitoredthestarclosetoaminimuminitsactivitycycle.Despitethelowspotactivity, we still detected seven flarelike events whose estimated energies are among the highest ever observed in V471 Tau

Analysis of variability of TW Hya as observed by MOST and ASAS in 2009

As a continuation of our previous studies in 2007 and 2008, new photometric observations of the T Tauri star TW Hya obtained by the MOST satellite and the All Sky Automated Survey (ASAS) project over 40 d in 2009 with temporal resolution of 0.2 d are presented. A wavelet analysis of the combined MOST-ASAS data provides a rich picture of coherent, intermittent, variable-period oscillations, similarly as discovered in the 2008 data. The periods (1.3-10 d) and systematic period shortening on time-scales of weeks can be interpreted within the model of magnetorotationally controlled accretion processes in the inner accretion disc around the star. Within this model and depending on the assumed visibility of plasma parcels causing the oscillations, the observed shortest oscillation period may indicate the stellar rotation period of 1.3 or 2.6 d, synchronized with the disc at 4.5 or 7.1 R ⊙ , respectively.

Analysis of the MOST light curve of the heavily spotted K2IV component of the single-line spectroscopic binary II Pegasi★

Continuous photometric observations of the visible component of the single-line, K2IV spectroscopic binary II Peg carried out by the MOST satellite during 31 consecutive days in 2008 have been analyzed. On top of spot-induced brightness modulation, eleven flares were detected of three distinct types characterized by different values of rise, decay and duration times. The flares showed a preference for occurrence at rotation phases when the most spotted hemisphere is directed to the observer, confirming previous similar reports. An attempt to detect a grazing primary minimum caused by the secondary component transiting in front of the visible star gave a negative result. The brightness variability caused by spots has been interpreted within a cold spot model. An assumption of differential rotation of the primary component gave a better fit to the light curve than a solid-body rotation model.

Analysis of MOST light curves of five young stars in Taurus–Auriga and Lupus 3 star-forming regions

Continuous photometric observations of five young stars obtained by the MOST satellite in 2009 and 2010 in the Taurus and Lupus star formation regions are presented. Using light-curve modelling under the assumption of internal invariability of spots, we obtained small values of the solar-type differential-rotation parameter (k = 0.0005–0.009) for three spotted weak-line T Tauri stars, V410 Tau, V987 Tau and Lupus 3-14; for another spotted weak-line T Tauri star (WTTS), Lupus 3-48, the data are consistent with a rigidly rotating surface (k = 0). Three flares of similar rise (4 min and 30 s) and decay (1 h and 45 min) times were detected in the light curve of Lupus 3-14. The brightness of the classical T Tauri star RY Tau continuously decreased over 3 weeks of its observations with a variable modulation not showing any obvious periodic signal.