R. Howe

Asteroseismology of the solar analogs 16 Cyg A and B from Kepler observations

The evolved solar-type stars 16 Cyg A and B have long been studied as solar analogs, yielding a glimpse into the future of our own Sun. The orbital period of the binary system is too long to provide meaningful dynamical constraints on the stellar properties, but asteroseismology can help because the stars are among the brightest in the Kepler field. We present an analysis of three months of nearly uninterrupted photometry of 16 Cyg A and B from the Kepler space telescope. We extract a total of 46 and 41 oscillation frequencies for the two components, respectively, including a clear detection of octupole (l = 3) modes in both stars. We derive the properties of each star independently using the Asteroseismic Modeling Portal, fitting the individual oscillation frequencies and other observational constraints simultaneously. We evaluate the systematic uncertainties from an ensemble of results generated by a variety of stellar evolution codes and fitting methods. The optimal models derived by fitting each component individually yield a common age (t = 6.8 ± 0.4 Gyr) and initial composition (Z i = 0.024 ± 0.002, Y i = 0.25 ± 0.01) within the uncertainties, as expected for the components of a binary system, bolstering our confidence in the reliability of asteroseismic techniques. The longer data sets that will ultimately become available will allow future studies of differential rotation, convection zone depths, and long-term changes due to stellar activity cycles.

Oscillation mode frequencies of 61 main sequence and subgiant stars observed by Kepler

Context. Solar-like oscillations have been observed by Kepler and CoRoT in several solar-type stars, thereby providing a way to probe the stars using asteroseismology Aims. We provide the mode frequencies of the oscillations of various stars required to perform a comparison with those obtained from stellar modelling. Methods. We used a time series of nine months of data for each star. The 61 stars observed were categorised in three groups: simple, F-like, and mixed-mode. The simple group includes stars for which the identification of the mode degree is obvious. The F-like group includes stars for which the identification of the degree is ambiguous. The mixed-mode group includes evolved stars for which the modes do not follow the asymptotic relation of low-degree frequencies. Following this categorisation, the power spectra of the 61 main-sequence and subgiant stars were analysed using both maximum likelihood estimators and Bayesian estimators, providing individual mode characteristics such as frequencies, linewidths, and mode heights. We developed and describe a methodology for extracting a single set of mode frequencies from multiple sets derived by different methods and individual scientists. We report on how one can assess the quality of the fitted parameters using the likelihood ratio test and the posterior probabilities. Results. We provide the mode frequencies of 61 stars (with their 1-σ error bars), as well as their associated echelle diagrams.

PROPERTIES OF 42 SOLAR-TYPE KEPLER TARGETS FROM THE ASTEROSEISMIC MODELING PORTAL

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. 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.

A determination of the crystal structure of ice XI

Below 72 K ice doped with KOH transforms to an ordered phase known as ice XI. A powdered sample of such doped ice annealed for several days below 72 K was studied by neutron diffraction and found to be a two‐phase mixture of ice Ih and ice XI. The ice XI has an orthorhombic unit cell, and the diffraction profile is consistent with the predicted Cmc21 structure.

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

Context. 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 μHz and 0.7 μHz, 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.

Deciphering solar magnetic activity. I. On the relationship between the sunspot cycle and the evolution of small magnetic features

Sunspots are a canonical marker of the Suns internal magnetic field which flips polarity every ~22 yr. The principal variation of sunspots, an ~11 yr variation, modulates the amount of the magnetic field that pierces the solar surface and drives significant variations in our stars radiative, particulate, and eruptive output over that period. This paper presents observations from the Solar and Heliospheric Observatory and Solar Dynamics Observatory indicating that the 11 yr sunspot variation is intrinsically tied to the spatio-temporal overlap of the activity bands belonging to the 22 yr magnetic activity cycle. Using a systematic analysis of ubiquitous coronal brightpoints and the magnetic scale on which they appear to form, we show that the landmarks of sunspot cycle 23 can be explained by considering the evolution and interaction of the overlapping activity bands of the longer-scale variability.

On the variation of the peak asymmetry of low-ł solar p modes

The resonant peaks of solar p modes show small amounts of asymmetry in frequency. Here, we use five independent sets of contemporaneous data, collected over a 8 yr period, to investigate whether peak asymmetry in low angular degree p modes changes over the solar activity cycle. Three of the data sets are from instruments on board the ESA/NASA SOHO spacecraft (GOLF, MDI, and VIRGO/SPM); and two are from ground-based networks (BiSON and GONG). Evidence for variation in asymmetry, well correlated with the activity cycle, is uncovered in the GOLF and BiSON Doppler velocity data. Suggestions of a similar trend are present in the GONG Doppler velocity data. Apparent changes in the MDI Doppler velocity data are somewhat less significant. Meanwhile, analysis of the SPM intensity data failed to uncover any evidence for significant change of the asymmetry parameter.

The High-latitude Branch of the Solar Torsional Oscillation in the Rising Phase of Cycle 24

We use global heliseismic data from the Global Oscillation Network Group, the Michelson Doppler Imager on board the Solar and Heliospheric Observatory, and the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory, to examine the behavior, during the rising phase of Solar Cycle 24, of the migrating zonal flow pattern known as the torsional oscillation. Although the high-latitude part of the pattern appears to be absent in the new cycle when the flows are derived by subtracting a mean across a full solar cycle, it can be seen if we subtract the mean over a shorter period in the rising phase of each cycle, and these two mean rotation profiles differ significantly at high latitudes. This indicates that the underlying high-latitude rotation has changed; we speculate that this is in response to weaker polar fields, as suggested by a recent model.

The configurational entropy of partially ordered ice

A calculation is presented for the configurational entropy arising from partial disorder of the protons in ice. The result is given in terms of a single order parameter, and differs from the usually accepted formula of Nagle. The new result is justified by a simple argument for the case of almost complete ordering.

Oscillation mode linewidths and heights of 23 main-sequence stars observed by Kepler

Context. Solar-like oscillations have been observed by Kepler and CoRoT in many solar-type stars, thereby providing a way to probe the stars using asteroseismology.Aims. We provide the mode linewidths and mode heights of the oscillations of various stars as a function of frequency and of effective temperature.Methods. We used a time series of nearly two years of data for each star. The 23 stars observed belong to the simple or F-like category. The power spectra of the 23 main-sequence stars were analysed using both maximum likelihood estimators and Bayesian estimators, providing individual mode characteristics such as frequencies, linewidths, and mode heights. We study the source of systematic errors in the mode linewidths and mode heights, and we present a way to correct these errors with respect to a common reference fit.Results. Using the correction, we can explain all sources of systematic errors, which could be reduced to less than ±15% for mode linewidths and heights, and less than ±5% for amplitude, when compared to the reference fit. The effect of a different estimated stellar background and a different estimated splitting will provide frequency-dependent systematic errors that might affect the comparison with theoretical mode linewidth and mode height, therefore affecting the understanding of the physical nature of these parameters. All other sources of relative systematic errors are less dependent upon frequency. We also provide the dependence of the so-called linewidth dip in the middle of the observed frequency range as a function of effective temperature. We show that the depth of the dip decreases with increasing effective temperature. The dependence of the dip on effective temperature may imply that the mixing length parameter α or the convective flux may increase with effective temperature.