The effect of metallicity on the granulation activity in stars is still poorly understood. Available spectroscopic parameters from the updated APOGEE-\textit{Kepler} catalog, coupled with ...high-precision photometric observations from NASA's \textit{Kepler} mission spanning more than four years of observation, make oscillating red giant stars in open clusters crucial testbeds. We determine the role of metallicity on the stellar granulation activity by discriminating its effect from that of different stellar properties such as surface gravity, mass, and temperature. We analyze 60 known red giant stars belonging to the open clusters NGC 6791, NGC 6819, and NGC 6811, spanning a metallicity range from Fe/H \(\simeq -0.09\) to \(0.32\). The parameters describing the granulation activity of these stars and their \(\nu_\mathrm{max}\), are studied by considering the different masses, metallicities, and stellar evolutionary stages. We derive new scaling relations for the granulation activity, re-calibrate existing ones, and identify the best scaling relations from the available set of observations. We adopted the Bayesian code DIAMONDS for the analysis of the background signal in the Fourier spectra of the stars. We performed a Bayesian parameter estimation and model comparison to test the different model hypotheses proposed in this work and in the literature. Metallicity causes a statistically significant change in the amplitude of the granulation activity, with a dependency stronger than that induced by both stellar mass and surface gravity. We also find that the metallicity has a significant impact on the corresponding time scales of the phenomenon. The effect of metallicity on the time scale is stronger than that of mass. A higher metallicity increases the amplitude of granulation and meso-granulation signals and slows down their characteristic time scales toward longer periods.
Starting in December 2014, Kepler K2 observed Neptune continuously for 49 days at a 1-minute cadence. The goals consisted of studying its atmospheric dynamics (Simon et al. 2016), detecting its ...global acoustic oscillations (Rowe et al., submitted), and those of the Sun, which we report on here. We present the first indirect detection of solar oscillations in intensity measurements. Beyond the remarkable technical performance, it indicates how Kepler would see a star like the Sun. The result from the global asteroseismic approach, which consists of measuring the oscillation frequency at maximum amplitude "nu_max" and the mean frequency separation between mode overtones "Delta nu", is surprising as the nu_max measured from Neptune photometry is larger than the accepted value. Compared to the usual reference nu_max_sun = 3100 muHz, the asteroseismic scaling relations therefore make the solar mass and radius appear larger by 13.8 +/- 5.8 % and 4.3 +/- 1.9 % respectively. The higher nu_max is caused by a combination of the value of nu_max_sun, being larger at the time of observations than the usual reference from SOHO/VIRGO/SPM data (3160 +/- 10 muHz), and the noise level of the K2 time series, being ten times larger than VIRGO's. The peak-bagging method provides more consistent results: despite a low signal-to-noise ratio (SNR), we model ten overtones for degrees l=0,1,2. We compare the K2 data with simultaneous SOHO/VIRGO/SPM photometry and BiSON velocity measurements. The individual frequencies, widths, and amplitudes mostly match those from VIRGO and BiSON within 1 sigma, except for the few peaks with lowest SNR.
Binary star systems are important for understanding stellar structure and evolution, and are especially useful when oscillations can be detected and analysed with asteroseismology. However, only four ...systems are known in which solar-like oscillations are detected in both components. Here, we analyse the fifth such system, HD 176465, which was observed by Kepler. We carefully analysed the system's power spectrum to measure individual mode frequencies, adapting our methods where necessary to accommodate the fact that both stars oscillate in a similar frequency range. We also modelled the two stars independently by fitting stellar models to the frequencies and complementary parameters. We are able to cleanly separate the oscillation modes in both systems. The stellar models produce compatible ages and initial compositions for the stars, as is expected from their common and contemporaneous origin. Combining the individual ages, the system is about 3.0\(\pm\)0.5 Gyr old. The two components of HD 176465 are young physically-similar oscillating solar analogues, the first such system to be found, and provide important constraints for stellar evolution and asteroseismology.
$\theta$ Cygni is an F3 spectral-type main-sequence star with visual
magnitude V=4.48. This star was the brightest star observed by the original
Kepler spacecraft mission. Short-cadence (58.8 s) ...photometric data using a
custom aperture were obtained during Quarter 6 (June-September 2010) and
subsequently in Quarters 8 and 12-17. We present analyses of the solar-like
oscillations based on Q6 and Q8 data, identifying angular degree $l$ = 0, 1,
and 2 oscillations in the range 1000-2700 microHz, with a large frequency
separation of 83.9 plus/minus 0.4 microHz, and frequency with maximum amplitude
1829 plus/minus 54 microHz. We also present analyses of new ground-based
spectroscopic observations, which, when combined with angular diameter
measurements from interferometry and Hipparcos parallax, give T_eff = 6697
plus/minus 78 K, radius 1.49 plus/minus 0.03 solar radii, Fe/H = -0.02
plus/minus 0.06 dex, and log g = 4.23 plus/minus 0.03. We calculate stellar
models matching the constraints using several methods, including using the Yale
Rotating Evolution Code and the Asteroseismic Modeling Portal. The best-fit
models have masses 1.35-1.39 solar masses and ages 1.0-1.6 Gyr. theta Cyg's
T_eff and log g place it cooler than the red edge of the gamma Doradus
instability region established from pre-Kepler ground-based observations, but
just at the red edge derived from pulsation modeling. The pulsation models show
gamma Dor gravity-mode pulsations driven by the convective-blocking mechanism,
with frequencies of 1 to 3 cycles/day (11 to 33 microHz). However, gravity
modes were not detected in the Kepler data, one signal at 1.776 cycles/day
(20.56 microHz) may be attributable to a faint, possibly background, binary.
Asteroseismic studies of theta Cyg and other A-F stars observed by Kepler and
CoRoT, will help to improve stellar model physics and to test pulsation driving
mechanisms.
The Sloan Digital Sky Survey IV extended Baryonic Oscillation Spectroscopic Survey (SDSS-IV/eBOSS) will observe 195,000 emission-line galaxies (ELGs) to measure the Baryonic Acoustic Oscillation ...standard ruler (BAO) at redshift 0.9. To test different ELG selection algorithms, 9,000 spectra were observed with the SDSS spectrograph as a pilot survey based on data from several imaging surveys. First, using visual inspection and redshift quality flags, we show that the automated spectroscopic redshifts assigned by the pipeline meet the quality requirements for a reliable BAO measurement. We also show the correlations between sky emission, signal-to-noise ratio in the emission lines, and redshift error. Then we provide a detailed description of each target selection algorithm we tested and compare them with the requirements of the eBOSS experiment. As a result, we provide reliable redshift distributions for the different target selection schemes we tested. Finally, we determine an target selection algorithms that is best suited to be applied on DECam photometry because they fulfill the eBOSS survey efficiency requirements.
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