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Spectroscopic and seismic analysis of red giants in eclipsing binaries discovered by Kepler

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 Added by Patrick Gaulme
 Publication date 2021
  fields Physics
and research's language is English




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Eclipsing binaries (EBs) are unique benchmarks for stellar evolution. On the one hand, detached EBs hosting at least one star with detectable solar-like oscillations constitute ideal test objects to calibrate asteroseismic measurements. On the other hand, the oscillations and surface activity of stars that belong to EBs offer unique information about the evolution of binary systems. This paper builds upon previous works dedicated to red giant stars (RG) in EBs -- 20 known systems so far -- discovered by the NASA Kepler mission. Here we report the discovery of 16 RGs in EBs also from the Kepler data. This new sample includes three SB2-EBs with oscillations and six close systems where the RG display a clear surface activity and complete oscillation suppression. Based on dedicated high-resolution spectroscopic observations (Apache Point Observatory, Observatoire de Haute Provence), we focus on three main aspects. From the extended sample of 14 SB2-EBs, we first confirm that the simple application of the asteroseismic scaling relations to RGs overestimates masses and radii of RGs, by about 15% and 5%. This bias can be reduced by employing either new asteroseismic reference values for RGs, or model-based corrections of the asteroseismic parameters. Secondly, we confirm that close binarity leads to a high level of photometric modulation (up to 10%), and a suppression of solar-like oscillations. In particular, we show that it reduces the lifetime of radial modes by a factor of up to 10. Thirdly, we use our 16 new systems to complement previous observational studies that aimed at constraining tidal dissipation in interacting binaries. In particular, we identify systems with circular orbits despite relatively young ages, which suggests exploring complementary tidal dissipation mechanisms in the future. Finally, we report the measurements of mass, radius, and age of three M-dwarf companion stars.



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92 - Gang Li , Zhao Guo , Jim Fuller 2020
We systematically searched for gravity- and Rossby-mode period spacing patterns in Kepler eclipsing binaries with $gamma$ Doradus pulsators. These stars provide an excellent opportunity to test the theory of tidal synchronisation and angular momentum transport in F- and A-type stars. We discovered 35 systems that show clear patterns, including the spectroscopic binary KIC 10080943. Combined with 45 non-eclipsing binaries with $gamma$ Dor components that have been found using pulsation timing, we measured their near-core rotation rates and asymptotic period spacings. We find that many stars are tidally locked if the orbital periods are shorter than 10 days, in which the near-core rotation periods given by the traditional approximation of rotation (TAR) are consistent with the orbital period. Compared to the single stars, $gamma$ Dor stars in binaries tend to have slower near-core rotation rates, likely a consequence of tidal spin-down. We also find three stars that have extremely slow near-core rotation rates. To explain these, we hypothesise that unstable tidally excited oscillations can transfer angular momentum from the star to the orbit, and slow the star below synchronism, a process we refer to as `inverse tides.
The recently launched TESS mission is for the first time giving us the potential to perform inference asteroseismology across the whole sky. TESS observed the Kepler field entirely in its Sector 14 and partly in Sector 15. Here, we seek to detect oscillations in the red giants observed by TESS in the Kepler field of view. Using the full 4-yr Kepler results as the ground truth, we aim to characterise how well the seismic signal can be detected using TESS data. Because our data are based on one and two sectors of observation, our results will be representative of what one can expect for the vast majority of the TESS data. We detect clear oscillations in $sim$3000 stars with another $sim$1000 borderline (low S/N) cases, all of which yield a measurement of the frequency of maximum acoustic power, numax. In comparison, a simple calculation predicts $sim$4500 stars would show detectable oscillations. Of the clear detections we reliably measure the frequency separation between overtone radial modes, dnu, in 570 stars, meaning an overall dnu yield of 20%, which splits into a one-sector yield of 14% and a two-sector yield of 26%. These yields imply that typical (1-2 sector) TESS data will result in significant detection biases. Hence, to boost the number of stars, one might need to use only numax as the seismic input for stellar property estimation. On the up side, we find little or no bias in the seismic measurements and typical scatter relative to the Kepler `truth is about 5-6% in numax and 2-3% in dnu. These values, coupled with typical uncertainties in parallax, Teff, and Fe/H in a grid-based approach, would provide internal uncertainties of 3% in inferred stellar radius, 6% in mass and 20% in age. Finally, despite relatively large pixels of TESS, we find red giant seismology is not expected to be significantly affected by blending for stars with Tmag < 12.5.
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.
The granulation pattern that we observe on the surface of the Sun is due to hot plasma from the interior rising to the photosphere where it cools down, and descends back into the interior at the edges of granules. This is the visible manifestation of convection taking place in the outer part of the solar convection zone. Because red giants have deeper convection zones and more extended atmospheres than the Sun, we cannot a priori assume that granulation in red giants is a scaled version of solar granulation. Until now, neither observations nor 1D analytical convection models could put constraints on granulation in red giants. However, thanks to asteroseismology, this study can now be performed. The resulting parameters yield physical information about the granulation. We analyze sim1000 red giants that have been observed by Kepler during 13 months. We fit the power spectra with Harvey-like profiles to retrieve the characteristics of the granulation (time scale tau_gran and power P_gran). We also introduce a new time scale, tau_eff, which takes into account that different slopes are used in the Harvey functions. We search for a correlation between these parameters and the global acoustic-mode parameter (the position of maximum power, nu_max) as well as with stellar parameters (mass, radius, surface gravity (log g) and effective temperature (T_eff)). We show that tau_eff nu_max^{-0.89} and P_gran nu_max^{-1.90}, which is consistent with the theoretical predictions. We find that the granulation time scales of stars that belong to the red clump have similar values while the time scales of stars in the red-giant branch are spread in a wider range. Finally, we show that realistic 3D simulations of the surface convection in stars, spanning the (T_eff, log g)-range of our sample of red giants, match the Kepler observations well in terms of trends.
120 - D. R. Gies , R. A. Matson , Z. Guo 2015
Many short-period binary stars have distant orbiting companions that have played a role in driving the binary components into close separation. Indirect detection of a tertiary star is possible by measuring apparent changes in eclipse times of eclipsing binaries as the binary orbits the common center of mass. Here we present an analysis of the eclipse timings of 41 eclipsing binaries observed throughout the NASA Kepler mission of long duration and precise photometry. This subset of binaries is characterized by relatively deep and frequent eclipses of both stellar components. We present preliminary orbital elements for seven probable triple stars among this sample, and we discuss apparent period changes in seven additional eclipsing binaries that may be related to motion about a tertiary in a long period orbit. The results will be used in ongoing investigations of the spectra and light curves of these binaries for further evidence of the presence of third stars.
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