ABSTRACT We report the discovery of Kepler-432b, a giant planet ( , ) transiting an evolved star ( ) with an orbital period of days. Radial velocities (RVs) reveal that Kepler-432b orbits its parent ...star with an eccentricity of , which we also measure independently with asterodensity profiling (AP; ), thereby confirming the validity of AP on this particular evolved star. The well-determined planetary properties and unusually large mass also make this planet an important benchmark for theoretical models of super-Jupiter formation. Long-term RV monitoring detected the presence of a non-transiting outer planet (Kepler-432c; , days), and adaptive optics imaging revealed a nearby ( ), faint companion (Kepler-432B) that is a physically bound M dwarf. The host star exhibits high signal-to-noise ratio asteroseismic oscillations, which enable precise measurements of the stellar mass, radius, and age. Analysis of the rotational splitting of the oscillation modes additionally reveals the stellar spin axis to be nearly edge-on, which suggests that the stellar spin is likely well aligned with the orbit of the transiting planet. Despite its long period, the obliquity of the 52.5 day orbit may have been shaped by star-planet interaction in a manner similar to hot Jupiter systems, and we present observational and theoretical evidence to support this scenario. Finally, as a short-period outlier among giant planets orbiting giant stars, study of Kepler-432b may help explain the distribution of massive planets orbiting giant stars interior to 1 AU.
We present measurements of the dipole mode asymptotic period spacing (\(\Delta\Pi_1\)), the coupling factor between p- and g- modes (\(q\)), the g-mode phase offset (\(\epsilon_g\)), and the ...mixed-mode frequency rotational splitting (\(\delta\nu_{\mathrm{rot}}\)) for 1,074 low-luminosity red giants from the Kepler mission. Using oscillation mode frequencies extracted from each star, we apply Bayesian optimization to estimate \(\Delta\Pi_1\) from the power spectrum of the stretched period spectrum and to perform the subsequent forward modelling of the mixed-mode frequencies. With our measurements, we show that the mode coupling factor \(q\) shows significant anti-correlation with both stellar mass and metallicity, and can reveal highly metal-poor stars. We present the evolution of \(\epsilon_g\) up the lower giant branch up to before the luminosity bump, and find no significant trends in \(\epsilon_g\) or \(\delta\nu_{\mathrm{rot}}\) with stellar mass and metallicity in our sample. Additionally, we identify six new red giants showing anomalous distortions in their g-mode pattern. Our data products, code, and results are provided in a public repository.
We present HD-TESS, a catalog of 1,709 bright (\(V\sim3-10\)) red giants from the Henry Draper (HD) Catalog with asteroseismic measurements based on photometry from NASA's Transiting Exoplanet Survey ...Satellite (TESS). Using light curves spanning at least six months across a single TESS observing cycle, we provide measurements of global asteroseismic parameters (\(\nu_{\mathrm{max}}\) and \(\Delta\nu\)) and evolutionary state for each star in the catalog. We adopt literature values of atmospheric stellar parameters to estimate the masses and radii of the giants in our catalog using asteroseismic scaling relations, and observe that HD-TESS giants on average have larger masses compared to Kepler red giants. Additionally, we present the discovery of oscillations in 99 red giants in astrometric binary systems, including those with subdwarf or white dwarf companions. Finally, we benchmark radii from asteroseismic scaling relations against those measured using long-baseline interferometry for 18 red giants and find that correction factors to the scaling relations improve the agreement between asteroseismic and interferometric radii to approximately 3%.
Long, high-quality time-series data provided by previous space-missions such as CoRoT and \(\mathit{Kepler}\) have made it possible to derive the evolutionary state of red-giant stars, i.e. whether ...the stars are hydrogen-shell burning around an inert helium core or helium-core burning, from their individual oscillation modes. We utilise data from the \(\mathit{Kepler}\) mission to develop a tool to classify the evolutionary state for the large number of stars being observed in the current era of K2, TESS and for the future PLATO mission. These missions provide new challenges for evolutionary state classification given the large number of stars being observed and the shorter observing duration of the data. We propose a new method, \(\mathtt{Clumpiness}\), based upon a supervised classification scheme that uses "summary statistics" of the time series, combined with distance information from the Gaia mission to predict the evolutionary state. Applying this to red giants in the APOKASC catalogue, we obtain a classification accuracy of ~91% for the full 4 years of \(\mathit{Kepler}\) data, for those stars that are either only hydrogen-shell burning or also helium-core burning. We also applied the method to shorter \(\mathit{Kepler}\) datasets, mimicking CoRoT, K2 and TESS achieving an accuracy >91% even for the 27 day time series. This work paves the way towards fast, reliable classification of vast amounts of relatively short-time-span data with a few, well-engineered features.
With the observations of an unprecedented number of oscillating subgiant stars expected from NASA's TESS mission, the asteroseismic characterization of subgiant stars will be a vital task for stellar ...population studies and for testing our theories of stellar evolution. To determine the fundamental properties of a large sample of subgiant stars efficiently, we developed a deep learning method that estimates distributions of fundamental parameters like age and mass over a wide range of input physics by learning from a grid of stellar models varied in eight physical parameters. We applied our method to four Kepler subgiant stars and compare our results with previously determined estimates. Our results show good agreement with previous estimates for three of them (KIC 11026764, KIC 10920273, KIC 11395018). With the ability to explore a vast range of stellar parameters, we determine that the remaining star, KIC 10005473, is likely to have an age 1 Gyr younger than its previously determined estimate. Our method also estimates the efficiency of overshooting, undershooting, and microscopic diffusion processes, from which we determined that the parameters governing such processes are generally poorly-constrained in subgiant models. We further demonstrate our method's utility for ensemble asteroseismology by characterizing a sample of 30 Kepler subgiant stars, where we find a majority of our age, mass, and radius estimates agree within uncertainties from more computationally expensive grid-based modelling techniques.
Detecting the presence and characteristic scale of a signal is a common problem in data analysis. We develop a fast statistical test of the null hypothesis that a Fourier-like power spectrum is ...consistent with noise. The null hypothesis is rejected where the local "coefficient of variation" (CV)---the ratio of the standard deviation to the mean---in a power spectrum deviates significantly from expectations for pure noise (CV~1.0 for a Chi^2 2-degrees-of-freedom distribution). This technique is of particular utility for detecting signals in power spectra with frequency-dependent noise backgrounds, as it is only sensitive to features that are sharp relative to the inspected frequency bin width. We develop a CV-based algorithm to quickly detect the presence of solar-like oscillations in photometric power spectra that are dominated by stellar granulation. This approach circumvents the need for background fitting to measure the frequency of maximum solar-like oscillation power, nu_max. In this paper, we derive the basic method and demonstrate its ability to detect the pulsational power excesses from the well-studied APOKASC-2 sample of oscillating red giants observed by Kepler. We recover the cataloged nu_max values with an average precision of 2.7% for 99.4% of the stars with 4 years of Kepler photometry. Our method produces false positives for <1% of dwarf stars with nu_max well above the long-cadence Nyquist frequency. The algorithm also flags spectra that exhibit astrophysically interesting signals in addition to single, solar-like oscillation power excesses, which we catalog as part of our characterization of the Kepler light curves of APOKASC-2 targets.
We present the first near all-sky yield of oscillating red giants from the prime mission data of NASA's Transiting Exoplanet Survey Satellite (TESS). We apply machine learning towards long-cadence ...TESS photometry from the first data release by the MIT Quick-Look Pipeline to automatically detect the presence of red giant oscillations in frequency power spectra. The detected targets are conservatively vetted to produce a total of 158,505 oscillating red giants, which is an order of magnitude increase over the yield from Kepler and K2 and a lower limit to the possible yield of oscillating giants across TESS's nominal mission. For each detected target, we report effective temperatures and radii derived from colors and Gaia parallaxes, as well as estimates of their frequency at maximum oscillation power. Using our measurements, we present the first near all-sky Gaia-asteroseismology mass map, which shows global structures consistent with the expected stellar populations of our Galaxy. To demonstrate the strong potential of TESS asteroseismology for Galactic archeology even with only one month of observations, we identify 354 new candidates for oscillating giants in the Galactic halo, display the vertical mass gradient of the Milky Way disk, and visualize correlations of stellar masses with kinematic phase space substructures, velocity dispersions, and \(\alpha\)-abundances.
Asteroseismology is playing an increasingly important role in the
characterization of red-giant host stars and their planetary systems. Here, we
conduct detailed asteroseismic modeling of the evolved ...red-giant branch (RGB)
hosts KOI-3886 and $\iota$ Draconis, making use of end-of-mission Kepler
(KOI-3886) and multi-sector TESS ($\iota$ Draconis) time-series photometry. We
also model the benchmark star KIC 8410637, a member of an eclipsing binary,
thus providing a direct test to the seismic determination. We test the impact
of adopting different sets of observed modes as seismic constraints. Inclusion
of $\ell=1$ and 2 modes improves the precision on the stellar parameters,
albeit marginally, compared to adopting radial modes alone, with $1.9$-$3.0\%$
(radius), $5$-$9\%$ (mass), and $19$-$25\%$ (age) reached when using all
p-dominated modes as constraints. Given the very small spacing of adjacent
dipole mixed modes in evolved RGB stars, the sparse set of observed g-dominated
modes is not able to provide extra constraints, further leading to highly
multimodal posteriors. Access to multi-year time-series photometry does not
improve matters, with detailed modeling of evolved RGB stars based on
(lower-resolution) TESS data sets attaining a precision commensurate with that
based on end-of-mission Kepler data. Furthermore, we test the impact of varying
the atmospheric boundary condition in our stellar models. We find mass and
radius estimates to be insensitive to the description of the near-surface
layers, at the expense of substantially changing both the near-surface
structure of the best-fitting models and the values of associated parameters
like the initial helium abundance, $Y_{\rm i}$. Attempts to measure $Y_{\rm i}$
from seismic modeling of red giants may thus be systematically dependent on the
choice of atmospheric physics.
When main-sequence stars expand into red giants, they are expected to engulf close-in planets. Until now, the absence of planets with short orbital periods around post-expansion, core-helium-burning ...red giants has been interpreted as evidence that short-period planets around Sun-like stars do not survive the giant expansion phase of their host stars. Here we present the discovery that the giant planet 8 Ursae Minoris b orbits a core-helium-burning red giant. At a distance of only 0.5 au from its host star, the planet would have been engulfed by its host star, which is predicted by standard single-star evolution to have previously expanded to a radius of 0.7 au. Given the brief lifetime of helium-burning giants, the nearly circular orbit of the planet is challenging to reconcile with scenarios in which the planet survives by having a distant orbit initially. Instead, the planet may have avoided engulfment through a stellar merger that either altered the evolution of the host star or produced 8 Ursae Minoris b as a second-generation planet. This system shows that core-helium-burning red giants can harbour close planets and provides evidence for the role of non-canonical stellar evolution in the extended survival of late-stage exoplanetary systems.
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