In asteroseismology, the surface effect refers to a disparity between the observed and the modelled frequencies in stars with solar-like oscillations. It originates from improper modelling of the ...surface layers. Correcting the surface effect usually requires using functions with free parameters, which are conventionally fitted to the observed frequencies. On the basis that the correction should vary smoothly across the H--R diagram, we parameterize it as a simple function of surface gravity, effective temperature, and metallicity. We determine this function by fitting a wide range of stars. The absolute amount of the surface correction decreases with luminosity, but the ratio between it and \(\nu_{\rm max}\) increases, suggesting the surface effect is more important for red giants than dwarfs. Applying the prescription can eliminate unrealistic surface correction, which improves parameter estimations with stellar modelling. Using two open clusters, we found a reduction of scatter in the model-derived ages for each star in the same cluster. As an important application, we provide a new revision for the \(\Delta\nu\) scaling relation that, for the first time, accounts for the surface correction. The values of the correction factor, \(f_{\Delta\nu}\), are up to 2\% smaller than those determined without the surface effect considered, suggesting decreases of up to 4\% in radii and up to 8\% in masses when using the asteroseismic scaling relations. This revision brings the asteroseismic properties into an agreement with those determined from eclipsing binaries. The new correction factor and the stellar models with the corrected frequencies are available at {https://www.github.com/parallelpro/surface}.
Detached eclipsing binaries allow stellar masses and radii to be measured
with unrivalled accuracy. While inspecting light curves obtained with the
Transiting Exoplanet Survey Satellite (TESS), we ...noticed that the A0 III star
$\alpha$ Dra shows clear and well-separated primary and secondary eclipses.
This star is known to be a single-lined spectroscopic binary, with a period of
51.5 d and an eccentricity of 0.43. The currently available TESS observations
cover two 27-d sectors and the light curve shows a primary eclipse (depth 9%)
and a secondary eclipse (depth 2%), separated in time by 38.5 days. Additional
TESS observations of $\alpha$ Dra will come from TESS Sectors 16, 21 and 22,
and we predict that an eclipse will be visible in each of these. With a $V$
magnitude of 3.68, $\alpha$ Dra is one of the brightest known detached
eclipsing binaries.
Gyrochronology, a valuable tool for determining ages of low-mass stars where other techniques fail, relies on accurate calibration. We present a sample of 327 wide ($>$$100\(\,au) white dwarf + main ...sequence (WD + MS) binary systems. Total ages of WDs are computed using all-sky survey photometry, Gaia parallaxes, and current hydrogen atmosphere WD models. Using a magnetic braking law calibrated against open clusters, along with assumptions about initial conditions and angular momentum transport, we construct gyrochrones to predict the rotation periods of the MS stars. Both data and models show that, near the fully convective boundary, MS stars with WD ages up to 7.5\,Gyr experience a rotation period increase by up to a factor of \)\approx$$3\( within a \)<50\,\mathrm{K}\( effective temperature range. We suggest that rapid braking at this boundary is driven by a sharp rise in the convective overturn timescale (\)\tau_{\mathrm{cz}}\() caused by structural changes between partially and fully convective stars and the \)^3 \textrm{He}\( instability occurring at this boundary. While the specific location in mass (or temperature) of this feature varies with model physics, we argue that its existence remains consistent. Stars along this feature exhibit rotation periods that can be mapped, within 1\)\sigma\(, to a range of gyrochrones spanning \)\approx 6\(\, Gyr. Due to current temperature errors (\)\simeq$$50\,\mathrm{K}$), this implies that a measured rotation period cannot be uniquely associated to a single gyrochrone, implying that gyrochronology may not be feasible for M dwarfs very close to the fully convective boundary.
We search for transits around all known pulsating {\delta} Sct variables (6500 K < Teff < 10 000 K) in the long-cadence Kepler data after subtracting the pulsation signal through an automated ...routine. To achieve this, we devise a simple and computationally inexpensive method for distinguishing between low-frequency pulsations and transits in light curves. We find 3 new candidate transit events that were previously hidden behind the pulsations, but caution that they are likely to be false positive events. We also examined the Kepler Objects of Interest catalog and identify 13 additional host stars which show {\delta} Sct pulsations. For each star in our sample, we use the non-detection of pulsation timing variations for a planet that is known to be transiting a {\delta} Sct variable to obtain both an upper limit on the mass of the planet and the expected radial velocity semi-amplitude of the host star. Simple injection tests of our pipeline imply 100% recovery for planets of 0.5 RJup or greater. Extrapolating our number of Kepler {\delta} Sct stars, we expect 12 detectable planets above 0.5 RJup in TESS. Our sample contains some of the hottest known transiting planets around evolved stars, and is the first complete sample of transits around {\delta} Sct variables. We make available our code and pulsation-subtracted light curves to facilitate further analysis.
A star expands to become a red giant when it has fused all the hydrogen in its core into helium. If the star is in a binary system, its envelope can overflow onto its companion or be ejected into ...space, leaving a hot core and potentially forming a subdwarf-B star. However, most red giants that have partially transferred envelopes in this way remain cool on the surface and are almost indistinguishable from those that have not. Among \(\sim\)7000 helium-burning red giants observed by NASA's Kepler mission, we use asteroseismology to identify two classes of stars that must have undergone dramatic mass loss, presumably due to stripping in binary interactions. The first class comprises about 7 underluminous stars with smaller helium-burning cores than their single-star counterparts. Theoretical models show that these small cores imply the stars had much larger masses when ascending the red giant branch. The second class consists of 32 red giants with masses down to 0.5 M\(_\odot\), whose implied ages would exceed the age of the universe had no mass loss occurred. The numbers are consistent with binary statistics, and our results open up new possibilities to study the evolution of post-mass-transfer binary systems.
We describe the discovery of a solar neighborhood (d=468 pc) binary system with a main-sequence sunlike star and a massive non-interacting black hole candidate. The spectral energy distribution (SED) ...of the visible star is described by a single stellar model. We derive stellar parameters from a high signal-to-noise Magellan/MIKE spectrum, classifying the star as a main-sequence star with \(T_{\rm eff} = 5972 \rm K\), \(\log{g} = 4.54\), and \(M = 0.91\) \msun. The spectrum shows no indication of a second luminous component. To determine the spectroscopic orbit of the binary, we measured radial velocities of this system with the Automated Planet Finder, Magellan, and Keck over four months. We show that the velocity data are consistent with the \textit{Gaia} astrometric orbit and provide independent evidence for a massive dark companion. From a combined fit of our spectroscopic data and the astrometry, we derive a companion mass of \(11.39^{+1.51}_{-1.31}\)\msun. We conclude that this binary system harbors a massive black hole on an eccentric \((e =0.46 \pm 0.02)\), \(185.4 \pm 0.1\) d orbit. These conclusions are independent of \cite{ElBadry2022Disc}, who recently reported the discovery of the same system. A joint fit to all available data (including \cite{ElBadry2022Disc}'s) yields a comparable period solution, but a lower companion mass of \(9.32^{+0.22}_{-0.21} M_{\odot}\). Radial velocity fits to all available data produce a unimodal solution for the period that is not possible with either data set alone. The combination of both data sets yields the most accurate orbit currently available.
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.
We observed HD 19467 B with JWST's NIRCam in six filters spanning 2.5-4.6 \(\mu m\) with the Long Wavelength Bar coronagraph. The brown dwarf HD 19467 B was initially identified through a long-period ...trend in the radial velocity of G3V star HD 19467. HD 19467 B was subsequently detected via coronagraphic imaging and spectroscopy, and characterized as a late-T type brown dwarf with approximate temperature \(\sim1000\)K. We observed HD 19467 B as a part of the NIRCam GTO science program, demonstrating the first use of the NIRCam Long Wavelength Bar coronagraphic mask. The object was detected in all 6 filters (contrast levels of \(2\times10^{-4}\) to \(2\times10^{-5}\)) at a separation of 1.6 arcsec using Angular Differential Imaging (ADI) and Synthetic Reference Differential Imaging (SynRDI). Due to a guidestar failure during acquisition of a pre-selected reference star, no reference star data was available for post-processing. However, RDI was successfully applied using synthetic Point Spread Functions (PSFs) developed from contemporaneous maps of the telescope's optical configuration. Additional radial velocity data (from Keck/HIRES) are used to constrain the orbit of HD 19467 B. Photometric data from TESS are used to constrain the properties of the host star, particularly its age. NIRCam photometry, spectra and photometry from literature, and improved stellar parameters are used in conjunction with recent spectral and evolutionary substellar models to derive physical properties for HD 19467 B. Using an age of 9.4\(\pm\)0.9 Gyr inferred from spectroscopy, Gaia astrometry, and TESS asteroseismology, we obtain a model-derived mass of 62\(\pm 1M_{J}\), which is consistent within 2-\(\sigma\) with the dynamically derived mass of 81\(^{+14}_{-12}M_{J}\).
We report the discovery and characterisation of TIC 350842552 ("Zvrk"), an apparently isolated, rapidly-rotating (\(P_\text{rot} \sim 99\ \mathrm{d}\)) red giant observed by TESS in its Southern ...Continuous Viewing Zone. The star's fast surface rotation is independently verified by the use of p-mode asteroseismology, strong periodicity in TESS and ASAS-SN photometry, and measurements of spectroscopic rotational broadening. A two-component fit to APOGEE spectra indicates a coverage fraction of its surface features consistent with the amplitude of the photometric rotational signal. Variations in the amplitude of its photometric modulations over time suggest the evolution of its surface morphology, and therefore enhanced magnetic activity. We further develop and deploy new asteroseismic techniques to characterise radial differential rotation, and find weak evidence for rotational shear within Zvrk's convective envelope. This feature, in combination with such a high surface rotation rate, is incompatible with models of angular-momentum transport in single-star evolution. Spectroscopic abundance estimates also indicate a high lithium abundance, among other chemical anomalies. Taken together, all of these suggest a planet-ingestion scenario for the formation of this rotational configuration, various models for which we examine in detail.
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'.
