We present the first spectropolarimetric time-series analysis of the Maunder Minimum analog HD 166620, using 12 nights of data from CFHT/SPIRou and a single epoch from CFHT/ESPaDOnS. While individual Stokes V profiles exhibit no significant polarization signatures, we leverage the rotational coverage of the SPIRou dataset to compute a grand average LSD profile. Forward modeling of the cumulative Stokes V signal, assuming a purely axisymmetric dipole, yields a best-fit dipole field strength of Bdip=1.10-0.90+0.95 G (3 sigma). This field strength matches simulations of the solar dipole during the Maunder Minimum phase. Our results are consistent with independent constraints on the dipole field strength from an LBT/PEPSI snapshot and exclude the presence of strong nonaxisymmetric fields potentially missed by this single-epoch observation. These findings provide direct empirical evidence that the transition to weakened magnetic braking involves a weakening of the large-scale magnetic field and suggest that HD 166620 represents a state comparable to the Sun near the peak activity of a grand minimum.

We report revised mass and radius measurements for GJ 367 b, an ultra-short-period (7.7 hr) sub-Earth in a multiplanet system orbiting a nearby (similar to 9 pc) M dwarf host. Previous mass and radius measurements have suggested GJ 367 b has an anomalously high bulk density, close to that of solid iron. The existence of such an iron-rich planet is in tension with established planet formation scenarios. We utilized newly available TESS short-cadence photometry to constrain the radius of GJ 367 b to 0.736 +/- 0.035 R circle plus. We consider observational and modeling effects such as photometric dilution, stellar activity, and tidal distortion to account for possible inaccuracies in the star and planet radius measurements. From our radial velocity analysis using Very Large Telescope (VLT)/ESPRESSO data covering nearly the full orbit in a single night, we find a mass of 0.503 +/- 0.078 M circle plus, corresponding to a bulk density of 6.9 -1.4+1.6 g cm-3. We present a new tidal distortion and interior composition modeling framework to assess the iron mass fraction of GJ 367 b. Considering several different interior composition assumptions and radial aspect ratios, we find an iron fraction of similar to 50%-70%, which is broadly consistent with that of Mercury and not as iron-rich as previously suggested.

Recent observations have shown that sufficiently slow rotation disrupts the organization of large-scale magnetic field in older main-sequence stars, leading to weakened magnetic braking (WMB) and a collapse in the efficiency of the global stellar dynamo. Recent simulations predict a shift from solar-like to antisolar differential rotation (DR) at slower rotation rates, which typically do not occur on the main sequence due to WMB. However, physical expansion on the subgiant branch can eventually slow the stellar rotation beyond this threshold, yielding a noncycling large-scale field that revives magnetic braking. We combine asteroseismology from the Transiting Exoplanet Survey Satellite (TESS) with spectropolarimetry from the Large Binocular Telescope (LBT) to test these predictions in the old metal-rich subgiant 31 Aql. The LBT observations reveal a strong large-scale magnetic field in this star, and archival measurements of its chromospheric emission over 50 yr confirm that it is noncycling, as predicted. The star exhibits a variety of rotation periods during different observing seasons, consistent with DR but with no means of distinguishing between solar-like and antisolar patterns. We incorporate the TESS observations to estimate the current wind-braking torque of 31 Aql, demonstrating that it supports revived magnetic braking in this old subgiant. We also use rotational evolution modeling to place a preliminary constraint on the stellar Rossby number for the transition to antisolar DR. Future refinements in both asteroseismic observations and rotational modeling may yield improvements to this initial analysis.

In this paper we present the public release of the Yale Rotating Evolution Code (YREC). YREC is a stellar evolution code that covers brown dwarfs and stars across a wide range of masses, and evolutionary states from the pre-MS through helium burning. We summarize the key ingredients of the code, document the code performance, and discuss its strengths and limitations. We present libraries of input files, documentation, sample use cases, and scripts. In addition to usage as a research tool, we highlight the utility of the code for educational purposes.

There is an intricate relationship between the organization of large-scale magnetic fields by a stellar dynamo and the rate of angular momentum loss due to magnetized stellar winds. An essential ingredient for the operation of a large-scale dynamo is the Coriolis force, which imprints organizing flows on the global convective patterns and inhibits the complete cancellation of bipolar magnetic regions. Consequently, it is natural to expect a rotational threshold for large-scale dynamo action and for the efficient angular momentum loss that it mediates through magnetic braking. Here we present new observational constraints on magnetic braking for an evolutionary sequence of six early K-type stars. To determine the wind braking torque for each of our targets, we combine spectropolarimetric constraints on the large-scale magnetic field, Ly alpha or X-ray constraints on the mass-loss rate, as well as uniform estimates of the stellar rotation period, mass, and radius. As identified previously from similar observations of hotter stars, we find that the wind braking torque decreases abruptly by more than an order of magnitude at a critical value of the stellar Rossby number. Given that all of the stars in our sample exhibit clear activity cycles, we suggest that weakened magnetic braking may coincide with the operation of a subcritical stellar dynamo.

There is now a large sample of stars observed by the Kepler satellite with measured rotation periods and photometric activity index S _ph . We use this data, in conjunction with stellar interiors models, to explore the interplay of magnetism, rotation, and convection. Stellar activity proxies other than S _ph are correlated with the Rossby number, Ro, or ratio of rotation period to convective overturn timescale. We compute the latter using the Yale Rotating Evolution Code stellar models. We observe different S _ph –Ro relationships for different stellar spectral types. Though the overall trend of decreasing magnetic activity versus Rossby number is recovered, we find a localized dip in S _ph around Ro/Ro _⊙ ∼ 0.3 for the G and K dwarfs. F dwarfs show little to no dependence of S _ph on Rossby number due to their shallow convective zone, further accentuated as T _eff increases. The dip in activity for the G and K dwarfs corresponds to the intermediate rotation period gap, suggesting that the dip in S _ph could be associated with the redistribution of angular momentum between the core and convective envelope inside stars. For G-type stars, we observe enhanced magnetic activity above the solar Rossby number. Compared to other Sun-like stars with similar effective temperature and metallicity, we find that the Sun’s current level of magnetic activity is comparable to its peers and lies near the transition to increasing magnetic activity at high Rossby number. We confirm that metal-rich stars have a systematically larger S _ph level than metal-poor stars, which is likely a consequence of their deeper convective zones.

Weakened magnetic braking (WMB) was originally proposed in 2016 to explain anomalously rapid rotation in old field stars observed by the Kepler mission. The proximate cause was suggested to be a transition in magnetic morphology from larger to smaller spatial scales. In a series of papers over the past 5 yr, we have collected spectropolarimetric measurements to constrain the large-scale magnetic fields for a sample of stars spanning this transition, including a range of spectral types from late F to early K. During this time, we gradually improved our methods for estimating the wind braking torque in each of our targets, and for evaluating the associated uncertainties. Here, we reanalyze the entire sample with a focus on uniformity for the relevant observational inputs. We supplement the sample with two additional active stars to provide more context for the evolution of wind braking torque with stellar Rossby number (Ro). The results demonstrate unambiguously that standard spin-down models can reproduce the evolution of wind braking torque for active stars, but WMB is required to explain the subsequent abrupt decrease in torque as Ro approaches a critical value for dynamo excitation. This transition is seen in both the large-scale magnetic field and the X-ray luminosity, indicating weakened coronal heating. We interpret these transitions as evidence of a rotational threshold for the influence of Coriolis forces on global convective patterns and the resulting inefficiency of the global stellar dynamo.

For nearly a decade, observations have shown that many older Sun-like stars spin faster than predicted, a phenomenon known as weakened magnetic braking (WMB). The leading hypothesis for WMB is a weakening of the large-scale dipole field, which leads to a less efficient angular momentum loss. To test this hypothesis on a star known to be in the WMB regime, we present the first Zeeman Doppler imaging (ZDI) map of the Sun-like star τ Ceti, reconstructed using spectropolarimetric data from the Canada–France–Hawai‘i Telescope. Our ZDI analysis reveals a remarkably simple, stable, and weak (〈 B 〉 = 0.17 G) magnetic field, characterized by a predominantly dipolar (∼92% magnetic energy contained in l = 1 modes), and highly axisymmetric (∼88% magnetic energy contained in m < l /2 modes) morphology. We infer a dipole field strength of B _dip = 0.31 G, nearly an order of magnitude weaker than standard braking model predictions, providing direct confirmation of the weakened large-scale dipole predicted by the WMB hypothesis. This work establishes a new benchmark for ZDI, demonstrating that even extremely quiet stars in the WMB regime are accessible to this technique.

We present the first asteroseismic analysis of the K3 V planet host HD 219134 based on four consecutive nights of radial velocities collected with the Keck Planet Finder. We apply Gold deconvolution to the power spectrum to disentangle modes from side lobes in the spectral window and extract 25 mode frequencies with spherical degrees 0 ≤ ℓ ≤ 3. We derive the fundamental properties using five different evolutionary-modeling pipelines and report a mass of 0.763 ± 0.020 (stat) ± 0.007 (sys) M _⊙ , a radius of 0.748 ± 0.007 (stat) ± 0.002 (sys) R _⊙ , and an age of 10.151 ± 1.520 (stat) ± 0.810 (sys) Gyr. Compared to the interferometric radius 0.783 ± 0.005 R _⊙ , the asteroseismic radius is 4% smaller at the 4 σ level—a discrepancy not easily explained by known interferometric systematics, modeling assumptions on atmospheric boundary conditions and mixing lengths, magnetic fields, or tidal heating. HD 219134 is the first main-sequence star cooler than 5000 K with an asteroseismic age estimate and will serve as a critical calibration point for stellar spin-down relations. We show that existing calibrated prescriptions for angular momentum loss, incorporating weakened magnetic braking with asteroseismically constrained stellar parameters, accurately reproduce the observed rotation period. Additionally, we revised the masses and radii of the super-Earths in the system, which support their having Earth-like compositions. Finally, we confirm that the oscillation amplitude in radial velocity scales as ( L / M ) ^1.5 in K dwarfs, in contrast to the ( L / M ) ^0.7 relation observed in G dwarfs. These findings provide significant insights into the structure and angular momentum loss of K-type stars.

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.

Asteroseismic modeling is a powerful way to derive stellar properties. However, the derived quantities are limited by built-in assumptions used in stellar models. This work presents a detailed characterization of stellar model uncertainties in asteroseismic red giants, focusing on the mixing-length parameter alpha(MLT), the initial helium fraction Y-init, the solar abundance scale, and the overshoot parameters. First, we estimate error floors due to model uncertainties to be approximate to 0.4% in mass, approximate to 0.2% in radius, and approximate to 17% in age, primarily due to the uncertain state of alpha(MLT) and Y-init. The systematic uncertainties in age exceed typical statistical uncertainties, suggesting the importance of their evaluation in asteroseismic applications. Second, we demonstrate that the uncertainties from alpha(MLT) can be entirely mitigated by direct radius measurements or partially through nu(max). Utilizing radii from Kepler eclipsing binaries, we determined the alpha(MLT) values and calibrated the alpha(MLT)-[M/H] relation. The correlation observed between the two variables is positive, consistent with previous studies using 1D stellar models, but in contrast with outcomes from 3D simulations. Third, we explore the implications of using asteroseismic modeling to test the nu(max) scaling relation. We found that a perceived dependency of nu(max) on [M/H] from individual frequency modeling can be largely removed by incorporating the calibrated alpha(MLT)-[M/H] relation. Variations in Y-init can also affect nu(max) predictions. These findings suggest that nu(max) conveys information not fully captured by individual frequencies, and that it should be carefully considered as an important observable for asteroseismic modeling.

ABSTRACT We identify 182 flares on 158 stars within 100 pc of the Sun in both the near-ultraviolet (NUV; $1750\!-\!2750$ Å) and far-ultraviolet (FUV; $1350\!-\!1750$ Å) using high-cadence light curves from the Galaxy Evolution Explorer. Ultraviolet (UV) emission from stellar flares plays a crucial role in determining the habitability of exoplanetary systems. However, whether such UV emission promotes or threatens such life depends strongly on the energetics of these flares. Most studies assessing the effect of flares on planetary habitability assume a 9000 K blackbody spectral energy distribution that produces more NUV flux than FUV flux ($\mathcal {R} \equiv F_{\rm FUV} / F_{\rm NUV} \approx \frac{1}{6}$). Instead, we observe the opposite with the excess FUV reaching $\mathcal {R} \approx \frac{1}{2}\!-\!2$, roughly $3\!-\!12$ times the expectation of a 9000 K blackbody. The ratio of FUV to NUV time-integrated flare energies is 3.0 times higher on average than would be predicted by a constant 9000 K blackbody during the flare. Finally, we find that the FUV/NUV ratio at peak tentatively correlates (${\sim} 2 \sigma$ significance) both with total UV flare energy and with the G − RP colour of the host star. On average, we observe higher FUV/NUV ratios at peak in $E_{\text{UV}}\gt 10^{32}$ erg flares and in flares on fully convective stars.

The solar-type subgiant beta Hyi has long been studied as an old analog of the Sun. Although the rotation period has never been measured directly, it was estimated to be near 27 days. As a Southern Hemisphere target, it was not monitored by long-term stellar activity surveys, but archival International Ultraviolet Explorer data revealed a 12 yr activity cycle. Previous ground-based asteroseismology suggested that the star is slightly more massive and substantially larger and older than the Sun, so the similarity of both the rotation rate and the activity cycle period to solar values is perplexing. We use two months of precise time-series photometry from the Transiting Exoplanet Survey Satellite to detect solar-like oscillations in beta Hyi and determine the fundamental stellar properties from asteroseismic modeling. We also obtain a direct measurement of the rotation period, which was previously estimated from an ultraviolet activity-rotation relation. We then use rotational evolution modeling to predict the rotation period expected from either standard spin-down or weakened magnetic braking (WMB). We conclude that the rotation period of beta Hyi is consistent with WMB and that changes in stellar structure on the subgiant branch can reinvigorate the large-scale dynamo and briefly sustain magnetic activity cycles. Our results support the existence of a "born-again" dynamo in evolved subgiants-previously suggested to explain the cycle in 94 Aqr Aa-which can best be understood within the WMB scenario.

The degree of alignment between a star’s spin axis and the orbital plane of its planets (the stellar obliquity) is related to interesting and poorly understood processes that occur during planet formation and evolution. Hot Jupiters orbiting hot stars (≳6250 K) display a wide range of obliquities, while similar planets orbiting cool stars are preferentially aligned. Tidal dissipation is expected to be more rapid in stars with thick convective envelopes, potentially explaining this trend. Evolved stars provide an opportunity to test the damping hypothesis, particularly stars that were hot on the main sequence and have since cooled and developed deep convective envelopes. We present the first systematic study of the obliquities of hot Jupiters orbiting subgiants that recently developed convective envelopes using Rossiter–McLaughlin observations. Our sample includes two newly discovered systems in the Giants Transiting Giants survey (TOI-6029 b, TOI-4379 b). We find that the orbits of hot Jupiters orbiting subgiants that have cooled below ∼6250 K are aligned or nearly aligned with the spin axis of their host stars, indicating rapid tidal realignment after the emergence of a stellar convective envelope. We place an upper limit for the timescale of realignment for hot Jupiters orbiting subgiants at ∼500 Myr. Comparison with a simplified tidal evolution model shows that obliquity damping needs to be ∼4 orders of magnitude more efficient than orbital period decay to damp the obliquity without destroying the planet, which is consistent with recent predictions for tidal dissipation from inertial waves excited by hot Jupiters on misaligned orbits.

Despite a growing sample of precisely measured stellar rotation periods and ages, the strength of magnetic braking and the degree of departure from standard (Skumanich-like) spin-down have remained persistent questions, particularly for stars more evolved than the Sun. Rotation periods can be measured for stars older than the Sun by leveraging asteroseismology, enabling models to be tested against a larger sample of old field stars. Because asteroseismic measurements of rotation do not depend on starspot modulation, they avoid potential biases introduced by the need for a stellar dynamo to drive starspot production. Using a neural network trained on a grid of stellar evolution models and a hierarchical model-fitting approach, we constrain the onset of weakened magnetic braking (WMB). We find that a sample of stars with asteroseismically measured rotation periods and ages is consistent with models that depart from standard spin-down prior to reaching the evolutionary stage of the Sun. We test our approach using neural networks trained on model grids produced by separate stellar evolution codes with differing physical assumptions and find that the choices of grid physics can influence the inferred properties of the braking law. We identify the normalized critical Rossby number Ro _crit /Ro _⊙ = 0.91 ± 0.03 as the threshold for the departure from standard rotational evolution. This suggests that WMB poses challenges to gyrochronology for roughly half of the main-sequence lifetime of Sun-like stars.

ABSTRACT Determining the precise ages of young (tens to a few hundred Myr) kinematic (‘moving’) groups is important for placing star, protoplanetary disc, and planet observations on an evolutionary timeline. The nearby ∼25 Myr-old β Pictoris Moving Group (BPMG) is an important benchmark for studying stars and planetary systems at the end of the primordial disc phase. Gaia DR3 astrometry and photometry, combined with ground-based observations and more sophisticated stellar models, permit a systematic re-evaluation of BPMG membership and age. We combined Gaia astrometry with previously published radial velocities to evaluate moving group membership in a Bayesian framework. To minimize the effect of unresolved stellar multiplicity on age estimates, we identified and excluded multistar systems using Gaia astrometry, ground-based adaptive optics imaging, and multi-epoch radial velocities, as well as literature identifications. We estimated age using isochrone and lithium-depletion-boundary fitting with models that account for the effect of magnetic activity and spots on young, rapidly rotating stars. We find that age estimates are highly model-dependent; Dartmouth magnetic models with ages of 23 ± 8 and 33$^{+9}_{-11}$ Myr provide best fits to the lithium depletion boundary and Gaia MG versus BP–RP colour–magnitude diagram, respectively, whereas a Dartmouth standard model with an age of 11$^{+4}_{-3}$ Myr provides a best fit to the 2-Micron All-Sky Survey-Gaia$M_{K_S}$ versus BP–RP colour–magnitude diagram.

Stellar spin down is a critical yet poorly understood component of stellar evolution. In particular, results from the Kepler Mission imply that mature age, solar-type stars have inefficient magnetic braking, resulting in a stalled spin-down rate. However, a large number of precise asteroseismic ages are needed for mature (≥3 Gyr) stars in order to probe the regime where traditional and stalled spin-down models differ. In this paper, we present a new asteroseismic benchmark star for gyrochronology discovered using reprocessed Kepler short cadence data. KIC 11029516 (Papayu) is a bright ( Kp = 9.6 mag) solar-type star with a well-measured rotation period (21.1 ± 0.8 days) from spot modulation using 4 yr of Kepler long-cadence data. We combine asteroseismology and spectroscopy to obtain T eff = 5888 ± 100 K, [Fe/H] = 0.30 ± 0.06 dex, M = 1.24 ± 0.05 M ⊙ , R = 1.34 ± 0.02 R ⊙ , and age of 4.0 ± 0.4 Gyr, making Papayu one of the most similar stars to the Sun in terms of temperature and radius with an asteroseismic age and a rotation period measured from spot modulation. We find that Papayu sits at the transition of where traditional and weakened spin-down models diverge. A comparison with stars of similar zero-age main-sequence temperatures supports previous findings that weakened spin-down models are required to explain the ages and rotation periods of old solar-type stars.

White dwarf stars have been used for decades as precise and accurate age indicators. This work presents a test of the reliability of white dwarf total ages when spectroscopic observations are available. We conduct follow-up spectroscopy of 148 individual white dwarfs in widely separated double-white-dwarf (WD+WD) binaries. We supplement the sample with 264 previously published white dwarf spectra, as well as 1292 high-confidence white dwarf spectral types inferred from their Gaia XP spectra. We find that spectroscopic fits to optical spectra do not provide noticeable improvement to the age agreement among white dwarfs in wide WD+WD binaries. The median age agreement is ≈1.5 σ for both photometrically and spectroscopically determined total ages, for pairs of white dwarfs with each having a total age uncertainty < 20%. For DA white dwarfs, we further find that photometrically determined atmospheric parameters from spectral energy distribution fitting give better total age agreement (1.0 σ , 0.2 Gyr, or 14% of the binary’s average total age) compared to spectroscopically determined parameters from Balmer-line fits (agreement of 1.5 σ , 0.3 Gyr, or 28% of binary’s average total age). We find further evidence of a significant merger fraction among wide WD+WD binaries: across multiple spectroscopically identified samples, roughly 20% are inconsistent with a monotonically increasing initial–final mass relation. We recommend the acquisition of an identification spectrum to ensure the correct atmospheric models are used in photometric fits in order to determine the most accurate total age of a white dwarf star.

Multiple planets undergoing disk migration may be captured into a chain of mean-motion resonances with the innermost planet parked near the disk's inner edge. Subsequent dynamical evolution may disrupt these resonances, leading to the nonresonant configurations typically observed among Kepler planets that are Gyr old. In this scenario, resonant configurations are expected to be more common in younger systems. This prediction can now be tested, thanks to recent discoveries of young planets, in particular those in stellar clusters, by NASA's TESS mission. We divided the known planetary systems into three age groups: young (<100 Myr old), adolescent (0.1-1 Gyr old), and mature (>1 Gyr old). The fraction of neighboring planet pairs having period ratios within a few percent of a first-order commensurability (e.g., 4:3, 3:2, or 2:1) is 70% +/- 15% for young pairs, 24% +/- 8% for adolescent pairs, and 15% +/- 2% for mature pairs. The fraction of systems with at least one nearly commensurable pair (either first- or second-order) is 86% +/- 13% among young systems, 38% +/- 12% for adolescent systems, and 23% +/- 3% for mature systems. First-order commensurabilities prevail across all age groups, with an admixture of second-order commensurabilities. Commensurabilities are more common in systems with high planet multiplicity and low mutual inclinations. Observed period ratios often deviate from perfect commensurability by similar to 1% even among young planets, too large to be explained by resonant repulsion with equilibrium eccentricity tides. We also find that super-Earths in the radius gap (1.5-1.9R(circle plus)) are less likely to be near-resonant (11.9% +/- 2.0%) compared to Earth-sized planets (R-p < 1R(circle plus); 25.3% +/- 4.4%) or mini-Neptunes (1.9R(circle plus) <= R-p < 2.5R(circle plus); 14.4% +/- 1.8%).

Abstract The introduction of TESS’s 20 s cadence mode has given us the opportunity to examine the variation of stellar flux on shorter timescales than ever before. While we find no previously undetected variability at these timescales, we find oscillations outside of the published instability strip in TIC 349902873. We also quantify the stellar variability on specific timescales using the combined differential photometric precision (CDPP). We developed a light-curve creation pipeline for 57 bright stars in TESS’ SCVZ across the HR diagram with extant spectra. From the light curves, we reproduced the scaling previously seen between CDPP and magnitude at all timescales in our data set. We also find a potential correlation between CDPP on the shortest timescales and surface gravity, which may be related to convection and oscillations and impact the relative detectability of exoplanet features.