arXiv subjects

Lyra Cao

Publications and source records attributed to Lyra Cao.

At least 19 recordsLinked to original sources

arXiv2026

Open Clusters as Laboratories for Cool Star Evolution: Highlights from the Cool Stars 23 Splinter Session

Open clusters remain among the most powerful laboratories for calibrating how fundamental stellar properties such as rotation, magnetic activity, surface chemistry, and internal structure evolve over the pre-main-sequence to main-sequence and post-main-sequence stellar lifetime. Because cluster members share a common age, galactic environment, and initial composition, they provide empirical anchors upon which age-dating techniques such as gyrochronology, activity-age relations, and chemical clocks are built. This paper summarizes the Cool Stars 23 splinter session - Open Clusters as Laboratories for Cool Star Evolution, held on 15 June 2026 in Tokyo, Japan. The session comprised one invited review, ten contributed talks, and ten poster pop-up presentations, organized around three themes: the evolution of rotation, magnetic activity, and chemical abundances (Li-depletion, [C/N], [Y/Mg], and neutron-capture elements). We close with a summary of the open questions identified in the panel discussion and the observational and theoretical work that new surveys (e.g. Gaia DR4, 4MOST, WEAVE) and missions (e.g. PLATO, Roman) will enable over the coming years. Together, these efforts will help assess the current state of the field and shape its future directions.

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arXiv2026

The YREC Stellar Evolution Code: Public Data Release

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.

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arXiv2025

The Relationship of Stellar Radius Inflation to Rotation and Magnetic Starspots at 10--670 Myr

Active, low-mass stars are widely observed to have radii that are larger than predicted by standard stellar models. Proposed mechanisms for this radius inflation generally involve stellar magnetism, either in the form of added pressure support in the outer layers and/or suppression of convection via starspots. We have assembled a large sample of 261 low-mass stars in the young clusters Upper Scorpius, $\alpha$ Persei, Pleiades, and Praesepe (spanning ages 10--670 Myr) for which the data exist to empirically measure the stellar radii, rotation periods, and starspot covering fractions. We find a clear, strong relationship between the degree of radius inflation and stellar rotation as represented by the Rossby number; this inflation-rotation relationship bears striking resemblance to canonical activity-rotation relationships, including both the so-called linear and saturated regimes. We also demonstrate here for the first time that the radius inflation depends directly on the starspot covering fraction. We furthermore find that the stars' effective temperatures decrease with decreasing Rossby number as well, and that this temperature suppression balances the radius inflation so as to preserve the stellar bolometric luminosity. These relationships are consistent across the age range sampled here, which spans from the pre--main-sequence to the zero-age main sequence. The favorable comparison of our findings to the predictions of modern starspot-based stellar evolution models suggests that, while rotation is clearly the underlying driver, magnetism may be the most likely direct cause of the radius inflation phenomenon.

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arXiv2025

Rotational Variables: Kepler Versus ASAS-SN

Rotational variables are stars that vary in brightness due to star spots modulated by rotation. They are probes of stellar magnetism, binarity, and evolution. Phillips et al. (2023) explored distinct populations of ~50,000 high-amplitude rotational variables from the All-Sky Automated Survey for Supernovae (ASAS-SN), examining correlations between stellar rotation, binarity, and activity. Here, we carry out a similar analysis of ~50,000 much lower amplitude Kepler rotational variables. The Kepler population is dominated by slowly rotating, single, main sequence stars, with a striking absence of the rapidly rotating main sequence group in the ASAS-SN sample. The binary fractions of the Kepler rotators are significantly lower than for the ASAS-SN systems and they are significantly less spotted, as expected from their lower amplitudes. The scope of these statistical surveys will dramatically increase in the near future.

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arXiv2025

Rotationally Driven Ultraviolet Emission of Red Giant Stars II. Metallicity, Activity, Binarity and Sub-subgiants

Red Giant Branch (RGB) stars are overwhelmingly observed to rotate very slowly compared to main-sequence stars, but a few percent of them show rapid rotation and high activity, often as a result of tidal synchronizationn or other angular momentum transfer events. In this paper we build upon previous work using a sample of 7,286 RGB stars from APOGEE DR17 with measurable rotation. We derive an updated NUV excess vs $v \sin{i}$ rotation-activity relation that is consistent with our previous published version, but reduces uncertainty through the inclusion of a linear [M/H] correction term. We find that both single stars and binary stars generally follow our rotation-activity relation, but single stars seemingly saturate at $P_{\text{rot}}$/$\sin{i}$ $\sim$10 days while binary stars show no sign of saturation, suggesting they are able to carry substantially stronger magnetic fields. Our analysis reveals Sub-subgiant stars (SSGs) to be the most active giant binaries, with rotation synchronized to orbits with periods $\lesssim$ 20 days. Given their unusually high level of activity compared to other short-period synchronized giants we suspect the SSGs are most commonly overactive RS CVn stars. Using estimates of critical rotation we identify a handful giants rotating near break-up and determine tidal spin up to this level of rotation is highly unlikely and instead suggest planetary engulfment or stellar mergers in a fashion generally proposed for FK Comae stars.

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arXiv2025

Magnetic activity evolution of solar-like stars: II. $S_{\rm ph}$-Ro evolution of Kepler main-sequence targets

There is now a large sample of stars observed by the Kepler satellite with measured rotation periods and photometric activity index $S_{\rm ph}$. We use this data, in conjunction with stellar interiors models, to explore the interplay of magnetism, rotation, and convection. Stellar activity proxies other than $S_{\rm ph}$ 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 $S_{\rm ph}$-$Ro$ relationships for different stellar spectral types. Though the overall trend of decreasing magnetic activity versus $Ro$ is recovered, we find a localized dip in $S_{\rm ph}$ around $Ro/Ro_{\odot} \sim$\,0.3 for the G and K dwarfs. F dwarfs show little to no dependence of $S_{\rm ph}$ on $Ro$ due to their shallow convective zones; further accentuated as $T_{\rm eff}$ increases. The dip in activity for the G and K dwarfs corresponds to the intermediate rotation period gap, suggesting that the dip in $S_{\rm ph}$ 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 solar $Ro$. 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 $Ro$. We confirm that metal-rich stars have a systematically larger $S_{\rm ph}$ level than metal-poor stars, which is likely a consequence of their deeper convective zones.

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arXiv2024

Magnetic Fields of M Dwarfs from the Pleiades Open Cluster

Average magnetic field measurements are presented for 62 M-dwarf members of the Pleiades open cluster, derived from Zeeman-enhanced Fe I lines in the H-band. An MCMC methodology was employed to model magnetic filling factors using SDSS-IV APOGEE high-resolution spectra, along with the radiative transfer code SYNMAST, MARCS stellar atmosphere models, and the APOGEE DR17 spectral line list. There is a positive correlation between mean magnetic fields and stellar rotation, with slow-rotator stars (Rossby number, Ro$>$0.13) exhibiting a steeper slope than rapid-rotators (Ro$<$0.13). However, the latter sample still shows a positive trend between Ro and magnetic fields, which is given by $<$B$>$ = 1604 $\times$ Ro$^{-0.20}$. The derived stellar radii, when compared with physical isochrones, show that on average, our sample shows radius inflation, with median enhanced radii ranging from +3.0$\%$ to +7.0$\%$, depending on the model. There is a positive correlation between magnetic field strength and radius inflation, as well as with stellar spot coverage, correlations that together indicate that stellar spot-filling factors generated by strong magnetic fields might be the mechanism that drives radius inflation in these stars. We also compare our derived magnetic fields with chromospheric emission lines (H$α$, H$β$ and Ca II K), as well as with X-ray and H$α$ to bolometric luminosity ratios, and find that stars with higher chromospheric and coronal activity tend to be more magnetic.

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arXiv2024

Rotational Evolution of Classical T Tauri Stars: Models and Observations

We developed a grid of stellar rotation models for low-mass and solar-type Classical T Tauri stars (CTTS) ($0.3M_{\odot}<M_{\ast}<1.2M_{\odot}$). These models incorporate the star-disk interaction and magnetospheric ejections to investigate the evolution of the stellar rotation rate as a function of the mass of the star $M_{\ast}$, the magnetic field ($B_{\ast}$), and stellar wind ($\dot{M}_{wind}$). We compiled and determined stellar parameters for 208 CTTS, such as projected rotational velocity $v\sin(i)$, mass accretion rate $\dot{M}_{acc}$, stellar mass $M_{\ast}$, ages, and estimated rotational periods using TESS data. We also estimated a representative value of the mass-loss rate for our sample using the $[\text{O}\text{ I}]$ spectral line. Our results confirm that $v\sin(i)$ measurements in CTTS agree with the rotation rates provided by our spin models in the accretion-powered stellar winds (APSW) picture. In addition, we used the Approximate Bayesian Computation (ABC) technique to explore the connection between the model parameters and the observational properties of CTTS. We find that the evolution of $v\sin(i)$ with age might be regulated by variations in (1) the intensity of $B_{\ast}$ and (2) the fraction of the accretion flow ejected in magnetic winds, removing angular momentum from these systems. The youngest stars in our sample ($\sim $1 Myr) show a median branching ratio $\dot{M}_{wind}/\dot{M}_{acc}\sim$ $0.16$ and median $B_{\ast}\sim$ 2000 G, in contrast to $\sim 0.01$ and 1000 G, respectively, for stars with ages $\gtrsim 3$ Myr.

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arXiv2024

The Gasing Pangkah Collaboration: I. Asteroseismic Identification and Characterisation of a Rapidly-Rotating Engulfment Candidate

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.

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arXiv2024

TESS Stellar Rotation up to 80 days in the Southern Continuous Viewing Zone

The TESS mission delivers time-series photometry for millions of stars across the sky, offering a probe into stellar astrophysics, including rotation, on a population scale. However, light curve systematics related to the satellite's 13.7-day orbit have prevented stellar rotation searches for periods longer than 13 days, putting the majority of stars beyond reach. Machine learning methods have the ability to identify systematics and recover robust signals, enabling us to recover rotation periods up to 35 days for GK dwarfs and 80 days for M dwarfs. We present a catalog of 7245 rotation periods for cool dwarfs in the Southern Continuous Viewing Zone, estimated using convolutional neural networks. We find evidence for structure in the period distribution consistent with prior Kepler and K2 results, including a gap in 10--20-day cool star periods thought to arise from a change in stellar spin-down or activity. Using a combination of spectroscopic and gyrochronologic constraints, we fit stellar evolution models to estimate masses and ages for stars with rotation periods. We find strong correlations between the detectability of rotation in TESS and the effective temperature, age, and metallicity of the stars. Finally, we investigate the relationships between rotation and newly obtained spot filling fractions estimated from APOGEE spectra. Field star spot filling fractions are elevated in the same temperature and period regime where open clusters' magnetic braking stalls, lending support to an internal shear mechanism that can produce both phenomena.

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arXiv2023

Constraints on star formation in Orion A from Gaia

We develop statistical methods within a Bayesian framework to infer the star formation history from photometric surveys of pre-main sequence populations. Our procedures include correcting for biases due to extinction in magnitude-limited surveys, and using distributions from subsets of stars with individual extinction measurements. We also make modest corrections for unresolved binaries. We apply our methods to samples of populations with Gaia photometry in the Orion A molecular cloud. Using two well-established sets of evolutionary tracks, we find that, although our sample is incomplete at youngest ages due to extinction, star formation has proceeded in Orion A at a relatively constant rate between ages of about 0.3 and 5 Myr, in contrast to other studies suggesting multiple epochs of star formation. Similar results are obtained for a set of tracks that attempt to take the effects of strong magnetic fields into account. We also find no evidence for a well-constrained "birthline" that would result from low-mass stars appearing first along the deuterium-burning main sequence, especially using the magnetic evolutionary tracks. While our methods have been developed to deal with Gaia data, they may be useful for analyzing other photometric surveys of star-forming regions.

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arXiv2023

Seven Classes of Rotational Variables From a Study of 50,000 Spotted Stars with ASAS-SN, Gaia, and APOGEE

We examine the properties of $\sim50,000$ rotational variables from the ASAS-SN survey using distances, stellar properties, and probes of binarity from $\textit{Gaia}$ DR3 and the SDSS APOGEE survey. They have high amplitudes and span a broader period range than previously studied $\textit{Kepler}$ rotators. We find they divide into three groups of main sequence stars (MS1, MS2s, MS2b) and four of giants (G1/3, G2, G4s, and G4b). MS1 stars are slowly rotating (10-30 days), likely single stars with a limited range of temperatures. MS2s stars are more rapidly rotating (days) single stars spanning the lower main sequence up to the Kraft break. There is a clear period gap (or minimum) between MS1 and MS2s, similar to that seen for lower temperatures in the $\textit{Kepler}$ samples. MS2b stars are tidally locked binaries with periods of days. G1/3 stars are heavily spotted, tidally locked RS CVn with periods of tens of days. G2 stars are less luminous, heavily spotted, tidally locked sub-subgiants with periods of $\sim10$ days. G4s stars have intermediate luminosities to G1/3 and G2, slow rotation periods (approaching 100 days) and are almost certainly all merger remnants. G4b stars have similar rotation periods and luminosities to G4s, but consist of sub-synchronously rotating binaries. We see no difference in indicators for the presence of very wide binary companions between any of these groups and control samples of photometric twin stars built for each group.

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arXiv2023

Core-envelope decoupling drives radial shear dynamos in cool stars

Differential rotation is thought to be responsible for the dynamo process in stars like our Sun, driving magnetic activity and star spots. We report that star spot measurements in the Praesepe open cluster are strongly enhanced only for stars which depart from standard models of rotational evolution. A decoupling of the spin down history between the core and envelope explains both the activity and rotation anomalies: surface rotational evolution is stalled by interior angular momentum redistribution, and the resultant radial shears enhance star spot activity. These anomalies provide evidence for an evolving front of shear-enhanced activity affecting the magnetic and rotational evolution of cool stars and the high-energy environments of their planetary companions for hundreds of millions to billions of years on the main sequence.

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arXiv2023

Stellar Characterization and Radius Inflation of Hyades M Dwarf Stars From the APOGEE Survey

We present a spectroscopic analysis of a sample of 48 M dwarf stars ($0.2 M_{\odot}< M < 0.6 M_{\odot}$) from the Hyades open cluster using high-resolution H-band spectra from the SDSS/APOGEE survey. Our methodology adopts spectrum synthesis with LTE MARCS model atmospheres, along with the APOGEE DR17 line list, to determine effective temperatures, surface gravities, metallicities, and projected rotational velocities. The median metallicity obtained for the Hyades M dwarfs is [M/H]= 0.09$\pm$0.03 dex, indicating a small internal uncertainty and good agreement with optical results for Hyades red-giants. Overall, the median radii are larger than predicted by stellar models by 1.6$\pm$2.3\% and 2.4$\pm$2.3\%, relative to a MIST and DARTMOUTH isochrone, respectively. We emphasize, however, that these isochrones are different and the fractional radius inflation for the fully- and partially-convective regimes have distinct behaviors depending on the isochrone. Using a MIST isochrone there is no evidence of radius inflation for the fully convective stars, while for the partially convective M-dwarfs the radii are inflated by 2.7$\pm$2.1\%, which is in agreement with predictions from models that include magnetic fields. For the partially-convective stars, rapid-rotators present on average higher inflation levels than slow-rotators. The comparison with SPOTS isochrone models indicates that the derived M dwarf radii can be explained by accounting for stellar spots in the photosphere of the stars, with 76\% of the studied M dwarfs having up to 20\% spot coverage, and the most inflated stars with $\sim$20 -- 40\% spot coverage.

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arXiv2023

Spectroscopic identification of rapidly rotating red giant stars in APOKASC-3 and APOGEE DR16

Rapidly rotating red giant stars are astrophysically interesting but rare. In this paper we present a catalog of 3217 active red giant candidates in the APOGEE DR16 survey. We use a control sample in the well-studied Kepler fields to demonstrate a strong relationship between rotation and anomalies in the spectroscopic solution relative to typical giants. Stars in the full survey with similar solutions are identified as candidates. We use vsin\textiti measurements to confirm 50+/- 1.2% of our candidates as definite rapid rotators, compared to 4.9+/-0.2% in the Kepler control sample. In both the Kepler control sample and a control sample from DR16, we find that there are 3-4 times as many giants rotating with 5 < vsini < 10 km s$^{-1}$ compared to vsini > 10 km s$^{-1}$, the traditional threshold for anomalous rotation for red giants. The vast majority of intermediate rotators are not spectroscopically anomalous. We use binary diagnostics from APOGEE and \textit{Gaia} to infer a binary fraction of 73+/-2.4%. We identify a significant bias in the reported metallicity for candidates with complete spectroscopic solutions, with median offsets of 0.37 dex in [M/H] from a control sample. As such, up to 10% of stars with reported [M/H]<-1 are not truly metal poor. Finally, we use Gaia data to identify a sub-population of main sequence photometric binaries erroneously classified as giants.

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arXiv2023

ABYSS I: Targeting strategy for APOGEE & BOSS young star survey in SDSS-V

The fifth iteration of the Sloan Digital Sky Survey (SDSS-V) is set to obtain optical and near-infrared spectra of $\sim$5 million stars of all ages and masses throughout the Milky Way. As a part of these efforts, APOGEE & BOSS Young Star Survey (ABYSS) will observe $\sim10^5$ stars with ages $<$30 Myr that have been selected using a set of homogeneous selection functions that make use of different tracers of youth. The ABYSS targeting strategy we describe in this paper is aimed to provide the largest spectroscopic census of young stars to-date. It consists of 8 different types of selection criteria that take the position on the HR diagram, infrared excess, variability, as well as the position in phase space in consideration. The resulting catalog of $\sim$200,000 sources (of which a half are expected to be observed) provides representative coverage of the young Galaxy, including both nearby diffuse associations as well as more distant massive complexes, reaching towards the inner Galaxy and the Galactic center.

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arXiv2022

pynucastro: A Python Library for Nuclear Astrophysics

We describe pynucastro 2.0, an open source library for interactively creating and exploring astrophysical nuclear reaction networks. We demonstrate new methods for approximating rates and using detailed balance to create reverse rates, show how to build networks and determine whether they are appropriate for a particular science application, and discuss the changes made to the library over the past few years. Finally, we demonstrate the validity of the networks produced and share how we use pynucastro networks in simulation codes.

astro-ph.IM↗

arXiv2022

Starspots and Magnetism: Testing the Activity Paradigm in the Pleiades and M67

We measure starspot filling fractions for 240 stars in the Pleiades and M67 open star clusters using APOGEE high-resolution H-band spectra. For this work we developed a modified spectroscopic pipeline which solves for starspot filling fraction and starspot temperature contrast. We exclude binary stars, finding that the large majority of binaries in these clusters (80%) can be identified from Gaia DR3 and APOGEE criteria -- important for field star applications. Our data agree well with independent activity proxies, indicating that this technique recovers real starspot signals. In the Pleiades, filling fractions saturate at a mean level of 0.248$\pm$0.005 for active stars with a decline at slower rotation; we present fitting functions as a function of Rossby number. In M67, we recover low mean filling fractions of 0.030$\pm$0.008 and 0.003$\pm$0.002 for main sequence GK stars and evolved red giants respectively, confirming that the technique does not produce spurious spot signals in inactive stars. Starspots also modify the derived spectroscopic effective temperatures and convective overturn timescales. Effective temperatures for active stars are offset from inactive ones by -109$\pm$11 K, in agreement with the Pecaut & Mamajek empirical scale. Starspot filling fractions at the level measured in active stars changes their inferred overturn timescale, which biases the derived threshold for saturation. Finally, we identify a population of stars statistically discrepant from mean activity-Rossby relations and present evidence that these are genuine departures from a Rossby scaling. Our technique is applicable to the full APOGEE catalog, with broad applications to stellar, galactic, and exoplanetary astrophysics.

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