Wei-Chun Jao

We present the visual orbits and dynamical masses of two longer period spectroscopic binary stars, HD 210763 and HD 221950, using long baseline interferometry with the CHARA Array and high resolution spectroscopy with the APO 3.5 m and CTIO 1.5 m telescopes. By combining the astrometric and radial velocity observations, we solve for the full, three-dimensional orbits and determine the stellar masses to within 0.5% uncertainty and the distance to within 0.2% uncertainty. For HD 210763, we found component masses of M ₁ = 1.748 ± 0.008 M _⊙ , M ₂ = 1.492 ± 0.006 M _⊙ . For HD 221950, we found masses of M ₁ =1.098 ± 0.006 M _⊙ , M ₂ = 1.031 ± 0.005 M _⊙ . We then estimate the effective temperature and radius of each component star through disentangling and spectral energy distribution analyses. We compare the observed stellar parameters to the predictions of the stellar evolution models and estimate the system ages. The primary component of HD 210763 is at the end of the main sequence while the secondary component is on the main sequence, providing a tight age constraint for this system at 1.6 ± 0.1 Gyr. Both components of HD 221950 are still on the main sequence with an age of 3.76 ± 0.38 Gyr. These systems have longer orbital periods, beyond the tidal circularization limit, and therefore are better proxies for single stars and tests of stellar evolution models than short-period, eclipsing systems.

Previous studies of β Piscis Austrinus (PsA) have speculated that the narrow and saddle-like shapes of some of its weak metallic lines are a consequence of it being a rapidly rotating star viewed nearly pole-on. Here we use the fastrot-spec spectral synthesis code to model high-dispersion ( R = 115,000) HARPS spectra of β PsA in order to determine its inclination and photospheric properties, with additional constraints on the surface temperature set by measures of Fe ii /Fe i line ratios. The analysis confirms that β PsA is oriented nearly pole-on ( i = 4.7 5 − 0.50 + 0.75 °) and experiences substantial gravity darkening caused by its rapid rotation (Ω/Ω _crit =0.93 ± 0.17). β PsA has a polar temperature of 1030 0 − 250 + 200 K that is 24% hotter than its equatorial temperature ( 827 5 − 400 + 317 K). This results in its apparent luminosity being 48% larger than its actual luminosity of 26.2 − 2.4 + 1.9 L _⊙ . When this methodology is applied to high-dispersion spectra of the star Vega, the analysis determines a nearly pole-on orientation that is consistent with interferometric measurements, validating the technique. Based on comparisons with PARSEC evolutionary models of stars rotating at similar velocities, β PsA has a mass of 2.20 ± 0.03 M _⊙ and an age of 14 1 − 49 + 113 Myr; this age is consistent with the age inferred for its G5V companion star, CD-32 17127, based on lithium depletion models. The analysis demonstrates the potential for both identifying and determining the stellar properties of rapidly rotating stars viewed nearly pole-on via spectroscopy alone.

We present 1066 speckle measurements of M dwarf multiples observed over 2021–2024, all taken with HRCam on the Southern Astrophysical Research 4.1 m telescope. Among these, 900 observations resolve companions in 212 pairs, with separations spanning 17 mas to 3 . ″ 4 and brightness differences ranging from 0 to 4.9 mag in the I filter. We have characterized the orbits of 54 of these companions, spanning periods of 0.67–30 yr, by combining our data with literature astrometry, radial velocities, and, in four cases, Hipparcos–Gaia accelerations. Among the orbits presented here are 28 that are the first-ever such characterizations for their systems and 26 that revise previously published orbits, thus providing a significant update to the observed dynamics of M dwarfs in the solar neighborhood. From these orbits, we provide new and updated dynamical total masses for these systems, precise to 0.7%–7% in nearly all cases. Future mass derivations for components in these systems will contribute to efforts in refining the mass–luminosity relation for the smallest stars and will enhance investigations of age, magnetism, and metallicity effects on luminosities at a given mass.

We present a spectroscopic characterization of 580 K dwarfs within 33 pc, observed with the CHIRON echelle spectrograph ( R = 80,000) on the SMARTS 1.5 m telescope. This volume-limited sample is part of the RKSTAR survey of ∼4400 K dwarf primaries within 50 pc. Using Empirical SpecMatch and the diagnostic lines H α (6562.8 Å) and Li I (6707.8 Å), we derive stellar properties, activity status, and age indicators calibrated against 35 benchmark K dwarfs with ages from 20 Myr to 5 Gyr. We find that 7.4% (43 stars) exhibit signatures of youth and/or chromospheric activity: 19 stars show lithium absorption indicating ages α emission. Kinematic analysis using BANYAN Σ identifies eight additional young stars through membership in the AB Doradus moving group and the Hyades cluster, bringing the total young/active population to 8.8% (51 stars). Stellar parameters span 3600–5500 K in T _eff , −0.60 to +0.55 dex in [Fe/H], and 25 km s ⁻¹ in v sin i . A metal-poor population ([Fe/H] ≤ −0.50 dex) comprises 4% of the sample. Galactic kinematics place 80% in the thin disk and 18.4% in the thick disk, with one halo member (HD 134439). Young and active stars are predominantly thin disk members, with two thick-disk exceptions. Cross-matching with NASA’s Exoplanet Archive reveals only 7.5% (44 stars) host confirmed planets as of 2025 July. Our results identify 529 mature, inactive K dwarfs as prime targets for terrestrial planet searches, providing a crucial resource for exoplanet habitability studies in the solar neighborhood.

We report on a study of 36 pairs of “twin” M dwarfs in wide binaries and assess how similarly the stars behave. Stars in each twin pair have BP, RP, J , H , and K _s differing by 21 % − 7 % + 14 % of twin pairs, in rotation amplitude for 67 % − 15 % + 10 % of pairs, in multiyear photometric variability for 33 % − 9 % + 12 % of pairs, and in H α activity for 21 % − 6 % + 9 % of pairs, with fully convective systems generally mismatched more often. Thus, roughly one out of five M dwarf twin sets does not match in rotation and/or activity despite otherwise identical fundamental parameters. Furthermore, we compile three key systems showing larger relative active/inactive H α mismatches. We propose the various mismatches likely stem from factors such as dynamo stochasticity, activity cycles, formative disk aspects, and/or star–planet interactions, depending on the system. These well-vetted twins offer ripe targets for many future investigations.

The discovery of infant (R _⊕ planet on a 14.8-day orbit. TOI-6448 was previously identified to be a likely member of Vela Population IV. We confirm the star’s membership and rederive the age of the cluster using isochrones, variability, and gyrochronology. We find the star, and thus planet, to be 34 ± 3 Myr. Like other young planets, TOI-6448 b lands in a region of parameter space with few older planets. While just one data point, this fits with prior findings of an excess of 5–11 R _⊕ planets around young stars far beyond what can be explained by reduced sensitivity at young ages. Our ongoing searches of Vela, Taurus-Auriga, Sco-Cen, and Orion are expected to reveal dozens more <50 Myr transiting planets.

The discovery of infant (< 50 Myr), close-in (<30-day period) planets is vital in understanding the formation mechanisms that lead to the distribution of mature transiting planets as discovered by Kepler. Despite several discoveries in this age bin, the sample is still too small for a robust statistical comparison to older planets. Here we report the validation of TOI-6448 b, an 8.8 +/- 0.8 Re planet on a 14.8 day orbit. TOI-6448 was previously identified to be a likely member of Vela Population IV. We confirm the star's membership and re-derive the age of the cluster using isochrones, variability, and gyrochronology. We find the star, and thus planet, to be 34 +/- 3 Myr. Like other young planets, TOI-6448 b lands in a region of parameter space with few older planets. While just one data point, this fits with prior findings of an excess of 5-11Re planets around young stars far beyond what can be explained by reduced sensitivity at young ages. Our ongoing search of Vela, Taurus-Auriga, Sco-Cen, and Orion are expected to reveal dozens more < 50 Myr transiting planets.

Metal-poor M subdwarfs are among the oldest stellar populations and carry valuable information about the chemical enrichment history of the Milky Way. The measurements of chemical abundances of these stars therefore provide essential insights into the nucleosynthesis in the early stages of the Galaxy’s formation. We present detailed spectroscopic analysis of a nearby metal-poor M subdwarf, LHS 174, from its high-resolution optical spectrum, and apply our previously developed spectral fitting code, AutoSpecFit, to measure the abundances of five elements: [O/H] = −0.519±0.081, [Ca/H] = −0.753 ± 0.177, [Ti/H] = −0.711 ± 0.144, [V/H] = −1.026 ± 0.077, and [Fe/H] = −1.170 ± 0.135. We compare the abundances of O, Ti, and Fe derived from this work and those from previous studies, and demonstrate the observed data are clearly better matched with the synthetic model generated based on our abundances than those from the other analyses. The accuracy of inferred stellar abundances strongly depends on the accuracy of physical parameters, which motivates us to develop a reliable technique to more accurately determine the parameters of low-mass M dwarfs and infer abundances with smaller uncertainties.