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The application of clustering algorithms to the Gaia astrometric catalog has revolutionized our census of stellar populations in the milky Way, including the discovery of many new dispersed structures. We focus on one such structure, Theia 456 (COIN-Gaia-13), a loosely bound collection of ∼320 stars spanning ∼120 pc that has previously been shown to exhibit kinematic, chemical, and gyrochronal coherency, indicating a common origin. We obtain follow-up radial velocities and supplement these with Gaia astrometry to perform an in-depth dynamical analysis of Theia 456. By integrating stellar orbits through a Milky Way potential, we find the currently dispersed structure coalesced into a small cluster in the past. Via Bayesian modeling, we derive a kinematic age of 245 ± 3 Myr (statistical), a half-mass–radius of 9 ± 2 pc, and an initial one-dimensional velocity dispersion of 0.14 ± 0.02 km s ^−1 . Our results are entirely independent of model isochrones, details of stellar evolution, and internal cluster dynamics, and the statistical precision in our age derivation rivals that of the most precise age-dating techniques known today, though our imperfect knowledge of the Milky Way potential and simple spherical model for Theia 456 at birth add additional uncertainties. Using posterior predictive checking, we confirm these results are robust under reasonable variations to the Milky Way potential. Such low-density structures that are disrupted by the Galactic tides before virializing may be ubiquitous, signifying that Theia 456 is a valuable benchmark for studying the dynamical history of stellar populations in the Milky Way.

We present a detailed model atmosphere analysis of massive white dwarfs with M > 0.9 M _⊙ and T _eff ≥ 11,000 K in the Montreal White Dwarf Database 100 pc sample and the Pan-STARRS footprint. We obtained follow-up optical spectroscopy of 109 objects with no previous spectral classification in the literature. Our spectroscopic follow-up is now complete for all 204 objects in the sample. We find 118 normal DA white dwarfs, including 45 massive DAs near the ZZ Ceti instability strip. There are no normal massive DBs: the six DBs in the sample are strongly magnetic and/or rapidly rotating. There are 20 massive DQ white dwarfs in our sample, and all are found in the crystallization sequence. In addition, 66 targets are magnetic (32% of the sample). We use magnetic white dwarf atmosphere models to constrain the field strength and geometry using offset dipole models. We also use magnetism, kinematics, and rotation measurements to constrain the fraction of merger remnant candidates among this population. The merger fraction of this sample increases from 25% for 0.9–1 M _⊙ white dwarfs to 49% for 1.2–1.3 M _⊙ . However, this fraction is as high as ${78}_{-7}^{+4}$ % for 1.1–1.2 M _⊙ white dwarfs. Previous works have demonstrated that 5%–9% of high-mass white dwarfs stop cooling for ∼8 Gyr due to the ^22 Ne distillation process, which leads to an overdensity of Q-branch stars in the solar neighborhood. We demonstrate that the overabundance of the merger remnant candidates in our sample is likely due to the same process.

Low-mass (≲1.2 M _⊙ ) main-sequence stars lose angular momentum over time, leading to a decrease in their magnetic activity. The details of this rotation–activity relation remain poorly understood, however. Using observations of members of the ≈700 Myr old Praesepe and Hyades open clusters, we aim to characterize the rotation–activity relation for different tracers of activity at this age. To complement published data, we obtained new optical spectra for 250 Praesepe stars, new X-ray detections for 10, and new rotation periods for 28. These numbers for Hyads are 131, 23, and 137, respectively. The latter increases the number of Hyads with periods by 50%. We used these data to measure the fractional H α and X-ray luminosities, L _H _α / L _bol and L _X / L _bol , and to calculate Rossby numbers R _o . We found that at ≈700 Myr almost all M dwarfs exhibit H α emission, with binaries having the same overall color–H α equivalent width distribution as single stars. In the R _o – L _H _α / L _bol plane, unsaturated single stars follow a power law with index β = −5.9 ± 0.8 for R _o > 0.3. In the R _o – L _X / L _bol plane, we see evidence for supersaturation for single stars with R _o ≲ 0.01, following a power law with index ${\beta }_{\sup }={0.5}_{-0.1}^{+0.2}$ , supporting the hypothesis that the coronae of these stars are being centrifugally stripped. We found that the critical R _o value at which activity saturates is smaller for L _X / L _bol than for L _H _α / L _bol . Finally, we observed an almost 1:1 relation between L _H _α / L _bol and L _X / L _bol , suggesting that both the corona and the chromosphere experience similar magnetic heating.

The current sample of 12 radio-quiet isolated neutron stars that emit strongly in X-rays (XINSs) is both small and heterogeneous, limiting its usefulness for understanding the physics of neutron star atmospheres and cooling rates and for constraining the equation of state of neutron degenerate matter. Utilizing the ROSAT 1RXS and 2RXS data sets, in conjunction with the Sloan Digital Sky Survey Data Release 17 and other companion multiwavelength surveys, we have extended previous searches for blank-field X-ray source candidate XINSs, ultimately recovering two known XINSs while identifying 46 new, unstudied candidate fields devoid of likely multiwavelength counterparts. In this publication, we describe our selection approach and provide detailed information regarding our sample of new candidate XINSs. Future opportunities to verify or to refute these X-ray sources as isolated neutron stars by obtaining more accurate X-ray source positions, quality X-ray spectra, or deeper optical imaging are also discussed.

We present the results from our ongoing spectroscopic survey targeting low-mass white dwarf binaries, focusing on the southern sky. We used a Gaia DR2- and eDR3-based selection and identified 28 new binaries, including 19 new extremely low-mass (ELM) white dwarfs, one short period, likely eclipsing, DABZ, and two potential LISA binaries. We present the orbital and atmospheric parameters for each new binary based on our spectroscopic follow up. Four of our new binaries show periodic photometric variability in TESS 2 minutes cadence data, including one new eclipsing double-lined spectroscopic binary. Three others show periodic photometric variability in ZTF, including one new eclipsing binary. We provide estimates for the inclinations and scaled component radii for these ZTF variables, based on light-curve modeling of our high-speed photometric follow-up observations. Our observations have increased the sample of ELM Survey binaries identified in the southern sky to 41, an increase of 64%. Future time domain surveys, such as BlackGEM and the Vera C. Rubin Observatory Legacy Survey of Space and Time, will efficiently identify photometric variables in the southern sky and significantly increase the population of southern sky low-mass white dwarf binaries, leading to a more complete all-sky population of these systems.

K2-136 is a late-K dwarf (0.742 ± 0.039 M _⊙ ) in the Hyades open cluster with three known, transiting planets and an age of 650 ± 70 Myr. Analyzing K2 photometry, we found that planets K2-136b, c, and d have periods of 8.0, 17.3, and 25.6 days and radii of 1.014 ± 0.050 R _⊕ , 3.00 ± 0.13 R _⊕ , and 1.565 ± 0.077 R _⊕ , respectively. We collected 93 radial velocity (RV) measurements with the High-Accuracy Radial-velocity Planet Searcher for the Northern hemisphere (HARPS-N) spectrograph (Telescopio Nazionale Galileo) and 22 RVs with the Echelle SPectrograph for Rocky Exoplanets and Stable Spectroscopic Observations (ESPRESSO) spectrograph (Very Large Telescope). Analyzing HARPS-N and ESPRESSO data jointly, we found that K2-136c induced a semi-amplitude of 5.49 ± 0.53 m s ^−1 , corresponding to a mass of 18.1 ± 1.9 M _⊕ . We also placed 95% upper mass limits on K2-136b and d of 4.3 and 3.0 M _⊕ , respectively. Further, we analyzed Hubble Space Telescope and XMM-Newton observations to establish the planetary high-energy environment and investigate possible atmospheric loss. K2-136c is now the smallest planet to have a measured mass in an open cluster and one of the youngest planets ever with a mass measurement. K2-136c has ∼75% the radius of Neptune but is similar in mass, yielding a density of ${3.69}_{-0.56}^{+0.67}$ g cm ^−3 (∼2–3 times denser than Neptune). Mass estimates for K2-136b (and possibly d) may be feasible with more RV observations, and insights into all three planets’ atmospheres through transmission spectroscopy would be challenging but potentially fruitful. This research and future mass measurements of young planets are critical for investigating the compositions and characteristics of small exoplanets at very early stages of their lives and providing insights into how exoplanets evolve with time.

Stellar rotation was proposed as a potential age diagnostic that is precise, simple, and applicable to a broad range of low-mass stars (1 ). Unfortunately, rotation period () measurements of low-mass members of open clusters have undermined the idea that stars spin down with a common age dependence (i.e., ): K dwarfs appear to spin down more slowly than F and G dwarfs. Agüeros et al. interpreted data for the ≈1.4 Gyr-old cluster NGC 752 differently, proposing that after having converged onto a slow-rotating sequence in their first 600–700 Myr (by the age of Praesepe), K dwarf stall on that sequence for an extended period of time. We use data from Gaia DR2 to identify likely single-star members of the ≈1 Gyr-old cluster NGC 6811 with Kepler light curves. We measure for 171 members, more than doubling the sample relative to the existing catalog and extending the mass limit from to ≈0.6 . We then apply a gyrochronology formula calibrated with Praesepe and the Sun to 27 single G dwarfs in NGC 6811 to derive a precise gyrochronological age for the cluster of 1.04 ± 0.07 Gyr. However, when our new low-mass rotators are included, NGC 6811's color– sequence deviates away from the naive 1 Gyr projection down to K (K5V, 0.7 ), where it clearly overlaps with Praesepe’s. Combining these data with for other clusters, we conclude that the assumption that mass and age are separable dependencies is invalid. Furthermore, the cluster data show definitively that stars experience a temporary epoch of reduced braking efficiency where stall, and that the duration of this epoch lasts longer for lower-mass stars. [ABSTRACT FROM AUTHOR]

We present the results of an abundance analysis of three G and two K dwarfs in the Praesepe open cluster based on high-resolution, moderate signal-to-noise-ratio spectra obtained with the ARC 3.5 m Telescope at Apache Point Observatory. Using a principal component analysis and the BACCHUS automated spectral analysis code, we determined stellar parameters and abundances of up to 24 elements for each of our targets, which range in temperature from 6000 to 4600 K. The average derived iron abundance for the three G stars is 0.17 ± 0.07 dex, consistent with the 0.12 ± 0.04 dex derived by Boesgaard et al. for their sample of 11 solar-type Praesepe members, which included these G stars. To investigate the efficacy of using automated routines to derive the abundances of cooler main-sequence stars, we compared the abundances of the K dwarfs to those of the G dwarfs. Our abundances are consistent to ≤0.1 dex for 13 of the 18 elements we report for all five of the stars, providing more evidence that G and K stars in a given open cluster are chemically homogeneous. The median difference between the mean G and K stars’ abundances is 0.08 ± 0.05 dex, despite serious challenges with the noisier data for the fainter K dwarfs. Our results are encouraging for chemical tagging, as they indicate that it may be possible to use automated abundance determination techniques to identify chemically related main-sequence stars across larger temperature ranges than are usually considered in these experiments. [ABSTRACT FROM AUTHOR]

Stars of a common origin are thought to have similar, if not nearly identical, chemistry. Chemical tagging seeks to exploit this fact to identify Milky Way subpopulations through their unique chemical fingerprints. In this work, we compare the chemical abundances of dwarf stars in wide binaries to test the abundance consistency of stars of a common origin. Our sample of 31 wide binaries is identified from a catalog produced by cross-matching Apache Point Observatory Galactic Evolution Experiment spectroscopic survey (APOGEE) stars with UCAC5 astrometry, and we confirm the fidelity of this sample with precision parallaxes from Gaia DR2. For as many as 14 separate elements, we compare the abundances between components of our wide binaries, finding they have very similar chemistry (typically within 0.1 dex). This level of consistency is more similar than can be expected from stars with different origins (which show typical abundance differences of 0.3–0.4 dex within our sample). For the best-measured elements, Fe, Si, K, Ca, Mn, and Ni, these differences are reduced to 0.05–0.08 dex when selecting pairs of dwarf stars with similar temperatures. Our results suggest that APOGEE dwarf stars may currently be used for chemical tagging at the level of ∼0.1 dex or at the level of ∼0.05 dex when restricting for the best-measured elements in stars of similar temperatures. Larger wide binary catalogs may provide calibration sets, in complement to open cluster samples, for ongoing spectroscopic surveys. [ABSTRACT FROM AUTHOR]

Theoretical models of stars constitute the fundamental bedrock upon which much of astrophysics is built, but large swaths of model parameter space remain uncalibrated by observations. The best calibrators are eclipsing binaries in clusters, allowing measurement of masses, radii, luminosities, and temperatures for stars of known metallicity and age. We present the discovery and detailed characterization of PTFEB132.707+19.810, a P = 6.0 day eclipsing binary in the Praesepe cluster (τ ∼ 600–800 Myr; [Fe/H] = 0.14 ± 0.04). The system contains two late-type stars (SpTP = M3.5 ± 0.2; SpTS = M4.3 ± 0.7) with precise masses ( M⊙; M⊙) and radii ( R⊙; R⊙). Neither star meets the predictions of stellar evolutionary models. The primary has the expected radius but is cooler and less luminous, while the secondary has the expected luminosity but is cooler and substantially larger (by 20%). The system is not tidally locked or circularized. Exploiting a fortuitous 4:5 commensurability between Porb and , we demonstrate that fitting errors from the unknown spot configuration only change the inferred radii by ≲1%–2%. We also analyze subsets of data to test the robustness of radius measurements; the radius sum is more robust to systematic errors and preferable for model comparisons. We also test plausible changes in limb darkening and find corresponding uncertainties of ∼1%. Finally, we validate our pipeline using extant data for GU Boo, finding that our independent results match previous radii to within the mutual uncertainties (2%–3%). We therefore suggest that the substantial discrepancies are astrophysical; since they are larger than those for old field stars, they may be tied to the intermediate age of PTFEB132.707+19.810. [ABSTRACT FROM AUTHOR]

We present the results of a spectroscopic survey to characterize chromospheric activity, as measured by emission, in low-mass members of the 500 Myr old open cluster M37. Combining our new measurements of luminosities () with previously cataloged stellar properties, we identify saturated and unsaturated regimes in the dependence of the -to-bolometric luminosity ratio, , on the Rossby number Ro. All rotators with Ro smaller than 0.03 ± 0.01 converge to an activity level of . This saturation threshold () is statistically smaller than that found in most studies of the rotation–activity relation. In the unsaturated regime, slower rotators have lower levels of chromospheric activity, with (Ro) following a power-law of index , slightly shallower than that found for a combined ≈650 Myr old sample of Hyades and Praesepe stars. By comparing this unsaturated behavior to that previously found for coronal activity in M37 (as measured via the X-ray luminosity, ), we confirm that chromospheric activity decays at a much slower rate than coronal activity with increasing Ro. While a comparison of and for M37 members with measurements of both reveals a nearly 1:1 relation, removing the mass-dependencies by comparing instead and does not provide clear evidence for such a relation. Finally, we find that is smaller for our chromospheric than for our coronal indicator of activity ( versus 0.09 ± 0.01). We interpret this as possible evidence for coronal stripping. [ABSTRACT FROM AUTHOR]

We use a 440.5 ks Chandra observation of the ≈500 Myr old open cluster M37 to derive the X-ray luminosity functions of its ≤1.2 stars. Combining detections of 162 M37 members with upper limits for 160 non-detections, we find that its G, K, and M stars have a similar median (0.5–7 keV) X-ray luminosity , whereas the -to-bolometric-luminosity ratio () indicates that M stars are more active than G and K stars by order of magnitude at 500 Myr. To characterize the evolution of magnetic activity in low-mass stars over their first , we consolidate X-ray and optical data from the literature for stars in six other open clusters: from youngest to oldest they are, the Orion Nebula Cluster (ONC), NGC 2547, NGC 2516, the Pleiades, NGC 6475, and the Hyades. For these, we homogenize the conversion of instrumental count rates to by applying the same one-temperature emission model as for M37, and obtain masses using the same empirical mass-absolute magnitude relation (except for the ONC). We find that for G and K stars X-ray activity decreases orders of magnitude over their first 600 Myr, and for M stars, ≈1.5. The decay rate of the median follows the relation , where for G stars, −0.82 ± 0.16 for K stars, and −0.40 ± 0.17 for M stars. In space, the slopes are −0.68 ± 0.12, −0.81 ± 0.19, and −0.61 ± 0.12, respectively. These results suggest that for low-mass stars the age-activity relation steepens after , consistent with the faster decay in activity observed in solar analogs at . [ABSTRACT FROM AUTHOR]

Stellar rotation periods (Prot) measured in open clusters have proved to be extremely useful for studying stars’ angular momentum content and rotationally driven magnetic activity, which are both age- and mass-dependent processes. While Prot measurements have been obtained for hundreds of solar-mass members of the Pleiades, measurements exist for only a few low-mass (<0.5 M⊙) members of this key laboratory for stellar evolution theory. To fill this gap, we report Prot for 132 low-mass Pleiades members (including nearly 100 with M ≤ 0.45 M⊙), measured from photometric monitoring of the cluster conducted by the Palomar Transient Factory in late 2011 and early 2012. These periods extend the portrait of stellar rotation at 125 Myr to the lowest-mass stars and re-establish the Pleiades as a key benchmark for models of the transport and evolution of stellar angular momentum. Combining our new Prot with precise BVIJHK photometry reported by Stauffer et al. and Kamai et al., we investigate known anomalies in the photometric properties of K and M Pleiades members. We confirm the correlation detected by Kamai et al. between a star's Prot and position relative to the main sequence in the cluster's color–magnitude diagram. We find that rapid rotators have redder (V − K) colors than slower rotators at the same V, indicating that rapid and slow rotators have different binary frequencies and/or photospheric properties. We find no difference in the photometric amplitudes of rapid and slow rotators, indicating that asymmetries in the longitudinal distribution of starspots do not scale grossly with rotation rate. [ABSTRACT FROM AUTHOR]

We present observational constraints on the initial-final mass relation (IFMR) using wide double white dwarfs (DWDs). We identify 65 new candidate wide DWDs within the Sloan Digital Sky Survey, bringing the number of candidate wide DWDs to 142. We then engage in a spectroscopic follow-up campaign and collect existing spectra for these objects; using these spectra, we derive masses and cooling ages for 54 hydrogen (DA) WDs in DWDs. We also identify one new DA/DB pair, four candidate DA/DC pairs, four candidate DA/DAH pairs, and one new candidate triple degenerate system. Because wide DWDs are co-eval and evolve independently, the difference in the pre-WD lifetimes should equal the difference in the WD cooling ages. We use this to develop a Bayesian hierarchical framework and construct a likelihood function to determine the probability that any particular IFMR fits a sample of wide DWDs. We then define a parametric model for the IFMR and find the best parameters indicated by our sample of DWDs. We place robust constraints on the IFMR for initial masses of 2–4 . The WD masses produced by our model for stars within this mass range differ from those predicted by semi-empirical fits to open cluster WDs. Within this mass range, where there are few constraining open cluster WDs and disagreements in the cluster ages, wide DWDs may provide more reliable constraints on the IFMR. Expanding this method to the many wide DWDs expected to be discovered by Gaia may transform our understanding of the IFMR. [ABSTRACT FROM AUTHOR]

Empirical calibrations of the stellar age-rotation–activity relation (ARAR) rely on observations of the co-eval populations of stars in open clusters. We used the ChandraX-ray Observatory to study M37, a 500-Myr-old open cluster that has been extensively surveyed for rotation periods (). M37 was observed almost continuously for five days, for a total of 440.5 ks, to measure stellar X-ray luminosities (), a proxy for coronal activity, across a wide range of masses. The cluster’s membership catalog was revisited to calculate updated membership probabilities from photometric data and each star’s distance to the cluster center. The result is a comprehensive sample of 1699 M37 members: 426 with , 278 with X-ray detections, and 76 with both. We calculate Rossby numbers, , where τ is the convective turnover time, and ratios of the X-ray-to-bolometric luminosity, , to minimize mass dependencies in our characterization of the rotation-coronal activity relation at 500 Myr. We find that fast rotators, for which , show saturated levels of activity, with log(). For , activity is unsaturated and follows a power law of the form , where β = . This is the largest sample available for analyzing the dependence of coronal emission on rotation for a single-aged population, covering stellar masses in the range 0.4–1.3 , in the range 0.4–12.8 days, and in the range . Our results make M37 a new benchmark open cluster for calibrating the ARAR at ages of Myr. [ABSTRACT FROM AUTHOR]