As protostars evolve from optically faint/infrared-bright (Class I) sources to optically bright/infrared-faint (Class II) the solid material in their surrounding disks accumulates into planetesimals ...and protoplanets. The nearby, young Ophiuchus star-forming region contains hundreds of protostars in a range of evolutionary states. Using the Atacama Large Millimeter Array to observe their millimeter continuum emission, we have measured masses of, or placed strong upper limits on, the dust content of 279 disks. The masses follow a log-normal distribution with a clear trend of decreasing mass from less to more evolved protostellar infrared class. The (logarithmic) mean Class I disk mass, M = 3.8 M⊕, is about 5 times greater than the mean Class II disk mass, but the dispersion in each class is so high, logM 0.8-1, that there is a large overlap between the two distributions. The disk mass distribution of flat-spectrum protostars lies in between Classes I and II. In addition, three Class III sources with little to no infrared excess are detected with low disk masses, M 0.3 M⊕. Despite the clear trend of decreasing disk mass with protostellar evolutionary state in this region, a comparison with surveys of Class II disks in other regions shows that masses do not decrease monotonically with age. This suggests that the cloud-scale environment may determine the initial disk mass scale or that there is substantial dust regeneration after 1 Myr.
The mass-luminosity relation for late-type stars has long been a critical tool for estimating stellar masses. However, there is growing need for both a higher-precision relation and a better ...understanding of systematic effects (e.g., metallicity). Here we present an empirical relationship between and M* spanning 0.075 M < M* < 0.70 M . The relation is derived from 62 nearby binaries, whose orbits we determine using a combination of Keck/NIRC2 imaging, archival adaptive optics data, and literature astrometry. From their orbital parameters, we determine the total mass of each system, with a precision better than 1% in the best cases. We use these total masses, in combination with resolved KS magnitudes and system parallaxes, to calibrate the -M* relation. The resulting posteriors can be used to determine masses of single stars with a precision of 2%-3%, which we confirm by testing the relation on stars with individual dynamical masses from the literature. The precision is limited by scatter around the best-fit relation beyond measured M* uncertainties, perhaps driven by intrinsic variation in the -M* relation or underestimated uncertainties in the input parallaxes. We find that the effect of Fe/H on the -M* relation is likely negligible for metallicities in the solar neighborhood (0.0% 2.2% change in mass per dex change in Fe/H). This weak effect is consistent with predictions from the Dartmouth Stellar Evolution Database, but inconsistent with those from MESA Isochrones and Stellar Tracks (at 5 ). A sample of binaries with a wider range of abundances will be required to discern the importance of metallicity in extreme populations (e.g., in the Galactic halo or thick disk).
ABSTRACT Open clusters and young stellar associations are attractive sites to search for planets and to test theories of planet formation, migration, and evolution. We present our search for, and ...characterization of, transiting planets in the 800 Myr old Praesepe (Beehive, M44) Cluster from K2 light curves. We identify seven planet candidates, six of which we statistically validate to be real planets, the last of which requires more data. For each host star, we obtain high-resolution NIR spectra to measure its projected rotational broadening and radial velocity, the latter of which we use to confirm cluster membership. We combine low-resolution spectra with the known cluster distance and metallicity to provide precise temperatures, masses, radii, and luminosities for the host stars. Combining our measurements of rotational broadening, rotation periods, and our derived stellar radii, we show that all planetary orbits are consistent with alignment to their host star's rotation. We fit the K2 light curves, including priors on stellar density to put constraints on the planetary eccentricities, all of which are consistent with zero. The difference between the number of planets found in Praesepe and Hyades (8 planets, Myr) and a similar data set for Pleiades (0 planets, 125 Myr) suggests a trend with age, but may be due to incompleteness of current search pipelines for younger, faster-rotating stars. We see increasing evidence that some planets continue to lose atmosphere past 800 Myr, as now two planets at this age have radii significantly larger than their older counterparts from Kepler.
All-sky photometric time-series missions have allowed for the monitoring of thousands of young (t(age) < 800 Myr) stars in order to understand the evolution of stellar activity. Here, we developed a ...convolutional neural network (CNN), stella, specifically trained to find flares in Transiting Exoplanet Survey Satellite (TESS) short-cadence data. We applied the network to 3200 young stars in order to evaluate flare rates as a function of age and spectral type. The CNN takes a few seconds to identify flares on a single light curve. We also measured rotation periods for 1500 of our targets and find that flares of all amplitudes are present across all spot phases, suggesting high spot coverage across the entire surface. Additionally, flare rates and amplitudes decrease for stars t(age) > 50 Myr across all temperatures T(eff) ≥ 4000 K, while stars from 2300 ≤ T(eff) < 4000 K show no evolution across 800 Myr. Stars of T(eff) ≤ 4000 K also show higher flare rates and amplitudes across all ages. We investigate the effects of high flare rates on photoevaporative atmospheric mass loss for young planets. In the presence of flares, planets lose 4%–7% more atmosphere over the first 1 Gyr. stella is an open-source Python toolkit hosted on GitHub and PyPI.
ABSTRACT We confirm and characterize a close-in ( = 5.425 days), super-Neptune sized ( ) planet transiting K2-33 (2MASS J16101473-1919095), a late-type (M3) pre-main-sequence (11 Myr old) star in the ...Upper Scorpius subgroup of the Scorpius-Centaurus OB association. The host star has the kinematics of a member of the Upper Scorpius OB association, and its spectrum contains lithium absorption, an unambiguous sign of youth ( Myr) in late-type dwarfs. We combine photometry from K2 and the ground-based MEarth project to refine the planet's properties and constrain the host star's density. We determine K2-33's bolometric flux and effective temperature from moderate-resolution spectra. By utilizing isochrones that include the effects of magnetic fields, we derive a precise radius (6%-7%) and mass (16%) for the host star, and a stellar age consistent with the established value for Upper Scorpius. Follow-up high-resolution imaging and Doppler spectroscopy confirm that the transiting object is not a stellar companion or a background eclipsing binary blended with the target. The shape of the transit, the constancy of the transit depth and periodicity over 1.5 yr, and the independence with wavelength rule out stellar variability or a dust cloud or debris disk partially occulting the star as the source of the signal; we conclude that it must instead be planetary in origin. The existence of K2-33b suggests that close-in planets can form in situ or migrate within ∼10 Myr, e.g., via interactions with a disk, and that long-timescale dynamical migration such as by Lidov-Kozai or planet-planet scattering is not responsible for all short-period planets.
(3200) Phaethon exhibits both comet- and asteroid-like properties, suggesting it could be a rare transitional object such as a dormant comet or previously volatile-rich asteroid. This justifies ...detailed study of (3200) Phaethon's physical properties as a better understanding of asteroid-comet transition objects can provide insight into minor body evolution. We therefore acquired time series photometry of (3200) Phaethon over 15 nights from 1994 to 2013, primarily using the Tektronix 2048 x 2048 pixel CCD on the University of Hawaii 2.2 m telescope. We utilized light curve inversion to (1) refine (3200) Phaethon's rotational period to P = 3.6032 + or - 0.0008 hr; (2) estimate a rotational pole orientation of lambda = +85degrees + or - 13degrees and beta = -20degrees + or - 10degrees; and (3) derive a shape model. We also used our extensive light curve data set to estimate the slope parameter of (3200) Phaethon's phase curve as G ~ 0.06, consistent with C-type asteroids. We discuss how this highly oblique pole orientation with a negative ecliptic latitude supports previous evidence for (3200) Phaethon's origin in the inner main asteroid belt as well as the potential for deeply buried volatiles fueling impulsive yet rare cometary outbursts.
ABSTRACT We confirm a 0.995 day periodic planetary transit-like signal, KOI 6705.01, in the Kepler light curve of the star KIC 6423922. Optical and infrared spectra show that this star is a mid ...M-type dwarf with an effective temperature K, metallicity Fe/H = −0.08 0.10, radius R , and mass = 0.28 0.05M . The star is pc away and its space motion, rotation period, and lack of H emission indicate it is an older member of the "thin disk" population. On the other hand, the star exhibits excess infrared emission suggesting a dust disk more typical of a very young star. If the KOI 6705.01 signal is produced by a planet, the transit depth of 60 ppm means its radius is only R⊕, or about the size of the Moon. However, the duration ( hr) and time variation of KOI 6705.01 are anomalous: the signal was undetected in the first two years of the mission and increased through the latter two years. These characteristics require implausible orbits and material properties for any planet and rule out such an explanation, although a dust cloud is possible. We excluded several false positive scenarios including background stars, scattered light from stars that are nearby on the sky, and electronic cross-talk between detector readout channels. We find the most likely explanation to be that KOI 6705.01 is a false positive created by charge transfer inefficiency in a detector column on which KIC 6423922 and a 1.99 day eclipsing binary both happened to fall.
Planets orbiting within the close-in habitable zones of M dwarf stars will be exposed to elevated high-energy radiation driven by strong magnetohydrodynamic dynamos during stellar youth. ...Near-ultraviolet (NUV) irradiation can erode and alter the chemistry of planetary atmospheres, and a quantitative description of the evolution of NUV emission from M dwarfs is needed when modeling these effects. We investigated the NUV luminosity evolution of early M-type dwarfs by cross-correlating the Lepine & Gaidos catalog of bright M dwarfs with the Galaxy Evolution Explorer (GALEX) catalog of NUV (1771-2831 Angstrom) sources. Of the 4805 sources with GALEX counterparts, 797 have NUV emission significantly (>2.5sigma) in excess of an empirical basal level. We inspected these candidate active stars using visible-wavelength spectra, high-resolution adaptive optics imaging, time-series photometry, and literature searches to identify cases where the elevated NUV emission is due to unresolved background sources or stellar companions; we estimated the overall occurrence of these "false positives" (FPs) as ~16%. We constructed an NUV luminosity function that accounted for FPs, detection biases of the source catalogs, and GALEX upper limits. We found the NUV luminosity function to be inconsistent with predictions from a constant star-formation rate and simplified age-activity relation defined by a two-parameter power law.
As international partnerships increasingly look to be the way forward for sustainable human space exploration, the need to think about language protocols becomes more pressing. Using the historical ...examples of three international human spaceflight missions, this viewpoint shows how each language protocol was dictated by political realities and how often difficulties arose during implementation as a result. It is argued that, in order to optimize operational environments in future human space exploration, the international space community should adopt a standardized, single-language protocol, similar to commercial aviation. While English may appear to be the most obvious candidate, other languages, particularly Russian and perhaps even Chinese, may also be worth considering.
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