We present ALMA observations of 106 G-, K-, and M-type stars in the Upper Scorpius OB Association hosting circumstellar disks. With these data, we measure the 0.88 mm continuum and 12 CO J=3-2 line fluxes of disks around low-mass (0.14-1.66 M e ) stars at an age of 5-11 Myr. Of the 75 primordial disks in the sample, 53 are detected in the dust continuum and 26 in CO. Of the 31 disks classified as debris/evolved transitional disks, five are detected in the continuum and none in CO. The lack of CO emission in approximately half of the disks with detected continuum emission can be explained if CO is optically thick but has a compact emitting area (40 au), or if the CO is heavily depleted by a factor of at least ∼1000 relative to interstellar medium abundances and is optically thin. The continuum measurements are used to estimate the dust mass of the disks. We find a correlation between disk dust mass and stellar host mass consistent with a power-law relation of * µ  M M dust 1.67 0.37 . Disk dust masses in Upper Sco are compared to those measured in the younger Taurus star-forming region to constrain the evolution of disk dust mass. We find that the difference in the mean of * M M log dust ( )between Taurus and Upper Sco is 0.64±0.09, such that M dust /M * is lower in Upper Sco by a factor of ∼4.5.

Theories of the formation and early evolution of planetary systems postulate that planets are born in circumstellar disks, and undergo radial migration during and after dissipation of the dust and gas disk from which they formed 1,2 . The precise ages of meteorites indicate that planetesimals -the building blocks of planets -are produced within the first million years of a star's life 3 . A prominent question is: how early can one find fully formed planets like those frequently detected on short orbital periods around mature stars? Some theories suggest the in situ formation of planets close to their host stars is unlikely and the existence of such planets is evidence for large scale migration 4,5 . Other theories posit that planet assembly at small orbital separations may be common [6][7][8] . Here we report on a newly-born, transiting planet orbiting its star every 5.4 days. The planet is 50% larger than Neptune, and its mass is less than 3.6 times Jupiter (at 99.7% confidence), with a true mass likely to be within a factor of several of Neptune's. The 5-10 million year old star has a tenuous dust disk extending in to about 2 times the Earth-Sun separation, in addition to the large planet located at less than 1/20 the Earth-Sun separation.USco 161014. 75-191909.3, hereafter K2-33, is a several million year old M-type star that was observed by NASA's Kepler Space Telescope during Campaign 2 of the K2 mission. The star was identified as one of more than 200 candidate planet hosts in a systematic search for transits in K2 data 9 . As part of our ongoing study of the pre-main sequence population of Upper Scorpius observed by K2, we independently verified and analyzed the planetary transit signal. We acquired radial velocity (RV) and high spatial resolution observations at the W. M. Keck Observatory to confirm the planet, hereafter K2-33b, and to measure its size and mass.2 Within the 77.5 day photometric time series of K2-33 (Kp = 14.3 mag), there are periodic dimmings of 0.23% lasting 4.2 hours and occurring every 5.4 d (Fig. 1). The ensemble of transits are detected at a combined signal-to-noise ratio of ≈ 32. During the K2 observations, cool, dark regions on the stellar surface (starspots) rotated in and out of view, producing semi-sinusoidal brightness variations of ∼3% peak-to-trough amplitude with a 6.3 ± 0.2 d periodicity (Extended Data Fig. 1). We removed the starspot variability prior to modeling the transit events. We fit the transit profiles using established methods 10, measuring the planet's size relative to its host star and its orbital geometry (Table 1).K2-33 is an established member of the Upper Scorpius OB association 11,12 , the nearest site , which we confirm from Keck spectra (Table 1). Furthermore, the stellar rotation rate we measure via broadening of absorption lines in the spectra and via the starspot period (Table 1), is rapid relative to field-age stars of similar mass 14 . We determined the star's systemic RV (Table 1) The inferred planet size and mass depend directly upon the host sta...

We present detailed modeling of the spatial distributions of gas and dust in 57 circumstellar disks in the Upper Scorpius OB Association observed with ALMA at submillimeter wavelengths. We fit power-law models to the dust surface density and CO J=3-2 surface brightness to measure the radial extent of dust and gas in these disks. We found that these disks are extremely compact: the 25 highest signal-to-noise disks have a median dust outer radius of 21 au, assuming an R 1 -dust surface density profile. Our lack of CO detections in the majority of our sample is consistent with these small disk sizes assuming the dust and CO share the same spatial distribution. Of seven disks in our sample with well-constrained dust and CO radii, four appear to be more extended in CO, although this may simply be due to the higher optical depth of the CO. Comparison of the Upper Sco results with recent analyses of disks in Taurus, Ophiuchus, and Lupus suggests that the dust disks in Upper Sco may be approximately three times smaller in size than their younger counterparts, although we caution that a more uniform analysis of the data across all regions is needed. We discuss the implications of these results for disk evolution.

We analyze the dust morphology of 29 transition disks (TDs) observed with Atacama Large (sub-)Millimeter Array (ALMA) at (sub-)millimeter emission. We perform the analysis in the visibility plane to characterize the total flux, cavity size, and shape of the ring-like structure. First, we found that the M dust -M å relation is much flatter for TDs than the observed trends from samples of class II sources in different star-forming regions. This relation demonstrates that cavities open in high (dust) mass disks, independent of the stellar mass. The flatness of this relation contradicts the idea that TDs are a more evolved set of disks. Two potential reasons (not mutually exclusive) may explain this flat relation: the emission is optically thick or/and millimeter-sized particles are trapped in a pressure bump. Second, we discuss our results of the cavity size and ring width in the context of different physical processes for cavity formation. Photoevaporation is an unlikely leading mechanism for the origin of the cavity of any of the targets in the sample. Embedded giant planets or dead zones remain as potential explanations. Although both models predict correlations between the cavity size and the ring shape for different stellar and disk properties, we demonstrate that with the current resolution of the observations, it is difficult to obtain these correlations. Future observations with higher angular resolution observations of TDs with ALMA will help discern between different potential origins of cavities in TDs.

The AB Dor Moving Group consists of a "nucleus" of ∼10 stars at d ≃ 20 pc, along with dozens of purported "stream" members distributed across the sky. We perform a chemical and kinematic analysis of a subsample of AB Dor stream stars to test whether they constitute a physical stellar group. We use the NEMO Galactic kinematic code to investigate the orbits of the stream members, and perform a chemical abundance analysis using high resolution spectra taken with the Magellan Clay 6.5-m telescope. Using a χ 2 test with the measured abundances for 10 different elements, we find that only half of the purported AB Dor stream members could possibly constitute a statistically chemically homogeneous sample. Some stream members with 3D velocities were hundreds of parsecs from the AB Dor nucleus ∼10 8 yr ago, and hence were unlikely to share a common origin. We conclude that the published lists of AB Dor moving group stream members are unlikely to represent the dispersed remnant of a single star formation episode. A subsample of the stream stars appears to be both statistically chemically homogeneous and in the vicinity of the AB Dor nucleus at birth. Their mean metallicity is [Fe/H] = 0.02 ± 0.02 dex, which we consider representative for the AB Dor group. Finally, we report a strong lower limit on the age of the AB Dor nucleus of >110 Myr based on the pre-MS contraction times for K-type members which have reached the main sequence.