We report trigonometric parallaxes for 134 low-mass stars and brown dwarfs, of which 38 have no previously published measurement and 79 more have improved uncertainties. Our survey focused on nearby targets, so 119 are closer than 30 pc. Of the 38 stars with new parallaxes, 14 are within 20 pc and seven are likely brown dwarfs (spectral types later than L0). These parallaxes are useful for studies of kinematics, multiplicity, and spectrophotometric calibration. Two objects with new parallaxes are confirmed as young stars with membership in nearby young moving groups: LP 870-65 in AB Doradus and G 161-71 in Argus. We also report the first parallax for the planet-hosting star GJ 3470; this allows us to refine the density of its Neptune-mass planet. T-dwarf 2MASS J12590470-4336243, previously thought to lie within 4 pc, is found to be at 7.8 pc, and the M-type star 2MASS J01392170-3936088 joins the ranks of nearby stars as it is found to be within 10 pc. Five stars that are overluminous and/or too red for their spectral types are identified and deserve further study as possible young stars.

The TW Hya Association (TWA) is a nearby stellar association with an age of similar to 5-10 Myr. This is an important age for studying the late stages of star and planet formation. We measure the parallaxes of 14 candidate members of TWA. That brings to 38 the total number of individual stars with fully measured kinematics, i.e., proper motion, radial velocity, and parallax, to describe their motions through the Galaxy. We analyze these kinematics to search for convergence to a smaller volume in the past, but we find that the association is never much more compact than it is at present. We show that it is difficult to measure traceback ages for associations such as TWA that have expected velocity dispersions of 1-2 km s(-1) with typical measurement uncertainties. We also use our stellar distances and pre-main-sequence evolutionary tracks to find the average age of the association of 7.9 +/- 1.0 Myr. Additionally, our parallax measurement of TWA 32 indicates that it should be considered a bona fide member of TWA. Two new candidate members have high membership probabilities, and we assign them TWA numbers: TWA 45 for 2MASS J11592786-4510192 and TWA 46 for 2MASS J12354615-4115531.

Observational evidence exists for the formation of gas giant planets on wide orbits around young stars by disk gravitational instability, but the roles of disk instability and core accretion for forming gas giants on shorter period orbits are less clear. The controversy extends to population synthesis models of exoplanet demographics and to hydrodynamical models of the fragmentation process. The latter refers largely to the handling of radiative transfer in three-dimensional (3D) hydrodynamical models, which controls heating and cooling processes in gravitationally unstable disks, and hence dense clump formation. A suite of models using the beta cooling approximation is presented here. The initial disks have masses of 0.091M(circle dot) and extend from 4 to 20 au around a 1M(circle dot) protostar. The initial minimum Toomre Qi values range from 1.3 to 2.7, while beta ranges from 1 to 100. We show that the choice of Qi is equal in importance to the beta value assumed: high Qi disks can be stable for small a, when the initial disk temperature is taken as a lower bound, while low Qi disks can fragment for high beta. These results imply that the evolution of disks toward low Qi must be taken into account in assessing disk fragmentation possibilities, at least in the inner disk, i.e., inside about 20 au. The models suggest that if low Qi disks can form, there should be an as yet largely undetected population of gas giants orbiting G dwarfs between about 6 au and 16 au.

Recent meteoritical analyses support an initial abundance of the short-lived radioisotope (SLRI) Fe-60 that may be high enough to require nucleosynthesis in a core-collapse supernova, followed by rapid incorporation into primitive meteoritical components, rather than a scenario where such isotopes were inherited from a well-mixed region of a giant molecular cloud polluted by a variety of supernovae remnants and massive star winds. This paper continues to explore the former scenario, by calculating three-dimensional, adaptive mesh refinement, hydrodynamical code (FLASH 2.5) models of the self-gravitational, dynamical collapse of a molecular cloud core that has been struck by a thin shock front with a speed of 40 km s(-1), leading to the injection of shock front matter into the collapsing cloud through the formation of Rayleigh-Taylor fingers at the shock-cloud intersection. These models extend the previous work into the nonisothermal collapse regime using a polytropic approximation to represent compressional heating in the optically thick protostar. The models show that the injection efficiencies of shock front materials are enhanced compared to previous models, which were not carried into the nonisothermal regime, and so did not reach such high densities. The new models, combined with the recent estimates of initial Fe-60 abundances, imply that the supernova triggering and injection scenario remains a plausible explanation for the origin of the SLRIs involved in the formation of our solar system.

Transit photometry of the M8V dwarf star TRAPPIST-1 (2MASS J23062928-0502285) has revealed the presence of at least seven planets with masses and radii similar to that of Earth, orbiting at distances that might allow liquid water to be present on their surfaces. We have been following TRAPPIST-1 since 2011 with the CAPSCam astrometric camera on the 2.5 m du Pont telescope at the Las Campanas Observatory in Chile. In 2016, we noted that TRAPPIST-1 lies slightly farther away than previously thought, at 12.49 pc, rather than 12.1 pc. Here, we examine 15 epochs of CAPSCam observations of TRAPPIST-1, spanning the five years from 2011 to 2016, and obtain a revised trigonometric distance of 12.56 +/- 0.12 pc. The astrometric data analysis pipeline shows no evidence for a long-period astrometric wobble of TRAPPIST-1. After proper motion and parallax are removed, residuals at the level of +/- 1.3 mas remain. The amplitude of these residuals constrains the masses of any long-period gas giant planets in the TRAPPIST-1 system: no planet more massive than similar to 4.6M(Jup) orbits with a 1 year period, and no planet more massive than similar to 1.6 M-Jup orbits with a 5 year period. Further refinement of the CAPSCam data analysis pipeline, combined with continued CAPSCam observations, should either detect any long-period planets, or put an even tighter constraint on these mass upper limits.

Upper Scorpius is a subgroup of the nearest OB association, Scorpius-Centaurus. Its young age makes it an important association to study star and planet formation. We present parallaxes to 52 low-mass stars in Upper Scorpius, 28 of which have full kinematics. We measure ages of the individual stars by combining our measured parallaxes with pre-main-sequence evolutionary tracks. We find a significant difference in the ages of stars with and without circumstellar disks. The stars without disks have a mean age of 4.9 +/- 0.8 Myr and those with disks have an older mean age of 8.2 +/- 0.9 Myr. This somewhat counterintuitive result suggests that evolutionary effects in young stars can dominate their apparent ages. We also attempt to use the 28 stars with full kinematics (i.e., proper motion, radial velocity (RV), and parallax) to trace the stars back in time to their original birthplace to obtain a trackback age. As expected, given the large measurement uncertainties on available RV measurements, we find that measurement uncertainties alone cause the group to diverge after a few Myr.

We report individual dynamical masses for the brown dwarfs e Indi B and C, which have spectral types of T1.5 and T6, respectively, measured from astrometric orbit mapping. Our measurements are based on a joint analysis of astrometric data from the Carnegie Astrometric Planet Search and the Cerro Tololo Inter-American Observatory Parallax Investigation, as well as archival high-resolution imaging, and use a Markov chain Monte Carlo method. We find dynamical masses of 75.0 +/- 0.82M(Jup) for the T1.5 B component and 70.1 +/- 0.68M(Jup) for the T6 C component. These masses are surprisingly high for. such cool objects and challenge our understanding of substellar structure and evolution. We discuss several evolutionary scenarios proposed in the literature and find that while none of them can provide conclusive explanations for the high substellar masses, evolutionary models incorporating lower atmospheric opacities come closer to approximating our results. We discuss the details of our astrometric model, its algorithm implementation, and how we determine parameter values via Markov chain Monte Carlo Bayesian inference.

Cosmochemical evaluations of the initial meteoritical abundance of the short-lived radioisotope (SLRI) Al-26 have remained fairly constant since 1976, while estimates for the initial abundance of the SLRI Fe-60 have varied widely recently. At the high end of this range, Fe-60 initial abundances have seemed to require Fe-60 nucleosynthesis in a core-collapse supernova, followed by incorporation into primitive meteoritical components within similar to 1 Myr. This paper continues the detailed exploration of this classical scenario, using models of the self-gravitational collapse of molecular cloud cores that have been struck by suitable shock fronts, leading to the injection of shock front gas into the collapsing cloud through Rayleigh-Taylor fingers formed at the shock-cloud interface. As before, these models are calculated using the FLASH three-dimensional, adaptive mesh refinement, gravitational hydrodynamical code. While the previous models used FLASH 2.5, the new models employ FLASH 4.3, which allows sink particles to be introduced to represent the newly formed protostellar object. Sink particles permit the models to be pushed forward farther in time to the phase where a similar to 1 M-circle dot protostar has formed, orbited by a rotating protoplanetary disk. These models are thus able to define what type of target cloud core is necessary for the supernova triggering scenario to produce a plausible scheme for the injection of SLRIs into the presolar cloud core: a similar to 3 M-circle dot cloud core rotating at a rate of similar to 3. x. 10(-14) rad s-1 or higher.

The ability to make independent detections of the signatures of exoplanets with complementary telescopes and instruments brings a new potential for robust identification of exoplanets and precision characterization. We introduce PEXO, a package for Precise EXOplanetology to facilitate the efficient modeling of timing, astrometry, and radial velocity data, which will benefit not only exoplanet science but also various astrophysical studies in general. PEXO is general enough to account for binary motion and stellar reflex motions induced by planetary companions and is precise enough to treat various relativistic effects both in the solar system and in the target system. We also model the post-Newtonian barycentric motion for future tests of general relativity in extrasolar systems. We benchmark PEXO with the pulsar timing package TEMPO2 and find that PEXO produces numerically similar results with timing precision of about 1 ns, space-based astrometry to a precision of 1 mu as, and radial velocity of 1 mu m s(-1) and improves on TEMPO2 for decade-long timing data of nearby targets, due to its consideration of third-order terms of Roemer delay. PEXO is able to avoid the bias introduced by decoupling the target system and the solar system and to account for the atmospheric effects that set a practical limit for ground-based radial velocities close to 1 cm s(-1). Considering the various caveats in barycentric correction and ancillary data Required to realize cm s(-1) modeling, we recommend the preservation of original observational data. The PEXO modeling package is available at GitHub (https://github.com/phillippro/pexo) and Zenodo (Feng et al. 2019).

Observational evidence suggests that gas disk instability may be responsible for the formation of at least some gas giant exoplanets, particularly massive or distant gas giants. With regard to close-in gas giants, Boss used the beta cooling approximation to calculate hydrodynamical models of inner gas disk instability, finding that provided disks with low values of the initial minimum Toomre stability parameter (i.e., Q(i) < 2 inside 20 au) form, fragmentation into self-gravitating clumps could occur even for beta as high as 100 (i.e., extremely slow cooling). Those results implied that the evolution of disks toward low Q(i) must be taken into account. This paper presents such models: initial disk masses of 0.091 M-circle dot extending from 4 to 20 au around a 1Me protostar, with a range (1-100) of beta cooling parameters, the same as in Boss, but with all the disks starting with Q(i) = 2.7, i.e., gravitationally stable, and allowed to cool from their initial outer disk temperature of 180 K to as low as 40 K. All the disks eventually fragment into at least one dense clump. The clumps were again replaced by virtual protoplanets (VPs) and the masses and orbits of the resulting ensemble of VPs compare favorably with those of Boss, supporting the claim that disk instability can form gas giants rapidly inside 20 au, provided that sufficiently massive protoplanetary disks exist.