We present the occurrence rates for rocky planets in the habitable zones (HZs) of main-sequence dwarf stars based on the Kepler DR25 planet candidate catalog and Gaia-based stellar properties. We provide the first analysis in terms of star-dependent instellation flux, which allows us to track HZ planets. We define eta(circle plus) as the HZ occurrence of planets with radii between 0.5 and 1.5 R-circle plus orbiting stars with effective temperatures between 4800 and 6300 K. We find that eta(circle plus) for the conservative HZ is between 0.37(-0.21)(+0.48) (errors reflect 68% credible intervals) and 0.60(-0.36)(+0.90) planets per star, while the optimistic HZ occurrence is between 0.58(-0.33)(+0.73) and 0.88(-0.51)(+1.28) planets per star. These bounds reflect two extreme assumptions about the extrapolation of completeness beyond orbital periods where DR25 completeness data are available. The large uncertainties are due to the small number of detected small HZ planets. We find similar occurrence rates between using Poisson likelihood Bayesian analysis and using Approximate Bayesian Computation. Our results are corrected for catalog completeness and reliability. Both completeness and the planet occurrence rate are dependent on stellar effective temperature. We also present occurrence rates for various stellar populations and planet size ranges. We estimate with 95% confidence that, on average, the nearest HZ planet around G and K dwarfs is similar to 6 pc away and there are similar to 4 HZ rocky planets around G and K dwarfs within 10 pc of the Sun.

Observations support the hypothesis that gas disk gravitational instability might explain the formation of massive or wide-orbit gas giant exoplanets. The situation with regard to Jupiter-mass exoplanets orbiting within similar to 20 au is more uncertain. Theoretical models yield divergent assessments often attributed to the numerical handling of the gas thermodynamics. Boss used the beta cooling approximation to calculate three-dimensional hydrodynamical models of the evolution of disks with initial masses of 0.091 M-circle dot extending from 4 to 20 au around 1 M-circle dot protostars. The models considered a wide range (1-100) of beta cooling parameters and started from an initial minimum Toomre stability parameter of Q(i) = 2.7 (gravitationally stable). The disks cooled down from initial outer disk temperatures of 180 K to as low as 40 K as a result of the beta cooling, leading to fragmentation into dense clumps, which were then replaced by virtual protoplanets (VPs) and evolved for up to similar to 500 yr. The present models test the viability of replacing dense clumps with VPs by quadrupling the spatial resolution of the grid once dense clumps form, sidestepping in most cases VP insertion. After at least similar to 200 yr of evolution, the new results compare favorably with those of Boss: similar numbers of VPs and dense clumps form by the same time for the two approaches. The results imply that VP insertion can greatly speed disk instability calculations without sacrificing accuracy.

To fully constrain the orbits of low-mass circumstellar companions, we conduct combined analyses of the radial velocity data and the Gaia and Hipparcos astrometric data for eight nearby systems. Our study shows that companion-induced position and proper motion differences between Gaia and Hipparcos are significant enough to constrain orbits of low-mass companions to a precision comparable with previous combined analyses of direct imaging and radial velocity data. We find that our method is robust to whether we use Gaia Data Release 2 or Gaia Early Data Release 3, as well as whether we use all of the data or just proper motion differences. In particular, we fully characterize the orbits of HD 190360 b and HD 16160 C for the first time. With a mass of 1.8 +/- 0.2 M-Jup and an effective temperature of 123-176K and orbiting around a Sun-like star, HD 190360 b is the smallest Jupiter-like planet with well-constrained mass and orbit, belonging to a small sample of fully characterized Jupiter analogues. It is separated from its primary star by 0.25 arcsec and thus may be suitable for direct imaging by the coronagraph instrument of the Roman Space Telescope.

While collisional accumulation is nearly universally accepted as the formation mechanism of rock and ice worlds, the situation regarding gas giant planet formation is more nuanced. Gas accretion by solid cores formed by collisional accumulation is the generally favored mechanism, but observations increasingly suggest that gas disk gravitational instability might explain the formation of at least the massive or wide-orbit gas giant exoplanets. This paper continues a series aimed at refining three-dimensional (3D) hydrodynamical models of disk instabilities, where the handling of the gas thermodynamics is a crucial factor. Boss (2017, 2021) used the beta cooling approximation to calculate 3D models of disks with initial masses of 0.091 M-circle dot extending from 4 to 20 au around 1 M-circle dot protostars. Here we employ 3D flux-limited diffusion (FLD) approximation models of the same disks, in order to provide a superior treatment of disk gas thermodynamics. The new models have quadrupled spatial resolution compared to previous 3D FLD models, in both the radial and azimuthal spherical coordinates, resulting in the highest spatial resolution 3D FLD models to date. The new models continue to support the hypothesis that such disks can form self-gravitating, dense clumps capable of contracting to form gas giant protoplanets, and suggest that the FLD models yield similar numbers of clumps as beta cooling models with beta similar to 1 to similar to 10, including the critical value of beta = 3 for fragmentation proposed by Gammie.

Cook et al. found that iron meteorites have an initial abundance ratio of the short-lived isotope Fe-60 to the stable isotope Fe-56 of Fe-60/Fe-56 similar to (6.4 +/- 2.0) x 10(-7). This appears to require the injection of live Fe-60 from a Type II supernova (SN II) into the presolar molecular cloud core, as the observed ratio is over a factor of 10 times higher than would be expected to be found in the ambient interstellar medium (ISM) as a result of galactic chemical evolution. The supernova triggering and injection scenario offers a ready explanation for an elevated initial Fe-60 level, and in addition provides a physical mechanism for explaining the noncarbonaceous-carbonaceous (NC-CC) dichotomy of meteorites. The NC-CC scenario hypothesizes the solar nebula first accreted material that was enriched in supernova-derived nuclides, and then later accreted material depleted in supernova-derived nuclides. While the NC-CC dichotomy refers to stable nuclides, not short-lived isotopes like Fe-60, the SN II triggering hypothesis provides an explanation for the otherwise unexplained change in nuclides being accreted by the solar nebula. Three-dimensional hydrodynamical models of SN II shock-triggered collapse show that after triggering collapse of the presolar cloud core, the shock front sweeps away the local ISM while accelerating the resulting protostar/disk to a speed of several kilometers per second, sufficient for the protostar/disk system to encounter within similar to 1 Myr the more distant regions of a giant molecular cloud complex that might be expected to have a depleted inventory of supernova-derived nuclides.

II Zw 23 (UGC 3179) is a luminous (M-B similar to -21) nearby compact narrow emission line starburst galaxy with blue optical colors and strong emission lines. We present a photometric and morphological study of II Zw 23 and its interacting companion, KPG103a, using data obtained with the WIYN 3.5m telescope in combination with a WFPC2 image from the Hubble Space Telescope archives. II Zw 23 has a highly disturbed outer structure with long trails of debris that may be contributing material toward the production of tidal dwarfs. Its central regions appear disky, a structure that is consistent with the overall rotation pattern observed in the Ha velocity field measured from Densepak observations obtained with WIYN. We find additional evidence for interaction in this system, including the discovery of a new tidal loop extending from an associated dwarf galaxy, which appears to be in the process of disrupting along its orbit. We also present Ha equivalent widths and discuss the relative star formation rates across this interacting system.