I show that a recently discovered star cluster near the center of the ultra-faint dwarf galaxy Eridanus II provides strong constraints on massive compact halo objects (MACHOs) of 5 M e as the main component of dark matter. MACHO dark matter will dynamically heat the cluster, driving it to larger sizes and higher velocity dispersions until it dissolves into its host galaxy. The stars in compact ultra-faint dwarf galaxies themselves will be subject to the same dynamical heating; the survival of at least 10 such galaxies places independent limits on MACHO dark matter of masses 10 M e . Both Eri II's cluster and the compact ultra-faint dwarfs are characterized by stellar masses of just a few thousand M e and half-light radii of 13 pc (for the cluster) and ∼30 pc (for the ultra-faint dwarfs). These systems close the ∼20-100 M e window of allowed MACHO dark matter and combine with existing constraints from microlensing, wide binaries, and disk kinematics to rule out dark matter composed entirely of MACHOs from ∼10 −7 M e up to arbitrarily high masses.

We present high-resolution, H-band, imaging observations, collected with Subaru/HiCIAO, of the scattered light from the transitional disk around SAO 206462 (HD 135344B). Although previous submm imagery suggested the existence of the dust-depleted cavity at r ≤ 46 AU, our observations reveal the presence of scattered light components as close as 0. ′′ 2 (∼ 28 AU) from the star. Moreover, we have discovered two small-scale spiral structures lying within 0. ′′ 5 (∼ 70 AU). We present models for the spiral structures using the spiral density wave theory, and derive a disk aspect ratio of h ∼ 0.1, which is consistent with previous sub-mm observations. This model can potentially give estimates of the temperature and rotation profiles of the disk based on dynamical processes, independently from sub-mm observations. It also predicts the evolution of the spiral structures, which can be observable on timescales of 10-20 years, providing conclusive tests of the model. While we cannot uniquely identify the origin of these spirals, planets embedded in the disk may be capable of exciting the observed morphology. Assuming that this is the case, we can make predictions on the locations and, possibly, the masses of the unseen planets. Such planets may be detected by future multi-wavelengths observations.

We derive the delay-time distribution (DTD) of Type Ia supernovae (SNe Ia) using a sample of 132 SNe Ia, discovered by the Sloan Digital Sky Survey II (SDSS2) among 66 000 galaxies with spectral-based star formation histories (SFHs). To recover the best-fitting DTD, the SFH of every individual galaxy is compared, using Poisson statistics, to the number of SNe that it hosted (zero or one), based on the method introduced in Maoz et al. This SN sample differs from the SDSS2 SN Ia sample analysed by Brandt et al., using a related, but different, DTD recovery method. Furthermore, we use a simulation-based SN detection-efficiency function, and we apply a number of important corrections to the galaxy SFHs and SN Ia visibility times. The DTD that we find has 4σ detections in all three of its time bins: prompt (τ < 0.42 Gyr), intermediate (0.42 < τ < 2.4 Gyr) and delayed (τ > 2.4 Gyr), indicating a continuous DTD, and it is among the most accurate and precise among recent DTD reconstructions. The best-fitting power-law form to the recovered DTD is t −1.07 ±0.07 , consistent with generic ∼t −1 predictions of SN Ia progenitor models based on the gravitational-wave-induced mergers of binary white dwarfs. The time-integrated number of SNe Ia per formed stellar mass is N SN /M = 0.00130 ± 0.00015 M −1 , or about 4 per cent of the stars formed with initial masses in the 3−8 M range. This is lower than, but largely consistent with, several recent DTD estimates based on SN rates in galaxy clusters and in local-volume galaxies, and is higher than, but consistent with N SN /M estimated by comparing volumetric SN Ia rates to cosmic SFH.

This paper presents a cross-calibrated catalog of Hipparcos and Gaia astrometry to enable their use in measuring changes in proper motion, i.e., accelerations in the plane of the sky. The final catalog adopts the reference frame of the second Gaia data release (DR2) and locally cross-calibrates both the scaled Hipparcos–Gaia DR2 positional differences and the Hipparcos proper motions themselves to this frame. This gives three nearly independent proper motion measurements per star, with the scaled positional difference usually being the most precise. We find that a linear combination of the two Hipparcos reductions is superior to either reduction on its own and address error inflation for both Hipparcos and Gaia DR2. Our adopted error inflation is additive (in quadrature) for Hipparcos and multiplicative for Gaia. We provide the covariance matrices along with the central epochs of all measurements. Our final proper motion differences are accurately Gaussian with the appropriate variances and are suitable for acceleration measurements and orbit fitting. The catalog is constructed with an eye toward completeness; it contains nearly 98% of the Hipparcos stars. It also includes a handful of spurious entries and a few stars with poor Hipparcos reductions that the user must vet by hand. Statistical distributions of accelerations derived from this catalog should be interpreted with caution.

We present the first near-IR scattered light detection of the transitional disk associated with the Herbig Ae star MWC 758 using data obtained as part of the Strategic Exploration of Exoplanets and Disks with Subaru, and 1.1µm HST/NICMOS data. While sub-millimeter studies suggested there is a dustdepleted cavity with r=0. ′′ 35, we find scattered light as close as 0.1 ′′ (20-28 AU) from the star, with no visible cavity at H, K', or K s . We find two small-scaled spiral structures which asymmetrically shadow the outer disk. We model one of the spirals using spiral density wave theory, and derive a disk aspect ratio of h∼0.18, indicating a dynamically warm disk. If the spiral pattern is excited by a perturber, we estimate its mass to be 5 +3 −4 M J , in the range where planet filtration models predict accretion continuing onto the star. Using a combination of non-redundant aperture masking data at L ′ and angular differential imaging with Locally Optimized Combination of Images at K ′ and K s , we exclude stellar or massive brown dwarf companions within 300 mas of the Herbig Ae star, and all but planetary mass companions exterior to 0. ′′ 5. We reach 5-σ contrasts limiting companions to planetary masses, 3-4 M J at 1. ′′ 0 and 2 M J at 1. ′′ 55 using the COND models. Collectively, these data strengthen the case for MWC 758 already being a young planetary system. Subject headings: circumstellar matter instrumentation: high angular resolution polarization planetary systems: protoplanetary disks stars: individual (MWC 758) waves