We present observations of the interstellar interloper 1I/2017 U1 ('Oumuamua) taken during its 2017 October flyby of Earth. The optical colors B -V=0.70±0.06, V -R=0.45±0.05, overlap those of the D-type Jovian Trojan asteroids and are incompatible with the ultrared objects that are abundant in the Kuiper Belt. With a mean absolute magnitude H V =22.95 and assuming a geometric albedo p V =0.1, we find an average radius of 55 m. No coma is apparent; we deduce a limit to the dust mass production rate of only ∼2×10 −4 kg s, ruling out the existence of exposed ice covering more than a few m 2 of the surface. Volatiles in this body, if they exist, must lie beneath an involatile surface mantle 0.5 m thick, perhaps a product of prolonged cosmic-ray processing in the interstellar medium. The light curve range is unusually large at ∼2.0±0.2 mag. Interpreted as a rotational light curve the body has axis ratio ³ -+ 6.3 1.1 1.3 :1 and semi-axes ∼230 m×35 m. A 6:1 axis ratio is extreme relative to most small solar system asteroids and suggests that albedo variations may additionally contribute to the variability. The light curve is consistent with a two-peaked period ∼8.26 hr, but the period is non-unique as a result of aliasing in the data. Except for its unusually elongated shape, 1I/2017 U1 is a physically unremarkable, sub-kilometer, slightly red, rotating object from another planetary system. The steady-state population of similar, ∼100 m scale interstellar objects inside the orbit of Neptune is ∼10 4 , each with a residence time of ∼10 years.

We present imaging and spectroscopic observations of 6478 Gault, a ∼6 km diameter inner main-belt asteroid currently exhibiting strong, comet-like characteristics. Three distinct tails indicate that ultra-slow dust (ejection speed 0.15±0.05 m s −1 ) was emitted from Gault in separate episodes beginning UT 2018 October 28±5 (Tail A), UT 2018 December 31±5 (Tail B), and UT 2019 February 10±7 (Tail C), with durations of ∆T ∼ 10 to 20 days. With a mean particle radius a ∼ 200 µm, the estimated masses of the tails are M A ∼ 4 × 10 7 kg, M B ∼ 6 × 10 6 kg and M C ∼ 6 × 10 5 kg, respectively, and the mass loss rates from the nucleus are 20 to 40 kg s −1 for Tail A, 4 to 6 kg s −1 for Tail B and ∼0.4 kg s −1 for Tail C. In its optical colors Gault is more similar to C-type asteroids than to S-types, even though the latter are numerically dominant in the inner asteroid belt. A spectroscopic upper limit to the production of gas is set at 1 kg s −1 . Discrete emission in three protracted episodes effectively rules out an impact origin for the observed activity. Sublimation driven activity is unlikely given the inner belt orbit and the absence of detectable gas. In any case, sublimation would not easily account for the observed multiple ejections. The closest similarity is between Gault and active asteroid 311P/(2013 P5), an object showing repeated but aperiodic ejections of dust over a 9 month period. While Gault is 10 times larger than 311P/(2013 P5), and the relevant timescale for spin-up by radiation torques is ∼100 times longer, its properties are likewise most consistent with episodic emission from a body rotating near breakup.

We present mid-infrared nulling interferometric and direct imaging observations of the Herbig Ae star HD 100546 obtained with the Magellan I (Baade) 6.5 m telescope. The observations show resolved circumstellar emission at 10.3, 11.7, 12.5, 18.0, and 24.5 µm. Through the nulling observations (10.3, 11.7 and 12.5 µm), we detect a circumstellar disk, with an inclination of 45 ± 15 degrees with respect to a face-on disk, a semimajor axis position angle of 150 ± 10 degrees (E of N), and a spatial extent of about 25 AU. The direct images (18.0 and 24.5 µm) show evidence for cooler dust with a spatial extent of 30-40 AU from the star. The direct images also show evidence for an inclined disk with a similar position angle as the disk detected by nulling. This morphology is consistent with models in which a flared circumstellar disk dominates the emission. However, the similarity in relative disk size we derive for different wavelengths suggests that the disk may have a large inner gap, possibly cleared out by the formation of a giant protoplanet. The existence of a protoplanet in the system also provides a natural explanation for the observed difference between HD 100546 and other Herbig Ae stars.

We determine the relative ages of the open cluster NGC 188 and selected Hipparcos Ðeld stars by isochrone Ðtting and compare them to the age of the thick-disk globular cluster 47 Tuc. The best-Ðt age for NGC 188 was determined to be 6.5^1.0 Gyr. The solar-metallicity Hipparcos Ðeld stars yielded a slightly older thin-disk age, 7.5^0.7 Gyr. Two slightly metal-poor ([Fe/H] \ [0.22) Ðeld stars whose kinematic and orbital parameters indicate that they are members of the thin disk were found to have an age of 9

We combine nulling interferometry at 10 µm using the MMT and Keck Telescopes with spectroscopy, imaging, and photometry from 3 to 100 µm using Spitzer to study the debris disk around β Leo over a broad range of spatial scales, corresponding to radii of 0.1 to ∼100 AU. We have also measured the close binary star o Leo with both Keck and MMT interferometers to verify our procedures with these instruments. The β Leo debris system has a complex structure: 1.) relatively little material within 1 AU; 2.) an inner component with a color temperature of ∼600 K, fitted by a dusty ring from about 2 to 3 AU; and 3.) a second component with a color temperature of ∼120 K fitted by a broad dusty emission zone extending from about ∼5 AU to ∼55 AU. Unlike many other A-type stars with debris disks, β Leo lacks a dominant outer belt near 100 AU.

We present the results of 10 micron nulling interferometric observations of 13 Herbig Ae stars using the Magellan I (Baade) and the MMT 6.5 m telescopes. A portion of the observations was completed with the adaptive secondary at the MMT. We have conclusively spatially resolved 3 of the 13 stars, HD 100546, AB Aur, and HD 179218, the latter two recently resolved using adaptive optics in combination with nulling interferometry. For the resolved objects we find that the 10 $\mu$m emitting regions have a spatial extent of 15-30 AU in diameter. We also have some evidence for resolved emission surrounding an additional 2 stars (V892 Tau and R CrA). For those objects in our study with mid-IR SEDs classifications from Meeus et al. (2001), we find that the Group I objects (those with constant to increasing mid-IR flux) are more likely to be resolved, within our limited sample. This trend is evident in correlations in the inferred disk sizes vs. the sub-millimeter SED slope and disk size vs. fractional infrared luminosity of the systems. We explore the spatial distribution and orientation of the warm dust in the resolved systems and constrain physical models which are consistent with their observational signatures.Comment: 28 pages, including 3 figures, Accepted for publication in The Astrophysical Journa

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We present the results of mid-infrared nulling interferometric observations of the main-sequence star a Lyr (Vega) using the 6.5 m MMT with its adaptive secondary mirror. From the observations at 10.6 mm, we find that there is no resolved emission from the circumstellar environment (at separations greater than 0.8 AU) above 2.1% (3 j limit) of the level of the stellar photospheric emission. Thus, we are able to place an upper limit on the density of dust in the inner system of 650 times that of our own solar system's zodiacal cloud. This limit is roughly 2.8 times better than those determined with photometric excess observations such as those by IRAS. Comparison with far-infrared observations by IRAS shows that the density of warm dust in the inner system (!30 AU) is significantly lower than cold dust at larger separations. We consider two scenarios for grain removal, the sublimation of ice grains and the presence of a planetary mass "sweeper." We find that if sublimation of ice grains is the only removal process, a large fraction (180%) of the material in the outer system is ice.