Scott S. Sheppard

A New Population of High-Redshift Short-Duration Gamma-Ray Bursts

The redshift distribution of the short-duration gamma-ray bursts (GRBs) is a crucial, but currently fragmentary, cluetothenatureoftheirprogenitors.HerewepresentopticalobservationsofnineshortGRBsobtainedwithGemini, Magellan, and the Hubble Space Telescope. We detect the afterglows and host galaxies of two short bursts, and host galaxiesfortwoadditionalburstswithknownopticalafterglowpositions,andfivewithX-raypositions(P6 00 radius).In eightoftheninecaseswefindthatthemostprobablehostgalaxiesarefaint,R � 23 26:5mag,andarethereforestarkly different from the first few short GRB hosts with R � 17 22 mag and z P0:5. Indeed, we measure spectroscopic redshifts ofz � 0:4 1:1for the four brightest hosts. A comparison to largefield galaxy samples, as well as the hosts of longGRBsandpreviousshortGRBs,indicatesthatthefainterhostslikelyresideatz k1. Ourmostconservativelimit is that at least half of the five hosts without a known redshift reside at z > 0:7 (97% confidence level), suggesting that about 1 to 2 of all short GRBs originate at higher redshifts than previously determined. This has two important implications: (1) we constrain the acceptable age distributions to a wide lognormal (� k1) with � � � 4 8G yr, or to a powerlaw,P(� ) / � n ,with � 1Pn P0;and(2)theinferredisotropicenergies,E�; iso � 1050 10 52 ergs,aresignificantly larger than � 10 48 ‐10 49 ergs for the low-redshift, short GRBs, indicating a large spread in energy release or jet opening angles. Finally, we reiterate the importance of short GRBs as potential gravitational-wave sources and find a conservative detection rate with the advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) of � 2‐6 yr � 1 .

A Sedna-like body with a perihelion of 80 astronomical units

The observable Solar System can be divided into three distinct regions: the rocky terrestrial planets including the asteroids at 0.39 to 4.2 astronomical units (au) from the Sun (where 1 au is the mean distance between Earth and the Sun), the gas giant planets at 5 to 30 au from the Sun, and the icy Kuiper belt objects at 30 to 50 au from the Sun. The 1,000-kilometre-diameter dwarf planet Sedna was discovered ten years ago and was unique in that its closest approach to the Sun (perihelion) is 76 au, far greater than that of any other Solar System body. Formation models indicate that Sedna could be a link between the Kuiper belt objects and the hypothesized outer Oort cloud at around 10,000 au from the Sun. Here we report the presence of a second Sedna-like object, 2012 VP113, whose perihelion is 80 au. The detection of 2012 VP113 confirms that Sedna is not an isolated object; instead, both bodies may be members of the inner Oort cloud, whose objects could outnumber all other dynamically stable populations in the Solar System.

Time-resolved Photometry of Kuiper Belt Objects: Rotations, Shapes, and Phase Functions

We present a systematic investigation of the rotational light curves of trans-Neptunian objects based on extensive optical data from Mauna Kea. Four of 13 objects (corresponding to 31%) in our sample [(33128) 1998 BU48, 2000 GN171, (20000) Varuna, and (40314) 1999 KR16] were found to exhibit light curves with peak-to-peak range � 0.15 mag. In a larger sample obtained by combining our data with reliably determined light curves from the literature, seven of 22 objects (32%) display significant (� 0.15 mag range) light curves. About 23% of the sampled objects have light-curve ranges � 0.4 mag. Curiously, the objects are very large (e250 km diameter, assuming an albedo of 0.04) and, in the absence of rotation, should be nearly spherical as a result of self-compression. We propose that the large-amplitude, short-period objects are rotationally distorted, low-density rubble piles. Statistically, the trans-Neptunian objects are less spherical than their main-belt asteroid counterparts, indicating a higher specific angular momentum, perhaps resulting from the formation epoch. In addition to the rotational light curves, we measured phase darkening for seven Kuiper belt objects in the 0 � to 2 � phase-angle range. Unlike Pluto, the measured values show steep slopes and moderate opposition surge, indicating backscatter from low-albedo porous surface materials.

THE ALBEDO DISTRIBUTION OF JOVIAN TROJAN ASTEROIDS

We present radiometrically derived V-band geometric albedos and effective radii for 32 Jovian Trojan asteroids, using near-simultaneous mid-infrared and visible observations. We sampled the large end of the group’s size distribution, down to a radius of 25 km, using 14 objects in the L4 swarm and 18 in the L5 swarm. We find that the albedo distribution is much narrower than previously derived from IRAS measurements. The Trojans, for the most part, have very similar albedos. The actual mean and standard deviation of the distribution depend on the average Trojan beaming parameter � . The ‘‘ standard ’’ value of 0.756, which was used for the IRAS analysis, yields a mean albedo of 0.056 � 0.003 and a standard deviation of 0.009. However, a value of � = 0.94, which we found represented our data better, yields 0.041 � 0.002 and a standard deviation of just 0.007. The thermal behavior of the Trojans seems to follow the ‘‘ slow rotator ’’ model, and the thermal inertia itself can be no greater than about half the Moon’s value. The Kolmogorov-Smirnov test was used to compare the Trojans’ albedo distribution with that of cometary nuclei, dead-comet candidates, and outer solar system objects. We find that the Trojan distribution is similar only to the cometary ones, and only if the Trojans’ � � 1. Observations of the binary (617) Patroclus reveal that its albedo is rather typical among the distribution. We have also discovered that (4709) Ennomos has an extremely elevated albedo, about 0.15. This object may have a very unusual thermal behavior or have recently suffered a large impact that excavated the surface down to a layer of highly reflective, pristine ice.

High albedos of low inclination Classical Kuiper belt objects

Abstract We present observations of thermal emission from fifteen transneptunian objects (TNOs) made using the Spitzer Space Telescope . Thirteen of the targets are members of the Classical population: six dynamically hot Classicals, five dynamically cold Classicals, and two dynamically cold inner Classical Kuiper belt objects (KBOs). We fit our observations using thermal models to determine the sizes and albedos of our targets finding that the cold Classical KBOs have distinctly higher visual albedos than the hot Classicals and other TNO dynamical classes. The cold Classicals are known to be distinct from other TNOs in terms of their color distribution, size distribution, and binarity fraction. The Classical objects in our sample all have red colors yet they show a diversity of albedos which suggests that there is not a simple relationship between albedo and color. As a consequence of high albedos, the mass estimate of the cold Classical Kuiper belt is reduced from approximately 0.01 M ⊕ to approximately 0.001 M ⊕ . Our results also increase significantly the sample of small Classical KBOs with known albedos and sizes from 21 to 32 such objects.

A STUDY OF THE DIVERSE T DWARF POPULATION REVEALED BY WISE

We report the discovery of 87 new T dwarfs uncovered with the Wide-field Infrared Survey Explorer (WISE) and 3 brown dwarfs with extremely red near-infrared colors that exhibit characteristics of both L and T dwarfs. Two of the new T dwarfs are likely binaries with L7 ± 1 primaries and mid-type T secondaries. In addition, our follow-up program has confirmed 10 previously identified T dwarfs and 4 photometrically selected L and T dwarf candidates in the literature. This sample, along with the previous WISE discoveries, triples the number of known brown dwarfs with spectral types later than T5. Using the WISE All-Sky Source Catalog we present updated color-color and color-type diagrams for all the WISE-discovered T and Y dwarfs. Near-infrared spectra of the new discoveries are presented along with spectral classifications. To accommodate later T dwarfs we have modified the integrated flux method of determining spectral indices to instead use the median flux. Furthermore, a newly defined J-narrow index differentiates the early-type Y dwarfs from late-type T dwarfs based on the J-band continuum slope. The K/J indices for this expanded sample show that 32% of late-type T dwarfs have suppressed K-band flux and are blue relative to the spectral standards, while only 11% are redder than the standards. Comparison of the Y/J and K/J index to models suggests diverse atmospheric conditions and supports the possible re-emergence of clouds after the L/T transition. We also discuss peculiar brown dwarfs and candidates that were found not to be substellar, including two young stellar objects and two active galactic nuclei. The substantial increase in the number of known late-type T dwarfs provides a population that will be used to test models of cold atmospheres and star formation. The coolest WISE-discovered brown dwarfs are the closest of their type and will remain the only sample of their kind for many years to come.

Low Albedos Among Extinct Comet Candidates

We present radiometric effective radii and visual geometric albedos for six asteroids in comet-like orbits. Our sample has three of the four known retrograde asteroids (1999 LE31, 2000 DG8, and 2000 HE46) and three objects [(18916) 2000 OG44, 2000 PG3, and 2000 SB1] on prograde but highly elliptical orbits. These measurements more than double the number of known albedos for asteroids with a Tisserand invariant in the cometary regime. We find that all six of our objects, and nine of the 10 now known, have albedos that are as low as those of active cometary nuclei, which is consistent with their supposed evolutionary connection to that group. This albedo distribution is distinct from that of the whole near-Earth and unusual asteroid population, and the strong correlation between Tisserand invariant and albedo suggests that there is a significant cometary contribution to this asteroid population.

Physical Properties of Trans-Neptunian Object (20000) Varuna

We present new time-resolved photometric observations of the bright trans-Neptunian object (20000) Varuna and use them to study the rotation period, shape, and color. In observations from 2001 February and April, we find a best-fit two-peaked light curve with period 6.3442 ± 0.0002 hr. The peak-to-peak photometric range in the R band is 0.42 ± 0.02 mag. We find no rotational variation in colors over the 0.45 μm ≤ λ ≤ 0.85 μm wavelength range. From the short, double-peaked period and large amplitude, we suggest that Varuna is an elongated, body, perhaps close in shape to one of the Jacobi ellipsoids. If so, the ratio of the axes projected into the plane of the sky is 1.5:1 and the density is near 1000 kg m-3. Varuna may be a rotationally distorted rubble pile, with a weak internal constitution due to fracturing by past impacts. The high specific angular momentum implied by our observations and recent detections of binary trans-Neptunian objects both point to an early, intense collisional epoch in which large trans-Neptunian objects were ~100 times more abundant than now. In order to maintain a cosmochemically plausible rock : ice mass ratio of ~0.5, Varuna must be internally porous.

ALBEDOS OF ASTEROIDS IN COMET-LIKE ORBITS

We present the results of a mid-infrared survey of 26 asteroids in comet-like orbits, including six Damocloids and six near-Earth asteroids (NEAs). We define a ‘‘comet-like’’ orbit as one having a Tisserand invariant TJ under 3 (but only including objects that are NEAs or otherwise unusual). Visible-wavelength data were also obtained, so geometric albedos (in the Cousins R band) and effective radii are presented for 25 objects (plus one more with 3 � limits) as derived using the NEA Thermal Model. Nine of our objects were observed at two or more mid-infrared wavelengths, and in all cases the low-thermal inertia thermal model was found to be applicable, with various values of the beaming parameter. Our work more than quintuples the total number of observationally constrained albedos among TJ 3 NEAs. There are 10 NEAs among the 32 objects, and of those, 53% � 9% have comet-like albedos. With the current crop of NEAs, this implies that about 4% of all known NEAs are extinct comets. A comparison of the histogram of TJ < 3 asteroid albedos with that of active cometary nuclei shows that the former has a larger spread.

Thermal Properties of Centaurs Asbolus and Chiron

We have measured the mid-infrared thermal continua from two Centaurs, inactive (8405) Asbolus and active 95P = (2060) Chiron, and have constrained their geometric albedos, p, and effective radii, R, with the standard thermal model for slow rotators. These are the first such measurements of Asbolus; we find R = 33 ± 2 km and p = 0.12 ± 0.03. This albedo is higher than all of those confidently known for active cometary nuclei. The thermal inertia is comparable to or lower than those of main-belt asteroids, the Moon, and Chiron; lower than those of the icy Galilean satellites; and much lower than those of near-Earth asteroids. For Chiron, we find R = 74 ± 4 km and p = 0.17 ± 0.02. While this albedo is consistent with the established value, previous radiometry by others implied a larger radius. This discrepancy may be partially due to a varying infrared dust coma, but all data sets have too low signal to be sure. Four Centaur albedos (out of about 30 objects) are now known. They show a diversity greater than that of the active comets, to which they are evolutionarily linked.