84 research outputs found
Albedos of Small Jovian Trojans
We present thermal observations of 44 Jovian Trojan asteroids with diameters
(D) ranging from 5 to 24 km. All objects were observed at a wavelength of 24
microns with the Spitzer Space Telescope. Measurements of the thermal emission
and of scattered optical light, mostly from the University of Hawaii 2.2-meter
telescope, together allow us to constrain the diameter and geometric albedo of
each body. We find that the median R-band albedo of these small Jovian Trojans
is about 0.12, much higher than that of "large" Trojans with D > 57 km (0.04).
Also the range of albedos among the small Trojans is wider. We attribute the
Trojan albedos to an evolutionary effect: the small Trojans are more likely to
be collisional fragments and so their surfaces would be younger. A younger
surface means less cumulative exposure to the space environment, which suggests
that their surfaces would not be as dark as those of the large, primordial
Trojans. In support of this hypothesis is a statistically significant
correlation of higher albedo with smaller diameter in our sample alone and in a
sample that includes the larger Trojans.Comment: 41 pages, 6 figures, 4 tables, AASTe
Evidence of Fragmenting Dust Particles from Near-Simultaneous Optical and Near-IR Photometry and Polarimetry of Comet 73P/Schwassmann-Wachmann 3
We report imaging polarimetry of segments B and C of the Jupiter-family Comet
73P/Schwassmann-Wachmann 3 in the I and H bandpasses at solar phase angles of
approximately 35 and 85deg. The level of polarization was typical for active
comets, but larger than expected for a Jupiter-family comet. The polarimetric
color was slightly red (dP/dL = +1.2 +/- 0.4) at a phase angle of ~ 35deg and
either neutral or slightly blue at a phase angle of ~ 85deg. Observations
during the closest approach from 2006 May 11-13 achieved a resolution of 35 km
at the nucleus. Both segments clearly depart from a 1/rho surface brightness
for the first 50 - 200 km from the nucleus. Simulations of radiation driven
dust dynamics can reproduce some of the observed coma morphology, but only with
a wide distribution of initial dust velocities (at least a factor of 10) for a
given grain radius. Grain aggregate breakup and fragmentation are able to
reproduce the observed profile perpendicular to the Sun-Comet axis, but fit the
observations less well along this axis (into the tail). The required
fragmentation is significant, with a reduction in the mean grain aggregate size
by about a factor of 10. A combination of the two processes could possibly
explain the surface brightness profile of the comet.Comment: 40 pages including 11 figure
`Oumuamua as a messenger from the Local Association
7 pages, one table, two figures, accepted for publication by ApJL. © 2018. The American Astronomical Society. All rights reserved.With a hyperbolic trajectory around the Sun, 'Oumuamua is the first confirmed interstellar object. However, its origin is poorly known. By simulating the orbits of 0.23 million local stars, we find 109 encounters with periastron less than 5 pc. 'Oumuamua's low peculiar velocity is suggestive of its origin from a young stellar association with similar velocity. In particular, we find that 'Oumuamua would have had slow encounters with at least five young stars belonging to the Local Association, thus suggesting these as plausible sites for formation and ejection. In addition to an extremely elongated shape, the available observational data for 'Oumuamua indicates a red color, suggestive of a potentially organic-rich and activity-free surface. These characteristics seem consistent with formation through energetic collisions between planets and debris objects in the middle part of a young stellar system. We estimate an abundance of at least 6.0 × 10 -3 au -3 for such interstellar objects with mean diameter larger than 100 m and find that it is likely that most of them will be ejected into the Galactic halo. Our Bayesian analysis of the available light curves indicates a rotation period of 6.96 +1.45 -0.39, which is consistent with the estimation by Meech et al. and shorter than those in other literature. The codes and results are available on GitHub (https://github.com/phillippro/Oumuamua).Peer reviewe
The Diverse Solar Phase Curves of Distant Icy Bodies. II. The Cause of the Opposition Surges and Their Correlations
We collect well-measured opposition surge properties for many icy bodies
orbiting the Sun (mostly from our own observations) plus for many icy moons,
resulting in a data base of surface and orbital properties for 52 icy bodies.
(1) We put forward four criteria for determining whether the surge is being
dominated by shadow hiding (SH) or coherent backscattering (CB) based on
readily measured quantities. The CB surge mechanism dominates if the surge is
color dependent, the phase curve is steeper than 0.04 mag/deg, the phase curve
shape matches the CB model of Hapke, or if the albedo is higher than roughly
40%. (2) We find that virtually all of our sample have their phase curves
dominated by CB at low phase angles. (3) We present a graphical method to
determine the Hapke surge parameters B_C0 and h_C. (4) The Kuiper Belt Objects
(KBOs) and Centaurs have relatively high surge amplitudes, B_C0 > ~0.5 and
widths with h_C ~ 3 deg. (5) We find highly significant but loose correlations
between surge properties and the colors, albedos, and inclinations. We
interpret this as young surfaces tending to have low surge slopes, high albedo,
and gray colors. (6) Nereid has its surface properties similar to other icy
moons and greatly different from KBOs and Centaurs, so we conclude that Nereid
is likely a nearly-ejected inner Neptunian moon rather than a captured KBO.Comment: Astronomical Journal, in press, 41 pages, 5 figure
Properties of the Distant Kuiper Belt: Results from the Palomar Distant Solar System Survey
We present the results of a wide-field survey using the 1.2-m Samuel Oschin
Telescope at Palomar Observatory. This survey was designed to find the most
distant members of the Kuiper belt and beyond. We searched ~12,000 deg2 down to
a mean limiting magnitude of 21.3 in R. A total number of 52 KBOs and Centaurs
have been detected, 25 of which were discovered in this survey. Except for the
re-detection of Sedna, no additional Sedna-like bodies with perihelia greater
than 45 AU were detected despite sensitivity out to distances of 1000 AU. We
discuss the implications for a distant Sedna- like population beyond the Kuiper
belt, focusing on the constraints we can place on the embedded stellar cluster
environment the early Sun may be have been born in, where the location and
distribution of Sedna-like orbits sculpted by multiple stellar encounters is
indicative of the birth cluster size. We also report our observed latitude
distribution and implications for the size of the plutino population.Comment: 40 pages, 12 figures, 3 tables Accepted by Ap
Probing the Substellar Regime with SIRTF
One of the main scientific drivers of the Space InfraRed Telescope Facility
(SIRTF) is the search for brown dwarfs and extrasolar superplanets. We discuss
observational strategies for identification of these objects, and conclude that
an optimal strategy is a wide IRAC survey (18 deg) with a 5
sensitivity of 3.9 Jy in channel 2 (M). For this sensitivity,
we provide estimates of the number of low mass brown dwarfs and isolated
planets detected per square degree for power-law mass functions with
=1.5, and 1.0. Shallower surveys covering a larger area are inefficient
because of large overheads and detector noise. Deeper surveys covering a
smaller area become more and more affected by crowding with galaxies. A survey
like the one that we propose would determine the field mass function down to a
few Jupiter masses through the identification of a large sample of brown dwarfs
and isolated planets. The proposed SIRTF survey would also allow the first
detection of ultracool substellar objects with temperatures between 700 K and
200 K. The cooling curves of substellar objects with masses less than 20
Jupiters imply that they should spend most of their lifetimes at temperatures
below 700 K. Preliminary models indicate that their atmospheres could be
dominated by water clouds, which would diminish their optical and near-infrared
fluxes. The properties of those objects are still completely unexplored.Comment: 7 pages, 6 figures, submitted to PAS
Phase Curves of Nine Trojan Asteroids over a Wide Range of Phase Angles
We have observed well-sampled phase curves for nine Trojan asteroids in B-,
V-, and I-bands. These were constructed from 778 magnitudes taken with the
1.3-m telescope on Cerro Tololo as operated by a service observer for the
SMARTS consortium. Over our typical phase range of 0.2-10 deg, we find our
phase curves to be adequately described by a linear model, for slopes of
0.04-0.09 mag/deg with average uncertainty less than 0.02 mag/deg. (The one
exception, 51378 (2001 AT33), has a formally negative slope of -0.02 +- 0.01
mag/deg.) These slopes are too steep for the opposition surge mechanism to be
shadow hiding (SH), so we conclude that the dominant surge mechanism must be
coherent backscattering (CB). In a detailed comparison of surface properties
(including surge slope, B-R color, and albedo), we find that the Trojans have
surface properties similar to the P and C class asteroids prominent in the
outer main belt, yet they have significantly different surge properties (at a
confidence level of 99.90%). This provides an imperfect argument against the
traditional idea that the Trojans were formed around Jupiter's orbit. We also
find no overlap in Trojan properties with either the main belt asteroids or
with the small icy bodies in the outer Solar System. Importantly, we find that
the Trojans are indistinguishable from other small bodies in the outer Solar
System that have lost their surface ices (such as the gray Centaurs, gray
Scattered Disk Objects, and dead comets). Thus, we find strong support for the
idea that the Trojans originally formed as icy bodies in the outer Solar
System, were captured into their current orbits during the migration of the gas
giant planets, and subsequently lost all their surface ices.Comment: Icarus, in pres
Planetary Dynamics and Habitable Planet Formation In Binary Star Systems
Whether binaries can harbor potentially habitable planets depends on several
factors including the physical properties and the orbital characteristics of
the binary system. While the former determines the location of the habitable
zone (HZ), the latter affects the dynamics of the material from which
terrestrial planets are formed (i.e., planetesimals and planetary embryos), and
drives the final architecture of the planets assembly. In order for a habitable
planet to form in a binary star system, these two factors have to work in
harmony. That is, the orbital dynamics of the two stars and their interactions
with the planet-forming material have to allow terrestrial planet formation in
the habitable zone, and ensure that the orbit of a potentially habitable planet
will be stable for long times. We have organized this chapter with the same
order in mind. We begin by presenting a general discussion on the motion of
planets in binary stars and their stability. We then discuss the stability of
terrestrial planets, and the formation of potentially habitable planets in a
binary-planetary system.Comment: 56 pages, 29 figures, chapter to appear in the book: Planets in
Binary Star Systems (Ed. N. Haghighipour, Springer publishing company
TNOs are cool: a survey of the transneptunian region
Over one thousand objects have so far been discovered orbiting beyond Neptune. These trans-Neptunian objects (TNOs) represent the primitive remnants of the planetesimal disk from which the planets formed and are perhaps analogous to the unseen dust parent-bodies in debris disks observed around other main-sequence stars. The dynamical and physical properties of these bodies provide unique and important constraints on formation and evolution models of the Solar System. While the dynamical architecture in this region (also known as the Kuiper Belt) is becoming relatively clear, the physical properties of the objects are still largely unexplored. In particular, fundamental parameters such as size, albedo, density and thermal properties are difficult to measure. Measurements of thermal emission, which peaks at far-IR wavelengths, offer the best means available to determine the physical properties. While Spitzer has provided some results, notably revealing a large albedo diversity in this population, the increased sensitivity of Herschel and its superior wavelength coverage should permit profound advances in the field. Within our accepted project we propose to perform radiometric measurements of 139 objects, including 25 known multiple systems. When combined with measurements of the dust population beyond Neptune (e.g. from the New Horizons mission to Pluto), our results will provide a benchmark for understanding the Solar debris disk, and extra-solar ones as well
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