24 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
Strengthening the Case for Asteroidal Accrection: Evidence for Subtle and Diverse Disks at White Dwarfs
Spitzer Space Telescope IRAC 3-8 micron and AKARI IRC 2-4 micron photometry
are reported for ten white dwarfs with photospheric heavy elements; nine
relatively cool stars with photospheric calcium, and one hotter star with a
peculiar high carbon abundance. A substantial infrared excess is detected at HE
2221-1630, while modest excess emissions are identified at HE 0106-3253 and HE
0307+0746, implying these latter two stars have relatively narrow (Delta r <
0.1 Rsol) rings of circumstellar dust. A likely 7.9 micron excess is found at
PG 1225-079 and may represent, together with G166-58, a sub-class of dust ring
with a large inner hole. The existence of attenuated disks at white dwarfs
substantiates the connection between their photospheric heavy elements and the
accretion of disrupted minor planets, indicating many polluted white dwarfs may
harbor orbiting dust, even those lacking an obvious infrared excess.Comment: 13 pages, emulateapj, accepted to Ap
Asteroid Distributions in the Ecliptic
We present analysis of the asteroid surface density distribution of main belt
asteroids (mean perihelion AU) in five ecliptic latitude
fields, -17 \gtsimeq \beta(\degr) \ltsimeq +15, derived from deep
\textit{Large Binocular Telescope} (LBT) band (85% completeness limit mag) and \textit{Spitzer Space Telescope} IRAC 8.0 \micron (80%
completeness limit Jy) fields enabling us to probe the 0.5--1.0
km diameter asteroid population. We discovered 58 new asteroids in the optical
survey as well as 41 new bodies in the \textit{Spitzer} fields. The derived
power law slopes of the number of asteroids per square degree are similar
within each \degr{} ecliptic latitude bin with a mean value of . For the 23 known asteroids detected in all four IRAC channels mean
albedos range from to . No low albedo asteroids
( \ltsimeq 0.1) were detected in the \textit{Spitzer} FLS fields,
whereas in the SWIRE fields they are frequent. The SWIRE data clearly samples
asteroids in the middle and outer belts providing the first estimates of these
km-sized asteroids' albedos. Our observed asteroid number densities at optical
wavelengths are generally consistent with those derived from the Standard
Asteroid Model within the ecliptic plane. However, we find an over density at
\beta \gtsimeq 5\degr{} in our optical fields, while the infrared number
densities are under dense by factors of 2 to 3 at all ecliptic latitudes.Comment: 35 pages including 5 figures, accepted to The Astronomical Journa
Six White Dwarfs with Circumstellar Silicates
Spitzer Space Telescope spectra reveal 10 micron silicate emission from
circumstellar dust orbiting six externally-polluted white dwarfs. Micron-size
glasses with an olivine stoichiometry can account for the distinctively broad
wings that extend to 12 microns; these particles likely are produced by
tidal-disruption of asteroids. The absence of infrared PAH features is
consistent with a scenario where extrasolar rocky planets are assembled from
carbon-poor solids.Comment: 21 pages, 7 figures, AJ, in pres
The Dust Trail of Comet 67P/Churyumov-Gerasimenko
We report the detection of comet 67P/Churyumov-Gerasimenko's dust trail and
nucleus in 24 micron Spitzer Space Telescope images taken February 2004. The
dust trail is not found in optical Palomar images taken June 2003. Both the
optical and infrared images show a distinct neck-line tail structure, offset
from the projected orbit of the comet. We compare our observations to simulated
images using a Monte Carlo approach and a dynamical model for comet dust. We
estimate the trail to be at least one orbit old (6.6 years) and consist of
particles of size >~100 micron. The neck-line is composed of similar sized
particles, particles of size but younger in age. Together, our observations and
simulations suggest grains 100 micron and larger in size dominate the total
mass ejected from the comet. The radiometric effective radius of the nucleus is
1.87 +/- 0.08 km, derived from the Spitzer observation. The Rosetta spacecraft
is expected to arrive at and orbit this comet in 2014. Assuming the trail is
comprised solely of 1 mm radius grains, we compute a low probability (~10^-3)
of a trail grain impacting with Rosetta during approach and orbit insertion.Comment: Accepted for publication in Icaru
Physical Conditions in the Ionized Gas of 30 Doradus
We present a mid-infrared spectroscopic data cube of the central part of 30
Doradus, observed with Spitzer's IRS and MIPS/SED mode. Aromatic dust emission
features and emission lines from molecular and atomic hydrogen are detected but
not particularly strong. The dominant spectral features are emission lines from
moderately ionized species of argon, neon, and sulphur, which are used to
determine the physical conditions in the ionized gas. The ionized gas
excitation shows strong variations on parsec scales, some of which can
plausibly be associated with individual hot stars. We fit the ionic line
strengths with photoionization and shock models, and find that photoionization
dominates in the region. The ionization parameter U traces the rim of the
central bubble, as well as highlighting isolated sources of ionization, and at
least one quiescent clump. The hardness of the ionizing radiation field T_rad
reveals several "hot spots" that are either the result of individual very hot
stars or trace the propagation of the diffuse ionizing field through the
surrounding neutral cloud. Consistent with other measurements of giant
molecular hydrogen regions, log(U) ranges between -3 and -0.75, and T_rad
between 30000 and 85000K.Comment: 32 pages, 26 figures, ApJ accepted. A version with high-resolution
images can be found at
http://www.astro.virginia.edu/~ged3j/indebetouw20090125.pd
Unveiling short-period binaries in the inner VVV bulge
Most of our knowledge of the structure of the Milky Way has come from the study of variable stars. Among these variables, mimicking the periodic variation of pulsating stars, are eclipsing binaries. These stars are important in astrophysics because they allow us to directly measure the radii and masses of the components, as well as the distance to the system, thus being useful in studies of Galactic structure alongside pulsating RR Lyrae and Cepheids. Using the distinguishing features of their light curves, one can identify them using a semi-automated process. In this work, we present a strategy to search for eclipsing variables in the inner VVV bulge across an area of 13.4 deg2 within 1.68° < l < 7.53° and-3.73° < b <-1.44°, corresponding to the VVV tiles b293-b296 and b307-b310. We accurately classify 212 previously unknown eclipsing binaries, including six very reddened sources. A preliminary analysis suggests that these eclipsing binaries are located in the most obscured regions of the foreground disc and bulge of the Galaxy. This search is therefore complementary to other variable-star searches carried out at optical wavelengths.Fil: Botan, E.. Universidade Federal de Santa Catarina; Brasil. Universidade Federal de Mato Grosso; BrasilFil: Saito, Roberto. Universidade Federal de Santa Catarina; BrasilFil: Minniti, D.. Universidad Andrés Bello; Chile. Vatican Observatory; ItaliaFil: Kanaan, A.. Universidade Federal de Santa Catarina; BrasilFil: Contreras Ramos, R.. Pontificia Universidad Católica de Chile; Chile. Instituto Milenio de Astrofísica; ChileFil: Ferreira, T. S.. Universidade Federal de Santa Catarina; BrasilFil: Gramajo, Luciana Veronica. Universidad Nacional de Córdoba. Observatorio Astronómico de Córdoba. Departamento de Astrofísica Estelar; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba; ArgentinaFil: Navarro, M. G.. Universidad Andrés Bello; Chile. Università degli Studi di Roma "La Sapienza"; Italia. Instituto Milenio de Astrofísica; Chil
A Spitzer Study of Asymptotic Giant Branch Stars. III. Dust Production and Gas Return in Local Group Dwarf Irregular Galaxies
We present the third and final part of a census of Asymptotic Giant Branch
(AGB) stars in Local Group dwarf irregular galaxies. Papers I and II presented
the results for WLM and IC 1613. Included here are Phoenix, LGS 3, DDO 210, Leo
A, Pegasus dIrr, and Sextans A. Spitzer photometry at 3.6, 4.5, 5.8, and 8 are
presented, along with a more thorough treatment of background galaxy
contamination than was presented in papers I and II. We find that at least a
small population of completely optically obscured AGB stars exists in each
galaxy, regardless of the galaxy's metallicity, but that higher-metallicity
galaxies tend to harbor more stars with slight IR excesses. The optical
incompleteness increases for the redder AGB stars, in line with the expectation
that some AGB stars are not detected in the optical due to large amounts of
extinction associated with in situ dust production. Overall, there is an
underrepresentation of 30% - 40% in the optical AGB within the 1 sigma errors
for all of the galaxies in our sample. This undetected population is large
enough to affect star formation histories derived from optical color-magnitude
diagrams. As measured from the [3.6] - [4.5] color excesses, we find average
stellar mass-loss rates ranging from 3.1E-7 - 6.6E-6 solar masses per year, and
integrated galaxy mass-loss rates ranging from 4.4E-5 - 1.4E-3 solar masses per
year. The integrated mass-loss rate is sufficient to sustain the current star
formation rate in only LGS 3 and DDO 210, requiring either significant
non-dusty mass loss or gas accretion in Phoenix, Leo A, Pegasus dIrr, Sextans
A, WLM, and IC 1613 if they are to maintain their status as gas-rich galaxies.Comment: 25 pages, 17 figures, 9 tables, Accepted for publication in ApJ;
updated affiliation for Boye