122 research outputs found
Compositions of near-Earth asteroids
The goal is to determine whether any of the near-earth asteroids contain water-bearing phyllosilicate (clay) minerals. If these minerals are present, they would provide a readily available source of water for propellant generation and use in life support systems. Telescopic detection of water on the near-earth asteroids is complicated because thermal emission from the asteroid itself masks the diagnostic absorption features for objects this close to the sun. Sophisticated thermal models are necessary to determine whether the absorption features are present. This year, development of these models was continued and more telescopic data to test the models was obtained
Infrared observations of asteroids from earth and space
Infrared reflectances at wavelength between 1 and 4 micrometers are used for determining asteroid surface mineralogy, surface composition, diameters, and albedos. Thermal models were developed for analyzing infrared observations at longer wavelengths. The discovery of a spectral feature due to water of hydration on Ceres seems to contradict the mineralogy inferred from spectrophotometry
Observations of the 8 December 1987 occultation of AG+40 deg 0783 by 324 Bamberga
The occultation of AG+40 deg 0783 by 324 Bamberga on 8 December 1987 was observed at 13 sites in the United States, Japan, and China. At four sites the event was observed photoelectrically; the other observations were visual. A least-squares fit of a circular limb profile to the data gives a diameter of 227.6 + or - 1.9 km. However, this solution is inconsistent with a negative visual observation near the northern edge of the ground track. The inconsistency cannot be removed by assuming an elliptical profile. The data suggest that Bamberga, despite its low-amplitude lightcurve, may depart significantly from a spherical or ellipsoidal shape. The asteroid also appears to be at least 10 percent smaller than indicated by infrared radiometry
IR-dust observations of Comet Tempel 2 with CRAF VIMS
Measurement strategies are now being planned for using the Visual and Infrared Mapping Spectrometer (VIMS) to observe the asteroid Hestia, and the nucleus, and the gas and dust in the coma of comet P/Tempel 2 as part of the Comet Rendezvous Asteroid Flyby (CRAF) mission. The spectral range of VIMS will cover wavelengths from 0.35 to 5.2 micrometers, with a spectral resolution of 11 nm from 0.35 to 2.4 micrometers and of 22 nm from 2.4 to 5.2 micrometers. The instantaneous field of view (IFOV) provided by the foreoptics is 0.5 milliradians, and the current design of the instrument provides for a scanning secondary mirror which will scan a swath of length 72 IFOVs. The CRAF high resolution scan platform motion will permit slewing VIMS in a direction perpendicular to the swath. This enables the building of a two dimensional image in any or all wavelength channels. Important measurements of the dust coma will include the onset of early coma activity, the mapping of gas and dust jets and correlations with active nucleus areas, observations of the dust coma from various scattering phase angles, coverage of the low wavelength portion of the thermal radiation, and the 3.4 micrometer hydrocarbon emission. A description of the VIMS instrument is presented
65 Cybele in the thermal infrared: Multiple observations and thermophysical analysis
We investigated the physical and thermal properties of 65 Cybele}, one of the
largest main-belt asteroids. Based on published and recently obtained thermal
infrared observations, including ISO measurements, we derived through
thermophysical modelling (TPM) a size of 302x290x232 km (+/- 4 %) and an
geometric visible albedo of 0.050+/-0.005. Our model of a regolith covered
surface with low thermal inertia and "default" roughness describes the
wavelengths and phase angle dependent thermal aspects very well. Before/after
opposition effect and beaming behaviour can be explained in that way. We found
a constant emissivity of 0.9 at wavelengths up to about 100 micron and lower
values towards the submillimetre range, indicating a grain size distribution
dominated by 200 micron particle sizes. The spectroscopic analysis revealed an
emissivity increase between 8.0 and 9.5 micron. We compared this emissivity
behaviour with the Christiansen features of carbonaceous chondrite meteorites,
but a conclusive identification was not possible. A comparison between the
Standard Thermal Model (STM) and the applied TPM clearly demonstrates the
limitations and problems of the STM for the analysis of multi-epoch and
-wavelengths observations. While the TPM produced a unique diameter/albedo
solution, the calculated STM values varied by +/-30 % and showed clear trends
with wavelength and phase angle. Cybele can be considered as a nice textbook
case for the thermophysical analysis of combined optical and thermal infrared
observations.Comment: 11 pages, 8 figures, accepted for publication by A&
The size, shape, density, and albedo of Ceres from its occultation of BD+8 deg 471
The occultation of BD+8 degrees 471 by Ceres on 13 November 1984 was observed photoelectrically at 13 sites in Mexico, Florida, and the Caribbean. These observations indicate that Ceres is an oblate spheroid having an equatorial radius of 479.6 + or - 2.4 km and a polar radius of 453.4 + or - 4.5 km. The mean density of this minor planet is 2.7 gm/cubic cm + or - 5%, and its visual geometric albedo is 0.070. While the surface appears globally to be in hydrostatic equilibrium, firm evidence of real limb irregularities is seen in the data
Main Belt Asteroids with WISE/NEOWISE I: Preliminary Albedos and Diameters
We present initial results from the Wide-field Infrared Survey Explorer
(WISE), a four-band all-sky thermal infrared survey that produces data well
suited to measuring the physical properties of asteroids, and the NEOWISE
enhancement to the WISE mission allowing for detailed study of Solar system
objects. Using a NEATM thermal model fitting routine we compute diameters for
over 100,000 Main Belt asteroids from their IR thermal flux, with errors better
than 10%. We then incorporate literature values of visible measurements (in the
form of the H absolute magnitude) to determine albedos. Using these data we
investigate the albedo and diameter distributions of the Main Belt. As observed
previously, we find a change in the average albedo when comparing the inner,
middle, and outer portions of the Main Belt. We also confirm that the albedo
distribution of each region is strongly bimodal. We observe groupings of
objects with similar albedos in regions of the Main Belt associated with
dynamical breakup families. Asteroid families typically show a characteristic
albedo for all members, but there are notable exceptions to this. This paper is
the first look at the Main Belt asteroids in the WISE data, and only represents
the preliminary, observed raw size and albedo distributions for the populations
considered. These distributions are subject to survey biases inherent to the
NEOWISE dataset and cannot yet be interpreted as describing the true
populations; the debiased size and albedo distributions will be the subject of
the next paper in this series.Comment: Accepted to ApJ. Online table to also appear on the publisher's
websit
NEOWISE Observations of Near-Earth Objects: Preliminary Results
With the NEOWISE portion of the \emph{Wide-field Infrared Survey Explorer}
(WISE) project, we have carried out a highly uniform survey of the near-Earth
object (NEO) population at thermal infrared wavelengths ranging from 3 to 22
m, allowing us to refine estimates of their numbers, sizes, and albedos.
The NEOWISE survey detected NEOs the same way whether they were previously
known or not, subject to the availability of ground-based follow-up
observations, resulting in the discovery of more than 130 new NEOs. The
survey's uniformity in sensitivity, observing cadence, and image quality have
permitted extrapolation of the 428 near-Earth asteroids (NEAs) detected by
NEOWISE during the fully cryogenic portion of the WISE mission to the larger
population. We find that there are 98119 NEAs larger than 1 km and
20,5003000 NEAs larger than 100 m. We show that the Spaceguard goal of
detecting 90% of all 1 km NEAs has been met, and that the cumulative size
distribution is best represented by a broken power law with a slope of
1.320.14 below 1.5 km. This power law slope produces 1,900
NEAs with 140 m. Although previous studies predict another break in the
cumulative size distribution below 50-100 m, resulting in an increase in
the number of NEOs in this size range and smaller, we did not detect enough
objects to comment on this increase. The overall number for the NEA population
between 100-1000 m are lower than previous estimates. The numbers of near-Earth
comets will be the subject of future work.Comment: Accepted to Ap
The milliarcsecond-scale jet in the quasar J1625+4134
We present Very Long Baseline Array (VLBA) observations of the radio source
J1625+4134 at 22 and 15 GHz and analyze them in concurrence with other existing
VLBI data on this source. The high resolution images at 15 and 22 GHz show a
short and bending jet which has about 270\degr difference in position angle
with the northern jet detected at lower frequencies. The new high resolution
data, combined with the data available in the literature, allow us to estimate
the spectral index of the components and identify one of the compact components
as the VLBI core based on its flat spectrum between 5 and 22 GHz. Relative to
this core component, the jet appears to be bi-directional. The proper motion
measurement of the component C2 and the estimate of the Doppler boosting factor
suggest that the orientation of the jet is close to the line of sight. The
projection effect of an intrinsically sharply bending jet within a few mas from
the core or the erratic change in the nozzle direction of the jet may account
for the uncommon bi-directional structure of the jet in J1625+4134.Comment: accepted by Ap
Modeling the spectral energy distribution of ULIRGs I: the radio spectra
As a constraint for new starburst/AGN models of IRAS bright galaxies we
determine the radio spectra of 31 luminous and ultraluminous IRAS galaxies
(LIRGs/ULIRGs). We construct the radio spectra using both new and archival
data. From our sample of radio spectra we find that very few have a straight
power-law slope. Although some sources show a flattening of the radio spectral
slope at high frequencies the average spectrum shows a steepening of the radio
spectrum from 1.4 to 22.5 GHz. This is unexpected because in sources with high
rates of star formation we expect flat spectrum, free-free emission to make a
significant contribution to the radio flux at higher radio frequencies. Despite
this trend the radio spectral indices between 8.4 and 22.5 GHz are flatter for
sources with higher values of the FIR-radio flux density ratio q, when this is
calculated at 8.4 GHz. Therefore, sources that are deficient in radio emission
relative to FIR emission (presumably younger sources) have a larger thermal
component to their radio emission. However, we find no correlation between the
radio spectral index between 1.4 and 4.8 GHz and q at 8.4 GHz. Because the low
frequency spectral index is affected by free-free absorption, and this is a
function of source size for a given mass of ionized gas, this is evidence that
the ionized gas in ULIRGs shows a range of densities. The youngest LIRGs and
ULIRGs are characterized by a larger contribution to their high-frequency radio
spectra from free-free emission. However, the youngest sources are not those
that have the greatest free-free absorption at low radio frequencies. The
sources in which the effects of free-free absorption are strongest are instead
the most compact sources. Although these have the warmest FIR colours, they are
not necessarily the youngest sources.Comment: 16 pages. Submitted to A&A Re-submitted, with aesthetic improvements
to the text and figure
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