145 research outputs found
Albedo heterogeneity on the surface of (1943) Anteros
We have investigated the effect of rotation on the polarization of scattered
light for the near-Earth asteroid (1943) Anteros using the Dual Beam Imaging
Polarimeter on the University of Hawaii's 2.2 m telescope. Anteros is an L-type
asteroid that has not been previously observed polarimetrically. We find weak
but significant variations in the polarization of Anteros as a function of
rotation, indicating albedo changes across the surface. Specifically, we find
that Anteros has a background albedo of p_v = 0.18 +/- 0.02 with a dark spot of
p_v < 0.09 covering < 2% of the surface.Comment: Accepted to Icarus, 15 pages, 3 fig
The effect of the dust size distribution on asteroid polarization
We have developed a theoretical description of how of an asteroid's
polarization-phase curve will be affected by the removal of the dust from the
surface due to a size-dependent phenomenon such as radiation pressure-driven
escape of levitated particles. We test our calculations against new
observations of four small (D ~ 1 km) near-Earth asteroids [(85236), (142348),
(162900) and 2006 SZ_217] obtained with the Dual Beam Imaging Polarimeter on
the University of Hawaii's 2.2 m telescope, as well as previous observations of
(25143) Itokawa and (433) Eros. We find that the polarization of the light
reflected from an asteroid is controlled by the mineralogical and chemical
composition of the surface and is independent of dust particle. The relation
between the slope of the polarization-phase curve beyond the inversion angle
and the albedo of an asteroid is thus independent of the surface regolith size
distribution and is valid for both Main Belt and Near-Earth asteroids.Comment: accepted to A
The Milky Way halo as a QSO absorption-line system. New results from an HST/STIS absorption-line catalogue of Galactic high-velocity clouds
We use archival UV absorption-line data from HST/STIS to statistically
analyse the absorption characteristics of the high-velocity clouds (HVCs) in
the Galactic halo towards more than 40 extragalactic background sources. We
determine absorption covering fractions of low- and intermediate ions (OI, CII,
SiIII, MgII, FeII, SiIII, CIV, and SiIV) in the range fc = 0.20 - 0.70. For
detailed analysis we concentrate on SiII absorption components in HVCs, for
which we investigate the distribution of column densities, b-values, and radial
velocities. Combining information for SiII and MgII, and using a geometrical
HVC model we investigate the contribution of HVCs to the absorption cross
section of strong MgII absorbers in the local Universe. We estimate that the
Galactic HVCs would contribute on average ~52 % to the total strong MgII cross
section of the Milky Way, if our Galaxy were to be observed from an exterior
vantage point. We further estimate that the mean projected covering fraction of
strong MgII absorption in the Milky Way halo and disc from an exterior vantage
point is fc(sMgII) = 0.31 for a halo radius of R = 61 kpc. These numbers,
together with the observed number density of strong MgII absorbers at low
redshift, indicate that the contribution of infalling gas clouds (i.e., HVC
analogues) in the halos of Milky Way-type galaxies to the cross section of
strong MgII absorbers is <34 %. These findings are in line with the idea that
outflowing gas (e.g., produced by galactic winds) in the halos of more actively
star-forming galaxies dominate the absorption-cross section of strong MgII
absorbers in the local Universe
The Albedo Distribution of Near Earth Asteroids
The cryogenic WISE mission in 2010 was extremely sensitive to asteroids and
not biased against detecting dark objects. The albedos of 428 Near Earth
Asteroids (NEAs) observed by WISE during its fully cryogenic mission can be fit
quite well by a 3 parameter function that is the sum of two Rayleigh
distributions. The Rayleigh distribution is zero for negative values, and
follows for positive x. The peak
value is at x=\sigma, so the position and width are tied together. The three
parameters are the fraction of the objects in the dark population, the position
of the dark peak, and the position of the brighter peak. We find that 25.3% of
the NEAs observed by WISE are in a very dark population peaking at , while the other 74.7% of the NEAs seen by WISE are in a moderately dark
population peaking at . A consequence of this bimodal distribution
is that the Congressional mandate to find 90% of all NEAs larger than 140 m
diameter cannot be satisfied by surveying to H=22 mag, since a 140 m diameter
asteroid at the very dark peak has H=23.7 mag, and more than 10% of NEAs are
darker than p_V = 0.03.Comment: 7 pages LaTex, 4 figures, accepted for publication in the
Astronomical Journa
The Euphrosyne family's contribution to the low albedo near-Earth asteroids
The Euphrosyne asteroid family is uniquely situated at high inclination in
the outer Main Belt, bisected by the nu_6 secular resonance. This large, low
albedo family may thus be an important contributor to specific subpopulations
of the near-Earth objects. We present simulations of the orbital evolution of
Euphrosyne family members from the time of breakup to the present day, focusing
on those members that move into near-Earth orbits. We find that family members
typically evolve into a specific region of orbital element-space, with
semimajor axes near ~3 AU, high inclinations, very large eccentricities, and
Tisserand parameters similar to Jupiter family comets. Filtering all known NEOs
with our derived orbital element limits, we find that the population of
candidate objects is significantly lower in albedo than the overall NEO
population, although many of our candidates are also darker than the Euphrosyne
family, and may have properties more similar to comet nuclei. Followup
characterization of these candidates will enable us to compare them to known
family properties, and confirm which ones originated with the breakup of (31)
Euphrosyne.Comment: Accepted for publication in Ap
The Thousand Asteroid Light Curve Survey
We present the results of our Thousand Asteroid Light Curve Survey (TALCS)
conducted with the Canada-France-Hawaii Telescope in September 2006. Our
untargeted survey detected 828 Main Belt asteroids to a limiting magnitude of
g'~22.5 corresponding to a diameter range of 0.4 km <= D <= 10 km. Of these,
278 objects had photometry of sufficient quality to perform rotation period
fits. We debiased the observations and light curve fitting process to determine
the true distribution of rotation periods and light curve amplitudes of Main
Belt asteroids. We confirm a previously reported excess in the fraction of fast
rotators but find a much larger excess of slow rotating asteroids (~15% of our
sample). A few percent of objects in the TALCS size range have large light
curve amplitudes of ~1 mag. Fits to the debiased distribution of light curve
amplitudes indicate that the distribution of triaxial ellipsoid asteroid shapes
is proportional to the square of the axis-ratio, (b/a)^2, and may be bi-modal.
Finally, we find six objects with rotation periods that may be less than 2
hours with diameters between 400 m and 1.5 km, well above the break-up limit
for a gravitationally-bound aggregate. Our debiased data indicate that this
population represents <4% of the Main Belt in the 1-10 km size range.Comment: Accepted to Icarus. Full tables to appear there in electronic format,
or contact autho
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