268 research outputs found
Spectral evidence of size dependent space weathering processes on asteroid surfaces
Most compositional characterizations of the minor planets are derived from analysis of visible and near-infrared reflectance spectra. However, such spectra are derived from light which has only interacted with a very thin surface layer. Although regolith processes are assumed to mix all near-surface lithologic units into this layer, it has been proposed that space weathering processes can alter this surface layer to obscure the spectral signature of the bedrock lithology. It has been proposed that these spectral alteration processes are much less pronounced on asteroid surfaces than on the lunar surface, but the possibility of major spectral alteration of asteroidal optical surfaces has been invoked to reconcile S-asteroids with ordinary chondrites. The reflectance spectra of a large subset of the S-asteroid population have been analyzed in a systematic investigation of the mineralogical diversity within the S-class. In this sample, absorption band depth is a strong function of asteroid diameter. The S-asteroid band depths are relatively constant for objects larger than 100 km and increase linearly by factor of two toward smaller sizes (approximately 40 km). Although the S-asteroid surface materials includes a diverse variety of silicate assemblages, ranging from dunites to basalts, all compositional subtypes of the S-asteroids conform to this trend. The A-, R-, and V-type asteroids which are primarily silicate assemblages (as opposed to the metal-silicate mixtures of most S-asteroids) follow a parallel but displaced trend. Some sort of textural or regolith equilibrium appears to have been attained in the optical surfaces of asteroids larger than about 100 km diameter but not on bodies below this size. The relationships between absorption band depth, spectral slope, surface albedo and body size provide an intriguing insight into the nature of the optical surfaces of the S-asteroids and space weathering on these objects
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Life in the Atacama — Year 2: Geologic reconnaissance through long-range roving and implications on the search for life
The Life in the Atacama-2004 project, which included geological, morphological, and mineralogical mapping through combined satellite, field-based, and microscopic perspectives and long-range roving, led to the localization of potential habitats
Clustering and Sharing Incentives in BitTorrent Systems
Peer-to-peer protocols play an increasingly instrumental role in Internet
content distribution. Consequently, it is important to gain a full
understanding of how these protocols behave in practice and how their
parameters impact overall performance. We present the first experimental
investigation of the peer selection strategy of the popular BitTorrent protocol
in an instrumented private torrent. By observing the decisions of more than 40
nodes, we validate three BitTorrent properties that, though widely believed to
hold, have not been demonstrated experimentally. These include the clustering
of similar-bandwidth peers, the effectiveness of BitTorrent's sharing
incentives, and the peers' high average upload utilization. In addition, our
results show that BitTorrent's new choking algorithm in seed state provides
uniform service to all peers, and that an underprovisioned initial seed leads
to the absence of peer clustering and less effective sharing incentives. Based
on our observations, we provide guidelines for seed provisioning by content
providers, and discuss a tracker protocol extension that addresses an
identified limitation of the protocol
Milky Way potentials in CDM and MOND. Is the Large Magellanic Cloud on a bound orbit?
We compute the Milky Way potential in different cold dark matter (CDM) based
models, and compare these with the modified Newtonian dynamics (MOND)
framework. We calculate the axis ratio of the potential in various models, and
find that isopotentials are less spherical in MOND than in CDM potentials. As
an application of these models, we predict the escape velocity as a function of
the position in the Galaxy. This could be useful in comparing with future data
from planned or already-underway kinematic surveys (RAVE, SDSS, SEGUE, SIM,
GAIA or the hypervelocity stars survey). In addition, the predicted escape
velocity is compared with the recently measured high proper motion velocity of
the Large Magellanic Cloud (LMC). To bind the LMC to the Galaxy in a MOND
model, while still being compatible with the RAVE-measured local escape speed
at the Sun's position, we show that an external field modulus of less than
is needed.Comment: Accepted for publication in MNRAS, 13 pages, 7 figures, 3 table
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Spectroscopic results from the Life in the Atacama (LITA) project 2004 field season
Analysis of spectroscopy datasets from rover field tests in the Atacama Desert (Chile), focusing on the composition of the surface and identification of potential habitats for life
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Searching for life with rovers: exploration methods and science results from the 2004 field campaign of the “Life in the Atacama” project and applications to future Mars Missions
LITA develops and field tests a long-range automated rover and a science payload to search for microbial life in the Atacama. The Atacama's evolution provides a unique training ground for designing and testing exploration strategies and life detection methods for the search for life on Mars
The ACS Virgo Cluster Survey. VIII. The Nuclei of Early-Type Galaxies
(Abridged) The ACS Virgo Cluster Survey is an HST program to obtain
high-resolution, g and z-band images for 100 early-type members of the Virgo
Cluster, spanning a range of ~460 in blue luminosity. Based on this large,
homogeneous dataset, we present a sharp upward revision in the frequency of
nucleation in early-type galaxies brighter than M_B ~ -15 (66 < f_n < 82%), and
find no evidence for nucleated dwarfs to be more concentrated to the center of
Virgo than their non-nucleated counterparts. Resolved stellar nuclei are not
present in galaxies brighter than M_B ~ -20.5, however, there is no clear
evidence from the properties of the nuclei, or from the overall incidence of
nucleation, for a change at M_B ~ -17.6, the traditional dividing point between
dwarf and giant galaxies. On average, nuclei are ~3.5 mag brighter than a
typical globular cluster and have a median half-light radius ~4.2 pc. Nuclear
luminosities correlate with nuclear sizes and, in galaxies fainter than M_B ~
-17.6, nuclear colors. Comparing the nuclei to the "nuclear clusters" found in
late-type spiral galaxies reveals a close match in terms of size, luminosity
and overall frequency, pointing to a formation mechanism that is rather
insensitive to the detailed properties of the host galaxy. The mean
nuclear-to-galaxy luminosity ratio is indistinguishable from the mean
SBH-to-bulge mass ratio, calculated in early-type galaxies with detected
supermassive black holes (SBHs). We argue that compact stellar nuclei might be
the low-mass counterparts of the SBHs detected in the bright galaxies, and that
one should think in terms of "Central Massive Objects" -- either SBHs or
compact stellar nuclei -- that accompany the formation of almost all early-type
galaxies and contain a mean fraction ~0.3% of the total bulge mass.Comment: ApJ Supplements, accepted. Updated references. The manuscript is 61
pages, including 6 tables and 28 figures. Figures included in this submission
are low resolution; a version of the paper containing high-resolution color
figures can be downloaded from the ACSVCS website:
http://www.cadc.hia.nrc.gc.ca/community/ACSVCS/publications.html#acsvcs
Superconducting phase in the layered dichalcogenide 1T-TaS_{2} upon inhibition of the metal-insulator transition
When a Mott metal-insulator transition is inhibited by a small amount of disorder in the layered dichalcogenide 1T-TaS2, an inhomogeneous superconducting state arises below T=2.1 K and coexists with a nearly commensurate charge-density wave. By angle-resolved photoelectron spectroscopy, we show that it emerges from a bad metal state with strongly damped quasiparticles. Superconductivity is almost entirely suppressed by an external magnetic field of 0.1 T
Properties and Origin of the High-Velocity Gas Toward the Large Magellanic Cloud
In the spectra of 139 early-type Large Magellanic Cloud (LMC) stars observed
with FUSE and with deep radio Parkes HI 21-cm observations along those stars,
we search for and analyze the absorption and emission from high-velocity gas at
+90<v<+175 km/s. The HI column density of the high-velocity clouds (HVCs) along
these sightlines ranges from <10^18.4 to 10^19.2 cm^-2. The incidence of the
HVC metal absorption is 70%, significantly higher than the HI emission
occurrence of 32%. We find that the mean metallicity of the HVC is [OI/HI] =
-0.51 (+0.12,-0.16). There is no strong evidence for a large variation in the
HVC metallicity, implying that thes e HVCs have a similar origin and are part
of the same complex. The mean and scatter of the HVC metallicities are more
consistent with the present-day LMC oxygen abundance than that of the Small
Magellanic Cloud or the Milky Way. We find that on average [SiII/OI] = +0.48
(+0.15,- 0.25) and [FeII/OI] = +0.33 (+0.14,-0.21), implying that the HVC
complex is dominantly ionized. The HVC complex has a multiphase structure with
a neutral (OI, FeII), weakly ionized (FeII, NII), and highly ionized (OVI)
components, and has evidence of dust but no molecules. All the observed
properties of the HVC can be explained by an energetic outflow from the LMC.
This is the first example of a large (>10^6 M_sun) HVC complex that is linked
to stellar feedback occurring in a dwarf spiral galaxy.Comment: Accepted for publication in the Ap
Low-temperature spin dynamics of a valence bond glass in Ba<sub style="font-size: smaller; vertical-align: baseline; position: relative; top: 0.25em;">2YMoO<sub style="font-size: smaller; vertical-align: baseline; position: relative; top: 0.25em;">6</sub></sub>
We carried out AC magnetic susceptibility measurements and muon spin
relaxation spectroscopy on the cubic double perovskite Ba2YMoO6, down to 50 mK.
Below ~1 K the muon relaxation is typical of a magnetic insulator with a
spin-liquid type ground state, i.e. without broken symmetries or frozen
moments. However, the AC susceptibility revealed a dilute-spin-glass like
transition below ~ 1 K. Antiferromagnetically coupled Mo5+ 4d1 electrons in
triply degenerate t2g orbitals are in this material arranged in a geometrically
frustrated fcc lattice. Bulk magnetic susceptibility data has previously been
interpreted in terms of a freezing to a heterogeneous state with non-magnetic
sites where 4d^1 electrons have paired in spin-singlets dimers, and residual
unpaired Mo5+ 4d1 electrons. Based on the magnetic heat capacity data it has
been suggested that this heterogeneity is the result of kinetic constraints
intrinsic to the physics of the pure system (possibly due to topological
overprotection), leading to a self-induced glass of valence bonds between
neighbouring 4d1 electrons. The muSR relaxation unambiguously points to a
static heterogeneous state with a static arrangement of unpaired electrons
isolated by spin-singlet (valence bond) dimers between the majority of Mo5+ 4d
electrons. The AC susceptibility data indicate that the residual magnetic
moments freeze into a dilute-spin-glass-like state. This is in apparent
contradiction with the muon-spin decoupling at 50 mK in fields up to 200 mT,
which indicates that, remarkably, the time scale of the field fluctuations from
the residual moments is ~ 5 ns. Comparable behaviour has been observed in other
geometrically frustrated magnets with spin-liquid-like behaviour and the
implications of our observations on Ba2YMoO6 are discussed in this context.Comment: 11 pages, 3 Figures. Published in New Journal of Physic
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