539 research outputs found
Compositional Heterogeneity of Impact Melt Rocks at the Haughton Impact Structure, Canada: Implications for Planetary Processes and Remote Sensing
Connecting the surface expression of impact craterârelated lithologies to planetary or regional subsurface compositions requires an understanding of material transport during crater formation. Here, we use imaging spectroscopy of six clastârich impact melt rock outcrops within the wellâpreserved 23.5âMa, 23âkm diameter Haughton impact structure, Canada, to determine melt rock composition and spatial heterogeneity. We compare results from outcrop to outcrop, using clasts, groundmass, and integrated clastâgroundmass compositions as tracers of transport during craterâfill melt rock formation and cooling. Supporting laboratory imaging spectroscopy analyses of 91 meltâbearing breccia and clast samples and microscopic Xâray fluorescence elemental mapping of cut samples paired with spectroscopy of identical surfaces validate outcropâscale lithological determinations. Results show different clastârich impact melt rock compositions at three sites kilometers apart and an inverse correlation between silicaârich (sandstone, gneiss, and phyllosilicateârich shales) and gypsumârich rocks that suggests differences in source depth with location. In the target stratigraphy, gypsum is primarily sourced from ~1âkm depth, while gneiss is from >1.8âkm depth, sandstone from >1.3 km, and shales from ~1.6â1.7 km. Observed heterogeneities likely result from different excavation depths coupled with rapid quenching of the melt due to high content of cool clasts. Results provide quantitative constraints for numerical models of impact structure formation and give new details on melt rock heterogeneity important in interpreting mission data and planning sample return of impactites, particularly for bodies with impacts into sedimentary and volatileâbearing targets, e.g., Mars and Ceres
Hydrodynamic Simulations of Counterrotating Accretion Disks
Hydrodynamic simulations have been used to study accretion disks consisting
of counterrotating components with an intervening shear layer(s).
Configurations of this type can arise from the accretion of newly supplied
counterrotating matter onto an existing corotating disk. The grid-dependent
numerical viscosity of our hydro code is used to simulate the influence of a
turbulent viscosity of the disk. Firstly, we consider the case where the gas
well above the disk midplane rotates with angular rate +\Omega(r) and that well
below has the same properties but rotates with rate -\Omega(r). We find that
there is angular momentum annihilation in a narrow equatorial boundary layer in
which matter accretes supersonically with a velocity which approaches the
free-fall velocity and the average accretion speed of the disk can be
enormously larger than that for a conventional \alpha-disk rotating in one
direction. Secondly, we consider the case of a corotating accretion disk for
rr_t. In this case we observed, that
matter from the annihilation layer lost its stability and propagated inward
pushing matter of inner regions of the disk to accrete. Thirdly, we
investigated the case where counterrotating matter inflowing from large radial
distances encounters an existing corotating disk. Friction between the
inflowing matter and the existing disk is found to lead to fast boundary layer
accretion along the disk surfaces and to enhanced accretion in the main disk.
These models are pertinent to the formation of counterrotating disks in
galaxies and possibly in Active Galactic Nuclei and in X-ray pulsars in binary
systems.Comment: LaTeX, 18 pages, to appear in Ap
Detecting a small Kuiper Belt object using archival data of HST's Fine Guidance Sensor
The Kuiper Belt is a remnant of the primordial Solar System. Measurements of its size distribution constrain its accretion and collisional history, and the importance of material strength of Kuiper Belt objects. Small, sub-kilometer-sized, Kuiper Belt objects elude direct detection, but the signature of their occultations of background stars should be detectable. Such an occultation event
lasts typically a fraction of a second, thus making it a classical high time-resolution observation. Here we report an analysis of archival data of HSTâs Fine Guidance Sensors (FGS), that reveals an occultation by such a small object. The detection introduces the FGS as a valuable HTRA instrument. We discuss the statistical aspects regarding the validation of the detection claim, and its physical implications
Low Luminosity Companions to White Dwarfs
This paper presents results of a near-infrared imaging survey for low mass
stellar and substellar companions to white dwarfs. A wide field proper motion
survey of 261 white dwarfs was capable of directly detecting companions at
orbital separations between and 5000 AU with masses as low as 0.05
, while a deep near field search of 86 white dwarfs was capable of
directly detecting companions at separations between and 1100 AU with
masses as low as 0.02 . Additionally, all white dwarf targets were
examined for near-infrared excess emission, a technique capable of detecting
companions at arbitrarily close separations down to masses of 0.05 .
No brown dwarf candidates were detected, which implies a brown dwarf
companion fraction of % for white dwarfs. In contrast, the stellar
companion fraction of white dwarfs as measured by this survey is 22%,
uncorrected for bias. Moreover, most of the known and suspected stellar
companions to white dwarfs are low mass stars whose masses are only slightly
greater than the masses of brown dwarfs. Twenty previously unknown stellar
companions were detected, five of which are confirmed or likely white dwarfs
themselves, while fifteen are confirmed or likely low mass stars.
Similar to the distribution of cool field dwarfs as a function of spectral
type, the number of cool unevolved dwarf companions peaks at mid-M type. Based
on the present work, relative to this peak, field L dwarfs appear to be roughly
2-3 times more abundant than companion L dwarfs. Additionally, there is no
evidence that the initial companion masses have been altered by post main
sequence binary interactions.Comment: 149 pages, 59 figures, 11 tables, accepted to ApJ Supplement
Magnetically Torqued Thin Accretion Disks
We compute the properties of a geometrically thin, steady accretion disk
surrounding a central rotating, magnetized star. The magnetosphere is assumed
to entrain the disk over a wide range of radii. The model is simplified in that
we adopt two (alternate) ad hoc, but plausible, expressions for the azimuthal
component of the magnetic field as a function of radial distance. We find a
solution for the angular velocity profile tending to corotation close to the
central star, and smoothly matching a Keplerian curve at a radius where the
viscous stress vanishes. The value of this ''transition'' radius is nearly the
same for both of our adopted B-field models. We then solve analytically for the
torques on the central star and for the disk luminosity due to gravity and
magnetic torques. When expressed in a dimensionless form, the resulting
quantities depend on one parameter alone, the ratio of the transition radius to
the corotation radius. For rapid rotators, the accretion disk may be powered
mostly by spin-down of the central star. These results are independent of the
viscosity prescription in the disk. We also solve for the disk structure for
the special case of an optically thick alpha disk. Our results are applicable
to a range of astrophysical systems including accreting neutron stars,
intermediate polar cataclysmic variables, and T Tauri systems.Comment: 9 sharper figs, updated reference
On the development of QPOs in Bondi-Hoyle accretion flows
The numerical investigation of Bondi-Hoyle accretion onto a moving black hole
has a long history, both in Newtonian and in general-relativistic physics. By
performing new two-dimensional and general-relativistic simulations onto a
rotating black hole, we point out a novel feature, namely, that quasi-periodic
oscillations (QPOs) are naturally produced in the shock cone that develops in
the downstream part of the flow. Because the shock cone in the downstream part
of the flow acts as a cavity trapping pressure perturbations, modes with
frequencies in the integer ratios 2:1 and 3:1 are easily produced. The
frequencies of these modes depend on the black-hole spin and on the properties
of the flow, and scale linearly with the inverse of the black-hole mass. Our
results may be relevant for explaining the detection of QPOs in Sagittarius A*,
once such detection is confirmed by further observations. Finally, we report on
the development of the flip-flop instability, which can affect the shock cone
under suitable conditions; such an instability has been discussed before in
Newtonian simulations but was never found in a relativistic regime.Comment: 11 pages, 7 figure
Lattice strain distribution resolved by X-ray Bragg-surface diffraction in an Si matrix distorted by embedded FeSi2 nanoparticles
Out-of-plane and primarily in-plane lattice strain distributions, along the two perpendicular crystallographic directions on the subsurface of a silicon layer with embedded FeSi2 nanoparticles, were analyzed and resolved as a function of the synchrotron X-ray beam energy by using omega:phi mappings of the (111) and (111) Bragg-surface diffraction peaks. the nanoparticles, synthesized by ion-beam-induced epitaxial crystallization of Fe+-implanted Si(001), were observed to have different orientations and morphologies (sphere-and plate-like nanoparticles) within the implanted/recrystallized region. the results show that the shape of the synthesized material singularly affects the surrounding Si lattice. the lattice strain distribution elucidated by the nonconventional X-ray Bragg-surface diffraction technique clearly exhibits an anisotropic effect, predominantly caused by plate-shaped nanoparticles. This type of refined detection reflects a key application of the method, which could be used to allow discrimination of strains in distorted semiconductor substrate layers.Conselho Nacional de Desenvolvimento CientĂfico e TecnolĂłgico (CNPq)Fundação de Amparo Ă Pesquisa do Estado de SĂŁo Paulo (FAPESP)FAPEMACoordenação de Aperfeiçoamento de Pessoal de NĂvel Superior (CAPES)UNIFESP, Inst Ciencia & Tecnol ICT, BR-12231280 Sao Jose Dos Campos, SP, BrazilUniv Fed Maranhao, Dept Fis CCET, BR-65085580 Sao Luis, MA, BrazilUniv Fed Maranhao, CCSST, BR-65900410 Imperatriz, MA, BrazilUniv Fed Rio Grande do Sul, Inst Fis, Programa Posgrad Ciencias Mat PGCIMAT, BR-91501970 Porto Alegre, RS, BrazilCEA, Leti Minatec Campus, F-38054 Grenoble, FranceUniv Estadual Campinas, Inst Fis Gleb Wataghin IFGW, BR-13083859 Campinas, SP, BrazilUNIFESP, Inst Ciencia & Tecnol ICT, BR-12231280 Sao Jose Dos Campos, SP, BrazilCAPES: 2358-09-3Web of Scienc
An upper limit on the contribution of accreting white dwarfs to the type Ia supernova rate
There is wide agreement that Type Ia supernovae (used as standard candles for
cosmology) are associated with the thermonuclear explosions of white dwarf
stars. The nuclear runaway that leads to the explosion could start in a white
dwarf gradually accumulating matter from a companion star until it reaches the
Chandrasekhar limit, or could be triggered by the merger of two white dwarfs in
a compact binary system. The X-ray signatures of these two possible paths are
very different. Whereas no strong electromagnetic emission is expected in the
merger scenario until shortly before the supernova, the white dwarf accreting
material from the normal star becomes a source of copious X-rays for ~1e7 yr
before the explosion. This offers a means of determining which path dominates.
Here we report that the observed X-ray flux from six nearby elliptical galaxies
and galaxy bulges is a factor of ~30-50 less than predicted in the accretion
scenario, based upon an estimate of the supernova rate from their K-band
luminosities. We conclude that no more than ~5 per cent of Type Ia supernovae
in early type galaxies can be produced by white dwarfs in accreting binary
systems, unless their progenitors are much younger than the bulk of the stellar
population in these galaxies, or explosions of sub-Chandrasekhar white dwarfs
make a significant contribution to the supernova rate.Comment: 10 pages, 1 tabl
Measuring the Abundance of Sub-kilometer-sized Kuiper Belt Objects Using Stellar Occultations
We present here the analysis of about 19,500 new star hours of low ecliptic latitude observations (|b| †20°) obtained by the Hubble Space Telescope's Fine Guidance Sensors over a time span of more than nine years, which is in addition to the ~12, 000 star hours previously analyzed by Schlichting et al. Our search for stellar occultations by small Kuiper Belt Objects (KBOs) yielded one new candidate event corresponding to a body with a 530 ± 70 m radius at a distance of about 40 AU. Using bootstrap simulations, we estimate a probability of â5% that this event is due to random statistical fluctuations within the new data set. Combining this new event with the single KBO occultation reported by Schlichting et al. we arrive at the following results: (1) the ecliptic latitudes of 6Âș.6 and 14Âș.4 of the two events are consistent with the observed inclination distribution of larger, 100-km-sized KBOs. (2) Assuming that small, sub-kilometer-sized KBOs have the same ecliptic latitude distribution as their larger counterparts, we find an ecliptic surface density of KBOs with radii larger than 250 m of N(r > 250 m) = 1.1^(+1.5)_(â0.7) Ă 10^7 deg^(â2); if sub-kilometer-sized KBOs have instead a uniform ecliptic latitude distribution for â20° 250 m) = 4.4^(+5.8)_(â2.8) Ă 10^6 deg^(â2). This is the best measurement of the surface density of sub-kilometer-sized KBOs to date. (3) Assuming the KBO size distribution can be well described by a single power law given by N(> r) â r^(1âq) , where N(> r) is the number of KBOs with radii greater than r, and q is the power-law index, we find q = 3.8 ± 0.2 and q = 3.6 ± 0.2 for a KBO ecliptic latitude distribution that follows the observed distribution for larger, 100-km-sized KBOs and a uniform KBO ecliptic latitude distribution for â20° 4.0 at 2Ï, confirming a strong deficit of sub-kilometer-sized KBOs compared to a population extrapolated from objects with r > 45 km. This suggests that small KBOs are undergoing collisional erosion and that the Kuiper Belt is a true analog to the dust producing debris disks observed around other stars
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