57,178 research outputs found
Asteroid Cooling Rates Indicated by K-Feldspar Exsolution Textures in H4 Ordinary Chondrites
Undisturbed thermal metamorphism in ordinary chondrite (OC) asteroids, produced through the radioactive decay of 26Al, is expected to result in an onion-shell-like structure. In such a structure, the inner layers of the asteroid experience more extensive thermal metamorphism, as represented by higher petrologic type, than the exterior layers. Furthermore, cooling rates are expected to be slower for OCs of high petrologic type than those of low petrologic type. However, cooling rates determined using metallographic methods and pyroxene diffusion are inconsistent with onion-shell-style cooling and have resulted in new models. These models argue for the disruption of the asteroid after peak metamorphism followed by reaccretion into a rubble pile. Improved constraints on cooling rates would provide a better understanding of the timing and scale of disruptive events. Feldspar microtextures are another tool that can be used to determine asteroid cooling rates. In OC chondrules, plagioclase is present as either a primary phase, or a secondary phase forming from the crystallization of mesostasis glass through petrologic type 4, followed by chemical and textural equilibration. Potas-sium feldspar is observed in petrologic types 3.6-6, as either patches or lamellae exsolved from albite in a perthite texture, often near pores or fractures. Exsolution occurs most commonly, and most extensively, in petrologic type 4. Because the feldspar exsolution wavelength is related to the rate at which grains cooled from the solvus temperature, determined from the minerals bulk composition, the chondrite cooling rate can be measured from regions of exsolution. We have previously reported the perthite exsolution cooling rate of Avanhandava, an H4 chondrite, to be 1 C per 1-4 months over a temperature interval of 765-670 C. A peristerite exsolution texture was also present in the Na-rich lamellae for which we estimated a cooling rate of 1 C in 103-104 years from 570-540 C. Overall, the cooling rates determined from Avanhandava are consistent with pyroxene diffusion (fast cooling at high temperatures) and metallographic rates (slow cooling at low temperatures), hence with the rubble pile model of disruption and reaccretion. Here, we characterize feldspar microtextures in four additional H4 chondrites to test the consistency of feldspar cooling rates across a range of samples. We show that all H4s are similar and support rubble pile models
Recommended from our members
Radiative transfer modelling for the NOMAD-UVIS instrument on the ExoMars Trace Gas Orbiter mission
The NOMAD (Nadir and Occultation for MArs Discovery) instrument is a 3-channel (2 IR, 1 UV/Vis) spectrometer due to fly on the 2016 ExoMars Trace Gas Orbiter mission. A radiative transfer model for Mars has been developed providing synthetic spectra to simulate observations of the UVIS channel in both solar occultation and nadir viewing geometries. This will allow for the characterization and mitigation of the influence of dust on retrievals of ozone abundance
Microlensing of Broad Absorption Line Quasars: Polarization Variability
Roughly 10% of all quasars exhibit Broad Absorption Line (BAL) features which
appear to arise in material outflowing at high velocity from the active
galactic nucleus (AGN). The details of this outflow are, however, very poorly
constrained and the particular nature of the BAL material is essentially
unknown. Recently, new clues have become available through polarimetric studies
which have found that BAL troughs are more polarized than the quasar continuum
radiation. To explain these observations, models where the BAL material
outflows equatorially across the surface of the dusty torus have been
developed. In these models, however, several sources of the BAL polarization
are possible. Here, we demonstrate how polarimetric monitoring of
gravitationally lensed quasars, such as H 1413+117, during microlensing events
can not only distinguish between two currently popular models, but can also
provide further insight into the structure at the cores of BAL quasars.Comment: 17 pages, 3 figures, accepted to PAS
Seeing Star Formation Regions with Gravitational Microlensing
We qualitatively study the effects of gravitational microlensing on our view
of unresolved extragalactic star formation regions. Using a general
gravitational microlensing configuration, we perform a number of simulations
that reveal that specific imprints of the star forming region are imprinted,
both photometrically and spectroscopically, upon observations. Such
observations have the potential to reveal the nature and size of these star
forming regions, through the degree of variability observed in a monitoring
campaign, and hence resolve the star formation regions in distant galaxies
which are too small to be probed via more standard techniques.Comment: 7 pages, 8 figures, ApJ accepte
Uncovering CDM halo substructure with tidal streams
Models for the formation and growth of structure in a cold dark matter
dominated universe predict that galaxy halos should contain significant
substructure. Studies of the Milky Way, however, have yet to identify the
expected few hundred sub-halos with masses greater than about 10^6 Msun. Here
we propose a test for the presence of sub-halos in the halos of galaxies. We
show that the structure of the tidal tails of ancient globular clusters is very
sensitive to heating by repeated close encounters with the massive dark
sub-halos. We discuss the detection of such an effect in the context of the
next generation of astrometric missions, and conclude that it should be easily
detectable with the GAIA dataset. The finding of a single extended cold stellar
stream from a globular cluster would support alternative theories, such as
self-interacting dark matter, that give rise to smoother halos.Comment: 7 pages, 7 figures, submitted to MNRA
Anomalous isotopic predissociation in the F³Πu(v=1) state of O₂
Using a tunable, narrow-bandwidth vacuum-ultraviolet source based on third-harmonic generation from excimer-pumped dye-laser radiation, the F³Πu←X³Σg-(1,0)photoabsorption cross sections of ¹⁶O₂ and ¹⁸O₂ have been recorded in high resolution. Rotational analyses have been performed and the resultant F(v=1) term values fitted to the ³Π Hamiltonian of Brown and Merer [J. Mol. Spectrosc. 74, 488 (1979)]. A large rotationless isotope effect is observed in the F(v=1)predissociation, wherein the Lorentzian linewidth component for ¹⁸O₂ is a factor of ∼50 smaller than the corresponding ¹⁶O₂linewidth. This effect, a consequence of the nonadiabatic rotationless predissociation mechanism, is described using a coupled-channel treatment of the strongly Rydberg-valence-mixed 3Πu states. Significant J, e/f-parity, and sublevel dependencies observed in the isotopic F(v=1) rotational widths are found to derive from an indirect predissociation mechanism involving an accidental degeneracy with the E³Σ−u(v=3) level, itself strongly predissociated by ³Σ−u Rydberg-valence interactions, together with L-uncoupling (rotational) interactions between the Rydberg components of the F and E states. Transitions into the E(v=3) level are observed directly for the first time, specifically in the ¹⁸O₂ spectrumPartial support
was provided by an NSF International Opportunities for Scientists
and Engineers Program Grant No. INT-9513350, and
Visiting Fellowships for G.S. and J.B.W. at the Australian
National University
Quasar Microlensing at High Magnification and the Role of Dark Matter: Enhanced Fluctuations and Suppressed Saddlepoints
Contrary to naive expectation, diluting the stellar component of the lensing
galaxy in a highly magnified system with smoothly distributed ``dark'' matter
increases rather than decreases the microlensing fluctuations caused by the
remaining stars. For a bright pair of images straddling a critical curve, the
saddlepoint (of the arrival time surface) is much more strongly affected than
the associated minimum. With a mass ratio of smooth matter to microlensing
matter of 4:1, a saddlepoint with a macro-magnification of mu = 9.5 will spend
half of its time more than a magnitude fainter than predicted. The anomalous
flux ratio observed for the close pair of images in MG0414+0534 is a factor of
five more likely than computed by Witt, Mao and Schechter if the smooth matter
fraction is as high as 93%. The magnification probability histograms for
macro-images exhibit distinctly different structure that varies with the smooth
matter content, providing a handle on the smooth matter fraction. Enhanced
fluctuations can manifest themselves either in the temporal variations of a
lightcurve or as flux ratio anomalies in a single epoch snapshot of a multiply
imaged system. While the millilensing simulations of Metcalf and Madau also
give larger anomalies for saddlepoints than for minima, the effect appears to
be less dramatic for extended subhalos than for point masses. Morever,
microlensing is distinguishable from millilensing because it will produce
noticeable changes in the magnification on a time scale of a decade or less.Comment: As accepted for publication in ApJ. 17 pages. Substantial revisions
include a discussion of constant M/L models and the calculation of a
"photometric" dark matter fraction for MG0414+053
Discovery of three z>6.5 quasars in the VISTA Kilo-degree Infrared Galaxy (VIKING) survey
Studying quasars at the highest redshifts can constrain models of galaxy and
black hole formation, and it also probes the intergalactic medium in the early
universe. Optical surveys have to date discovered more than 60 quasars up to
z~6.4, a limit set by the use of the z-band and CCD detectors. Only one z>6.4
quasar has been discovered, namely the z=7.08 quasar ULAS J1120+0641, using
near-infrared imaging. Here we report the discovery of three new z>6.4 quasars
in 332 square degrees of the Visible and Infrared Survey Telescope for
Astronomy Kilo-degree Infrared Galaxy (VIKING) survey, thus extending the
number from 1 to 4. The newly discovered quasars have redshifts of z=6.60,
6.75, and 6.89. The absolute magnitudes are between -26.0 and -25.5, 0.6-1.1
mag fainter than ULAS J1120+0641. Near-infrared spectroscopy revealed the MgII
emission line in all three objects. The quasars are powered by black holes with
masses of ~(1-2)x10^9 M_sun. In our probed redshift range of 6.44<z<7.44 we can
set a lower limit on the space density of supermassive black holes of
\rho(M_BH>10^9 M_sun) > 1.1x10^(-9) Mpc^(-3). The discovery of three quasars in
our survey area is consistent with the z=6 quasar luminosity function when
extrapolated to z~7. We do not find evidence for a steeper decline in the space
density of quasars with increasing redshift from z=6 to z=7.Comment: 14 pages, 9 figures. Published in Ap
- …