119 research outputs found
How does viscosity contrast influence phase mixing and strain localization?
Author Posting. © American Geophysical Union, 2020. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Solid Earth 125 (2020): e2020JB020323, doi: 10.1029/2020JB020323.Ultramylonitesâintensely deformed rocks with fine grain sizes and wellâmixed mineral phasesâare thought to be a key component of Earthâlike plate tectonics, because coupled phase mixing and grain boundary pinning enable rocks to deform by grainâsizeâsensitive, selfâsoftening creep mechanisms over long geologic timescales. In isoviscous twoâphase composites, âgeometricâ phase mixing occurs via the sequential formation, attenuation (stretching), and disaggregation of compositional layering. However, the effects of viscosity contrast on the mechanisms and timescales for geometric mixing are poorly understood. Here, we describe a series of highâstrain torsion experiments on nonisoviscous calciteâfluorite composites (viscosity contrast, ηca/ηfl â 200) at 500°C, 0.75 GPa confining pressure, and 10â6â10â4 sâ1 shear strain rate. At low to intermediate shear strains (Îł †10), polycrystalline domains of the individual phases become sheared and form compositional layering. As layering develops, strain localizes into the weaker phase, fluorite. Strain partitioning impedes mixing by reducing the rate at which the stronger (calcite) layers deform, attenuate, and disaggregate. Even at very large shear strains (Îł â„ 50), grainâscale mixing is limited, and thick compositional layers are preserved. Our experiments (1) demonstrate that viscosity contrasts impede mechanical phase mixing and (2) highlight the relative inefficiency of mechanical mixing. Nevertheless, by employing laboratory flow laws, we show that âidealâ conditions for mechanical phase mixing may be found in the wet middle to lower continental crust and in the dry mantle lithosphere, where quartzâfeldspar and olivineâpyroxene viscosity contrasts are minimized, respectively.This work was funded through a National Science Foundation grant (EARâ1352306) awarded to P. S., with additional support for A. J. C. provided by the McDonnell Center for the Space Sciences (Washington University in St. Louis), the J. Lamar Worzel Assistant Scientist Fund (WHOI), and the Penzance Endowed Fund in Support of Assistant Scientists (WHOI). Partial support for electron microscopy was provided by the Institute of Materials Science and Engineering (Washington University in St. Louis).2021-02-0
Sirius B Imaged in the Mid-Infrared: No Evidence for a Remnant Planetary System
Evidence is building that remnants of solar systems might orbit a large
percentage of white dwarfs, as the polluted atmospheres of DAZ and DBZ white
dwarfs indicate the very recent accretion of metal-rich material. (Zuckerman et
al. 2010). Some of these polluted white dwarfs are found to have large
mid-infrared excesses from close-in debris disks that are thought to be
reservoirs for the metal accretion. These systems are coined DAZd white dwarfs
(von Hippel et al. 2007). Here we investigate the claims of Bonnet-Bidaud &
Pantin (2008) that Sirius B, the nearest white dwarf to the Sun, might have an
infrared excess from a dusty debris disk. Sirius B's companion, Sirius A is
commonly observed as a mid-infrared photometric standard in the Southern
hemisphere. We combine several years of Gemini/T-ReCS photometric standard
observations to produce deep mid-infrared imaging in five ~10 micron filters
(broad N + 4 narrowband), which reveal the presence of Sirius B. Our photometry
is consistent with the expected photospheric emission such that we constrain
any mid-infrared excess to <10% of the photosphere. Thus we conclude that
Sirius B does not have a large dusty disk, as seen in DAZd white dwarfs.Comment: 13 pages, 3 figures, accepted to Ap
High contrast imaging at the LBT: the LEECH exoplanet imaging survey
In Spring 2013, the LEECH (LBTI Exozodi Exoplanet Common Hunt) survey began its ~130-night campaign from the Large Binocular Telescope (LBT) atop Mt Graham, Arizona. This survey benefits from the many technological achievements of the LBT, including two 8.4-meter mirrors on a single fixed mount, dual adaptive secondary mirrors for high Strehl performance, and a cold beam combiner to dramatically reduce the telescopeâs overall background emissivity. LEECH neatly complements other high-contrast planet imaging efforts by observing stars at Lâ (3.8 ÎŒm), as opposed to the shorter wavelength near-infrared bands (1-2.4 ÎŒm) of other surveys. This portion of the spectrum offers deep mass sensitivity, especially around nearby adolescent (~0.1-1 Gyr) stars. LEECHâs contrast is competitive with other extreme adaptive optics systems, while providing an alternative survey strategy. Additionally, LEECH is characterizing known exoplanetary systems with observations from 3-5ÎŒm in preparation for JWST
High Spatial Resolution Thermal-Infrared Spectroscopy with ALES: Resolved Spectra of the Benchmark Brown Dwarf Binary HD 130948BC
We present 2.9-4.1 micron integral field spectroscopy of the L4+L4 brown
dwarf binary HD 130948BC, obtained with the Arizona Lenslets for Exoplanet
Spectroscopy (ALES) mode of the Large Binocular Telescope Interferometer
(LBTI). The HD 130948 system is a hierarchical triple system, in which the G2V
primary is joined by two co-orbiting brown dwarfs. By combining the age of the
system with the dynamical masses and luminosities of the substellar companions,
we can test evolutionary models of cool brown dwarfs and extra-solar giant
planets. Previous near-infrared studies suggest a disagreement between HD
130948BC luminosities and those derived from evolutionary models. We obtained
spatially-resolved, low-resolution (R~20) L-band spectra of HD 130948B and C to
extend the wavelength coverage into the thermal infrared. Jointly using JHK
photometry and ALES L-band spectra for HD 130948BC, we derive atmospheric
parameters that are consistent with parameters derived from evolutionary
models. We leverage the consistency of these atmospheric quantities to favor a
younger age (0.50 \pm 0.07 Gyr) of the system compared to the older age (0.79
\pm 0.22 Gyr) determined with gyrochronology in order to address the luminosity
discrepancy.Comment: 17 pages, 9 figures, Accepted to Ap
New Spatially Resolved Observations of the T Cha Transition Disk and Constraints on the Previously Claimed Substellar Companion
We present multi-epoch non-redundant masking observations of the T Cha
transition disk, taken at the VLT and Magellan in H, Ks, and L' bands. T Cha is
one of a small number of transition disks that host companion candidates
discovered by high-resolution imaging techniques, with a putative companion at
a position angle of 78 degrees, separation of 62 mas, and contrast at L' of 5.1
mag. We find comparable binary parameters in our re-reduction of the initial
detection images, and similar parameters in the 2011 L', 2013 NaCo L', and 2013
NaCo Ks data sets. We find a close-in companion signal in the 2012 NaCo L'
dataset that cannot be explained by orbital motion, and a non-detection in the
2013 MagAO/Clio2 L' data. However, Monte-carlo simulations show that the best
fits to the 2012 NaCo and 2013 MagAO/Clio2 followup data may be consistent with
noise. There is also a significant probability of false non-detections in both
of these data sets. We discuss physical scenarios that could cause the best
fits, and argue that previous companion and scattering explanations are
inconsistent with the results of the much larger dataset presented here.Comment: 25 pages, 22 figures, accepted for publication in Ap
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