2,323 research outputs found
Two Extraordinary Substellar Binaries at the T/Y Transition and the Y-Band Fluxes of the Coolest Brown Dwarfs
Using Keck laser guide star adaptive optics imaging, we have found that the
T9 dwarf WISE J1217+1626 and T8 dwarf WISE J1711+3500 are exceptional binaries,
with unusually wide separations (~0.8 arcsec, 8-15 AU), large near-IR flux
ratios (~2-3 mags), and small mass ratios (~0.5) compared to previously known
field ultracool binaries. Keck/NIRSPEC H-band spectra give a spectral type of
Y0 for WISE J1217+1626B, and photometric estimates suggest T9.5 for WISE
J1711+3500B. The WISE J1217+1626AB system is very similar to the T9+Y0 binary
CFBDSIR J1458+1013AB; these two systems are the coldest known substellar
multiples, having secondary components of ~400 K and being planetary-mass
binaries if their ages are <~1 Gyr. Both WISE J1217+1626B and CFBDSIR
J1458+1013B have strikingly blue Y-J colors compared to previously known T
dwarfs, including their T9 primaries. Combining all available data, we find
that Y-J color drops precipitously between the very latest T dwarfs and the Y
dwarfs. The fact that this is seen in (coeval, mono-metallicity) binaries
demonstrates that the color drop arises from a change in temperature, not
surface gravity or metallicity variations among the field population. Thus, the
T/Y transition established by near-IR spectra coincides with a significant
change in the ~1 micron fluxes of ultracool photospheres. One explanation is
the depletion of potassium, whose broad absorption wings dominate the far-red
optical spectra of T dwarfs. This large color change suggests that far-red data
may be valuable for classifying objects of <~500 K.Comment: ApJ, in press (accepted Aug 1, 2012). Small cosmetic changes in
version 2 to match final publicatio
Precise Dynamical Masses of Directly Imaged Companions from Relative Astrometry, Radial Velocities, and Hipparcos-Gaia DR2 Accelerations
We measure dynamical masses for five objects--three ultracool dwarfs, one
low-mass star, and one white dwarf--by fitting orbits to a combination of the
Hipparcos-Gaia Catalog of Accelerations, literature radial velocities, and
relative astrometry. Our approach provides precise masses without any
assumptions about the primary star, even though the observations typically
cover only a small fraction of an orbit. We also perform a uniform re-analysis
of the host stars' ages. Two of our objects, HD 4747B and HR 7672B, already
have precise dynamical masses near the stellar/substellar boundary and are used
to validate our approach. For Gl 758B, we obtain a mass of
, the most precise mass measurement of this
companion to date. Gl 758B is the coldest brown dwarf with a dynamical mass,
and the combination of our low mass and slightly older host-star age resolves
its previously noted discrepancy with substellar evolutionary models. HD
68017B, a late-M dwarf, has a mass of , consistent
with stellar theory and previous empirical estimates based on its absolute
magnitude. The progenitor of the white dwarf Gl 86B has been debated in the
literature, and our dynamical measurement of is
consistent with a higher progenitor mass and younger age for this
planet-hosting binary system. Overall, these case studies represent only five
of the thousands of accelerating systems identified by combining Hipparcos and
Gaia. Our analysis could be repeated for many of them to build a large sample
of companions with dynamical masses.Comment: 33 pages, 24 figures, 9 tables, AJ accepted with minor revision
Subducted and recycled lithosphere as the mantle source of ocean island basalts from southern Polynesia, central Pacific
L'étude géochimique et pétrographique des basaltes des archipels volcaniques de la Polynésie Française montre que ces roches dérivent d'une lithosphère ancienne ayant subi une subduction et un recyclage. La composition chimique et isotopique des basaltes montre l'existence d'hétérogénéités géochimiques pour la source magmatique : elles s'expliquent par un mélange de la croûte océanique avec une péridotite résiduelle
ADAPTIVE OPTICS IMAGING OF VHS 1256-1257: A LOW MASS COMPANION TO A BROWN DWARF BINARY SYSTEM
Recently, Gauza et al. (2015) reported the discovery of a companion to the
late M-dwarf, VHS J125601.92-125723.9 (VHS 1256-1257). The companion's absolute
photometry suggests its mass and atmosphere are similar to the HR 8799 planets.
However, as a wide companion to a late-type star, it is more accessible to
spectroscopic characterization. We discovered that the primary of this system
is an equal-magnitude binary. For an age Myr the A and B components
each have a mass of , and the b component
has a mass of , making VHS 1256-1257 only the third brown
dwarf triple system. There exists some tension between the spectrophotometric
distance of pc and the parallax distance of pc. At
12.7 pc VHS1256-1257 A and B would be the faintest known M7.5 objects, and are
even faint outliers among M8 types. If the larger spectrophotmetric distance is
more accurate than the parallax, then the mass of each component increases. In
particular, the mass of the b component increases well above the deuterium
burning limit to and the mass of each binary
component increases to . At 17.1 pc, the UVW
kinematics of the system are consistent with membership in the AB~Dor moving
group. The architecture of the system resembles a hierarchical stellar multiple
suggesting it formed via an extension of the star-formation process to low
masses. Continued astrometric monitoring will resolve this distance uncertainty
and will provide dynamical masses for a new benchmark system.Comment: Accepted to ApJ
A Model-Independent Mass and Moderate Eccentricity for β Pic b
We use a cross-calibration of Hipparcos and Gaia DR2 astrometry for
Pic to measure the mass of the giant planet Pic b in a comprehensive joint orbit analysis that includes published relative
astrometry and radial velocities. Our mass uncertainty is somewhat higher than
previous work because our astrometry from the Hipparcos-Gaia Catalog of
Accelerations accounts for the error inflation and systematic terms that are
required to bring the two data sets onto a common astrometric reference frame,
and because we fit freely for the host-star mass .
This first model-independent mass for a directly imaged planet is inconsistent
with cold-start models given the age of the Pic moving group
Myr) but consistent with hot- and warm-start models, concordant with past work.
We find a higher eccentricity for Pic b compared to
previous orbital fits. If confirmed by future observations, this eccentricity
may help explain inner edge, scale height, and brightness asymmetry of
Pic's disk. It could also potentially signal that Pic b has migrated
inward to its current location, acquiring its eccentricity from interaction
with the 3:1 outer Lindblad resonance in the disk.Comment: ApJ Letters, accepte
Low temperature transition to a superconducting phase in boron-doped silicon films grown on (001)-oriented silicon wafers
We report on a detailed analysis of the superconducting properties of
boron-doped silicon films grown along the 001 direction by Gas Immersion Laser
Doping. The doping concentration cB has been varied up to approx. 10 at.% by
increasing the number of laser shots to 500. No superconductivity could be
observed down to 40mK for doping level below 2.5 at.%. The critical temperature
Tc then increased steeply to reach 0.6K for cB = 8 at%. No hysteresis was found
for the transitions in magnetic field, which is characteristic of a type II
superconductor. The corresponding upper critical field Hc2(0) was on the order
of 1000 G, much smaller than the value previously reported by Bustarret et al.
in Nature (London) 444, 465 (2006).Comment: 4 pages including 4 figures, submitted to PRB-Rapid Communicatio
Biochemical characteristics and bacterial community structure of the sea surface microlayer in the South Pacific Ocean
The chemical and biological characteristics of the surface microlayer were determined during a transect across the South Pacific Ocean in October-December 2004. Concentrations of particulate organic carbon (1.3 to 7.6-fold) and nitrogen (1.4 to 7-fold), and POC:PON ratios were consistently higher in the surface microlayer as compared to surface waters (5 m). The large variability in particulate organic matter enrichment was negatively correlated to wind speed. No enhanced concentrations of dissolved organic carbon were detectable in the surface microlayer as compared to 5 m, but chromophoric dissolved organic matter was markedly enriched (by 2 to 4-fold) at all sites. Based on pigment analysis and cell counts, no consistent enrichment of any of the major components of the autotrophic and heterotrophic microbial community was detectable. CE-SSCP fingerprints and CARD FISH revealed that the bacterial communities present in the surface microlayer had close similarity (>76%) to those in surface waters. By contrast, bacterial heterotrophic production (<sup>3</sup>H-leucine incorporation) was consistently lower in the surface microlayer than in surface waters. By applying CARD-FISH and microautoradiography, we observed that <i>Bacteroidetes</i> and <i>Gammaproteobacteria</i> dominated leucine uptake in the surface microlayer, while in surface waters <i>Bacteroidetes</i> and <i>Alphaproteobacteria</i> were the major groups accounting for leucine incorporation. Our results demonstrate that the microbial community in the surface microlayer closely resembles that of the surface waters of the open ocean. Even a short residence in the surface microlayer influences leucine incorporation by different bacterial groups, probably as a response to the differences in the physical and chemical nature of the two layers
The Impact of Bayesian Hyperpriors on the Population-Level Eccentricity Distribution of Imaged Planets
Orbital eccentricities directly trace the formation mechanisms and dynamical
histories of substellar companions. Here, we study the effect of hyperpriors on
the population-level eccentricity distributions inferred for the sample of
directly imaged substellar companions (brown dwarfs and cold Jupiters) from
hierarchical Bayesian modeling (HBM). We find that the choice of hyperprior can
have a significant impact on the population-level eccentricity distribution
inferred for imaged companions, an effect that becomes more important as the
sample size and orbital coverage decrease to values that mirror the existing
sample. We reanalyse the current observational sample of imaged giant planets
in the 5-100 AU range from Bowler et al. (2020) and find that the underlying
eccentricity distribution implied by the imaged planet sample is broadly
consistent with the eccentricity distribution for close-in exoplanets detected
using radial velocities. Furthermore, our analysis supports the conclusion from
that study that long-period giant planets and brown dwarf eccentricity
distributions differ by showing that it is robust to the choice of hyperprior.
We release our HBM and forward modeling code in an open-source Python package,
ePop!, and make it freely available to the community.Comment: 18 pages, 11 figures. Accepted for publication in The Astronomical
Journa
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