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 $m=38.1_{-1.5}^{+1.7}$ $M_{Jup}$, 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 $m=0.147\pm 0.003$ $M_\odot$, 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 $m=0.595 \pm 0.010$ $M_\odot$ 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 $\sim300$ Myr the A and B components each have a mass of $64.6^{+0.8}_{-2.0}~M_{\mathrm{Jup}}$, and the b component has a mass of $11.2^{+9.7}_{-1.8}$, making VHS 1256-1257 only the third brown dwarf triple system. There exists some tension between the spectrophotometric distance of $17.2\pm2.6$ pc and the parallax distance of $12.7\pm1.0$ 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 $\sim35~M_{\mathrm{Jup}}$ and the mass of each binary component increases to $73^{+20}_{-17}~M_{\mathrm{Jup}}$. 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 $\beta$ Pic to measure the mass of the giant planet $\beta$ Pic b $(13\pm3$ $M_{\rm Jup})$ 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 $(1.84\pm0.05$ $M_{\odot})$. This first model-independent mass for a directly imaged planet is inconsistent with cold-start models given the age of the $\beta$ Pic moving group $(22\pm6$ Myr) but consistent with hot- and warm-start models, concordant with past work. We find a higher eccentricity $(0.24\pm0.06)$ for $\beta$ 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 $\beta$ Pic's disk. It could also potentially signal that $\beta$ 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 (&gt;76%) to those in surface waters. By contrast, bacterial heterotrophic production (&lt;sup&gt;3&lt;/sup&gt;H-leucine incorporation) was consistently lower in the surface microlayer than in surface waters. By applying CARD-FISH and microautoradiography, we observed that &lt;i&gt;Bacteroidetes&lt;/i&gt; and &lt;i&gt;Gammaproteobacteria&lt;/i&gt; dominated leucine uptake in the surface microlayer, while in surface waters &lt;i&gt;Bacteroidetes&lt;/i&gt; and &lt;i&gt;Alphaproteobacteria&lt;/i&gt; 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