274 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
Protracted Reproduction in Sunfish: the Temporal Dimension in Fish Recruitment Revisited
Understanding how life histories influence reproductive success under uncertain conditions is necessary to predict population dynamics. For many organisms, protracted reproduction may increase expected offspring recruitment in variable environments, requiring that temporal patterns of reproduction be considered when developing management or conservation strategies. We explored the interrelationships among birth date, production of embryos on nests, survival of larvae to the open-water stage, and survival of juveniles through the first fall and winter of life for bluegill and pumpkinseed sunfish (Lepomis macrochirus and L. gibbosus) in Lake Opinicon, Ontario, during May 1998 through May 1999. Age-0 sunfish were sampled with nesting surveys for embryos, surface tows for free-swimming larvae, and seining/trawling for juveniles. Age of juveniles (days post swim-up) was quantified using otoliths. The abundance of embryos on nests and the density of open-water larvae were unrelated across all dates and sites. Although 40% of larvae had appeared in the open water by 12 June, most juveniles sampled in the fall were produced after that time, suggesting that high mortality of early produced larvae occurred. Larval survival to the juvenile stage was generally unrelated to the abundance of edible zooplankton taxa during swim-up from nests. Larval survival was often highest at temperatures \u3e23.5°C. Fall length of age-0 sunfish increased with increasing age. Both age-specific length and mean lengths shifted positively between October 1998 and May 1999, suggesting that growth of all individuals and perhaps selective mortality of small juveniles occurred. Although early reproduction may increase sizes reached by fall and thereby improve overwinter survival, early hatched larvae are subject to variable environmental factors that may reduce survival. Late-hatched larvae may reach relatively smaller sizes by fall, but have higher survival probabilities during this life stage. Protracted reproduction appears to be a response to variable environmental factors influencing growth and survival across multiple life stages. As such, all reproducing adults, rather than those perceived to produce offspring during typically favorable times, must be protected from exploitation or other human-induced perturbations
Food Limitation in Larval Fish: Ontogenetic Variation in Feeding Scope and Its Potential Effect on Survival
We used the radiated shanny Ulvaria subbifurcataas a model species to explore the relative gut fullness from hatch to metamorphosis of wild larvae, and compared these values with those of laboratory-reared larvae fed at maximum rates. Ingestion rates of most wild larvae were above starvation levels but below the maximum feeding levels of laboratory-reared larvae. Twenty-six percent of freshly-hatched wild larvae and 11% of large, pre-settlement wild larvae had insufficient food in their stomach to satisfy metabolic requirements. These results, taken on their own, are consistent with the much hypothesized increased foraging performance and survival of larger larvae relative to smaller conspecifics. However, feeding scope-defined as the difference between the minimum daily food required for maintenance and the maximum set by gut capacity-increased disproportionately with size during ontogeny. As a result, none of the larger (\u3e 500 µg C) wild larvae fed at saturating levels. We conclude that environmental factors operating to increase encounters between larvae and prey (e.g. prey densities, turbulence, patch encounter) and ingestion rates will affect smaller and larger larvae differently. As a consequence, increased encounter rates should lead to increased survival in smaller larvae, and (primarily) to increased growth in larger larvae
The Binary White Dwarf LHS 3236
The white dwarf LHS 3236 (WD1639+153) is shown to be a double-degenerate
binary, with each component having a high mass. Astrometry at the U.S. Naval
Observatory gives a parallax and distance of 30.86 +/- 0.25 pc and a tangential
velocity of 98 km/s, and reveals binary orbital motion. The orbital parameters
are determined from astrometry of the photocenter over more than three orbits
of the 4.0-year period. High-resolution imaging at the Keck Observatory
resolves the pair with a separation of 31 and 124 mas at two epochs. Optical
and near-IR photometry give a set of possible binary components. Consistency of
all data indicates that the binary is a pair of DA stars with temperatures near
8000 and 7400 K and with masses of 0.93 and 0.91 M_solar; also possible, is a
DA primary and a helium DC secondary with temperatures near 8800 and 6000 K and
with masses of 0.98 and 0.69 M_solar. In either case, the cooling ages of the
stars are ~3 Gyr and the total ages are <4 Gyr. The combined mass of the binary
(1.66--1.84 M_solar) is well above the Chandrasekhar limit; however, the
timescale for coalescence is long.Comment: Accepted for the Astrophysical Journa
Host Subtraction, Filtering and Assembly Validations for Novel Viral Discovery Using Next Generation Sequencing Data.
The use of next generation sequencing (NGS) to identify novel viral sequences from eukaryotic tissue samples is challenging. Issues can include the low proportion and copy number of viral reads and the high number of contigs (post-assembly), making subsequent viral analysis difficult. Comparison of assembly algorithms with pre-assembly host-mapping subtraction using a short-read mapping tool, a k-mer frequency based filter and a low complexity filter, has been validated for viral discovery with Illumina data derived from naturally infected liver tissue and simulated data. Assembled contig numbers were significantly reduced (up to 99.97%) by the application of these pre-assembly filtering methods. This approach provides a validated method for maximizing viral contig size as well as reducing the total number of assembled contigs that require down-stream analysis as putative viral nucleic acids.This work was supported by Wellcome Trust WT091501MAThis is the author accepted manuscript. It is currently under an indefinite embargo pending publication by PLOS
6DOF Testing of the SLS Inertial Navigation Unit
The Navigation System on the NASA Space Launch System (SLS) Block 1 vehicle performs initial alignment of the Inertial Navigation System (INS) navigation frame through gyrocompass alignment (GCA). In lieu of direct testing of GCA accuracy in support of requirement verification, the SLS Navigation Team proposed and conducted an engineering test to, among other things, validate the GCA performance and overall behavior of the SLS INS model through comparison with test data. This paper will detail dynamic hardware testing of the SLS INS, conducted by the SLS Navigation Team at Marshall Space Flight Center's 6DOF Table Facility, in support of GCA performance characterization and INS model validation. A 6-DOF motion platform was used to produce 6DOF pad twist and sway dynamics while a simulated SLS flight computer communicated with the INS. Tests conducted include an evaluation of GCA algorithm robustness to increasingly dynamic pad environments, an examination of GCA algorithm stability and accuracy over long durations, and a long-duration static test to gather enough data for Allan Variance analysis. Test setup, execution, and data analysis will be discussed, including analysis performed in support of SLS INS model validation
Clouds in the Coldest Brown Dwarfs: FIRE Spectroscopy of Ross 458C
Condensate clouds are a salient feature of L dwarf atmospheres, but have been
assumed to play little role in shaping the spectra of the coldest T-type brown
dwarfs. Here we report evidence of condensate opacity in the near-infrared
spectrum of the brown dwarf candidate Ross 458C, obtained with the Folded-Port
Infrared Echellette (FIRE) spectrograph at the Magellan Telescopes. These data
verify the low-temperature nature of this source, indicating a T8 spectral
classification, log Lbol/Lsun = -5.62+/-0.03, Teff = 650+/-25 K, and a mass at
or below the deuterium burning limit. The data also reveal enhanced emission at
K-band associated with youth (low surface gravity) and supersolar metallicity,
reflecting the properties of the Ross 458 system (age = 150-800 Myr, [Fe/H] =
+0.2 to +0.3). We present fits of FIRE data for Ross 458C, the T9 dwarf ULAS
J133553.45+113005.2, and the blue T7.5 dwarf SDSS J141624.08+134826.7B, to
cloudless and cloudy spectral models from Saumon & Marley. For Ross 458C we
confirm a low surface gravity and supersolar metallicity, while the temperature
differs depending on the presence (635 [+25,-35] K) or absence (760 [+70,-45]
K) of cloud extinction. ULAS J1335+1130 and SDSS J1416+1348B have similar
temperatures (595 [+25,-45] K), but distinct surface gravities (log g = 4.0-4.5
cgs versus 5.0-5.5 cgs) and metallicities ([M/H] ~ +0.2 versus -0.2). In all
three cases, cloudy models provide better fits to the spectral data,
significantly so for Ross 458C. These results indicate that clouds are an
important opacity source in the spectra of young cold T dwarfs, and should be
considered when characterizing the spectra of planetary-mass objects in young
clusters and directly-imaged exoplanets. The characteristics of Ross 458C
suggest it could itself be regarded as a planet, albeit one whose cosmogony
does not conform with current planet formation theories.Comment: Accepted for publication to ApJ: 18 pages, 11 figures in emulateapj
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