182 research outputs found
A Young Planetary-Mass Object in the Ļ Oph Cloud Core
We report the discovery of a young planetary-mass brown dwarf in the Ļ Oph cloud core. The object was identified as such with the aid of a 1.5-2.4 Ī¼m low-resolution spectrum obtained using the NIRC instrument on the Keck I telescope. Based on the COND model, the observed spectrum is consistent with a reddened (A_V ~ 15-16) brown dwarf whose effective temperature is in the range 1200-1800 K. For an assumed age of 1 Myr, comparison with isochrones further constrains the temperature to ~1400 K and suggests a mass of ~2-3 Jupiter masses. The inferred temperature is suggestive of an early T spectral type, which is supported by spectral morphology consistent with weak methane absorption. Based on its inferred distance (~100 pc) and the presence of overlying visual absorption, it is very likely to be a Ļ Oph cluster member. In addition, given the estimated spectral type, it may be the youngest and least massive T dwarf found so far. Its existence suggests that the initial mass function for the Ļ Oph star-forming region extends well into the planetary-mass regime
Eighteen Leading Social Critics Comment: What is the Real Threat to World Peace and Social Security?
What is the real threat to world peace and social security? Is it the prevalent ideology of violence, aggressive nationalism, and militarism? Or is it the maldistribution of resources, technology, and social welfare benefits? How much of itsnational budget must the U.S. allocate to military expenditures? How can the national budget priorities be changed so there is a more realistic funding of social programs? Do we face as great a threat of nuclear annihilation in the \u2770\u27s and the \u2780\u27s as we did in the early \u2760\u27s?
These are some of the questions the editors asked of 96 experts on the warfare- welfare problem. In all, 18 persons responded to the invitation, including 22% of the social scientists (N=11), 10% of the writers and editors (N=l), 25% of the executives of peace organizations (N=5), 10% of the elected government officials (N=l), and none of the military leaders. The names and affiliations of participants are indicated on the following page
WISE Brown Dwarf Binaries: The Discovery of a T5+T5 and a T8.5+T9 System
The multiplicity properties of brown dwarfs are critical empirical constraints for formation theories, while multiples themselves provide unique opportunities to test evolutionary and atmospheric models and examine empirical trends. Studies using high-resolution imaging cannot only uncover faint companions, but they can also be used to determine dynamical masses through long-term monitoring of binary systems. We have begun a search for the coolest brown dwarfs using preliminary processing of data from the Wide-field Infrared Survey Explorer and have confirmed many of the candidates as late-type T dwarfs. In order to search for companions to these objects, we are conducting observations using the Laser Guide Star Adaptive Optics system on Keck II. Here we present the first results of that search, including a T5 binary with nearly equal mass components and a faint companion to a T8.5 dwarf with an estimated spectral type of T9
Hubble Space Telescope Spectroscopy of Brown Dwarfs Discovered with the Wide-field Infrared Survey Explorer
We present a sample of brown dwarfs identified with the {\it Wide-field
Infrared Survey Explorer} (WISE) for which we have obtained {\it Hubble Space
Telescope} ({\it HST}) Wide Field Camera 3 (WFC3) near-infrared grism
spectroscopy. The sample (twenty-two in total) was observed with the G141 grism
covering 1.101.70 m, while fifteen were also observed with the G102
grism, which covers 0.901.10 m. The additional wavelength coverage
provided by the G102 grism allows us to 1) search for spectroscopic features
predicted to emerge at low effective temperatures (e.g.\ ammonia bands) and 2)
construct a smooth spectral sequence across the T/Y boundary. We find no
evidence of absorption due to ammonia in the G102 spectra. Six of these brown
dwarfs are new discoveries, three of which are found to have spectral types of
T8 or T9. The remaining three, WISE J082507.35280548.5 (Y0.5), WISE
J120604.38840110.6 (Y0), and WISE J235402.77024015.0 (Y1) are the
nineteenth, twentieth, and twenty-first spectroscopically confirmed Y dwarfs to
date. We also present {\it HST} grism spectroscopy and reevaluate the spectral
types of five brown dwarfs for which spectral types have been determined
previously using other instruments.Comment: Accepted for publication in the Astrophysical Journal. 20 pages, 18
figures, 7 table
FIRE Spectroscopy of Five Late-type T Dwarfs Discovered with the Wide-field Infrared Survey Explorer
We present the discovery of five late-type T dwarfs identified with the
Wide-field Infrared Survey Explorer (WISE). Low-resolution near-infrared
spectroscopy obtained with the Magellan Folded-port InfraRed Echellette (FIRE)
reveal strong water and methane absorption in all five sources, and spectral
indices and comparison to spectral templates indicate classifications ranging
from T5.5 to T8.5:. The spectrum of the latest-type source, WISE J1812+2721, is
an excellent match to that of the T8.5 companion brown dwarf Wolf 940B.
WISE-based spectrophotometric distance estimates place these T dwarfs at 12-13
pc from the Sun, assuming they are single. Preliminary fits of the spectral
data to the atmosphere models of Saumon & Marley indicate effective
temperatures ranging from 600 K to 930 K, both cloudy and cloud-free
atmospheres, and a broad range of ages and masses. In particular, two sources
show evidence of both low surface gravity and cloudy atmospheres, tentatively
supporting a trend noted in other young brown dwarfs and exoplanets. In
contrast, the high proper motion T dwarf WISE J2018-7423 exhibits a suppressed
K-band peak and blue spectrophotometric J-K colors indicative of an old,
massive brown dwarf; however, it lacks the broadened Y-band peak seen in
metal-poor counterparts. These results illustrate the broad diversity of
low-temperature brown dwarfs that will be uncovered with WISE.Comment: 19 pages, 13 figures; accepted for publication to Ap
A T8.5 Brown Dwarf Member of the Xi Ursae Majoris System
The Wide-field Infrared Survey Explorer has revealed a T8.5 brown dwarf (WISE
J111838.70+312537.9) that exhibits common proper motion with a
solar-neighborhood (8 pc) quadruple star system - Xi Ursae Majoris. The angular
separation is 8.5 arc-min, and the projected physical separation is about 4000
AU. The sub-solar metallicity and low chromospheric activity of Xi UMa A argue
that the system has an age of at least 2 Gyr. The infrared luminosity and color
of the brown dwarf suggests the mass of this companion ranges between 14 and 38
Jupiter masses for system ages of 2 and 8 Gyr respectively.Comment: AJ in press, 12 pages LaTeX with 6 figures. More astrometric data and
a laser guide star adaptive optics image adde
Parallaxes and Proper Motions of Ultracool Brown Dwarfs of Spectral Types Y and Late T
We present astrometric measurements of 11 nearby ultracool brown dwarfs of spectral types Y and late-T, based on imaging observations from a variety of space-based and ground-based telescopes. These measurements have been used to estimate relative parallaxes and proper motions via maximum likelihood fitting of geometric model curves. To compensate for the modest statistical significance (ā²7) of our parallax measurements we have employed a novel Bayesian procedure for distance estimation which makes use of an a priori distribution of tangential velocities, V_tan, assumed similar to that implied by previous observations of T_dwarfs. Our estimated distances are therefore somewhat dependent on that assumption. Nevertheless, the results have yielded distances for five of our eight Y dwarfs and all three T dwarfs. Estimated distances in all cases are ā³ 3 pc. In addition, we have obtained significant estimates of V_tan for two of the Y dwarfs; both are <100 km s^(ā1), consistent with membership in the thin disk population. Comparison of absolute magnitudes with model predictions as a function of color shows that the Y dwarfs are significantly redder in J ā H than predicted by a cloud-free model
Deep Near-Infrared Imaging of the rho Oph Cloud Core: Clues to the Origin of the Lowest-Mass Brown Dwarfs
A search for young substellar objects in the rho Oph cloud core region has
been made using the deep-integration Combined Calibration Scan images of the
2MASS extended mission in J, H and Ks bands, and Spitzer IRAC images at 3.6,
4.5, 5.8 and 8.0 microns. The field of view of the combined observations was 1
deg x 9.3 arcmin, and the 5 sigma limiting magnitude at J was 20.5. Comparison
of the observed SEDs with the predictions of the COND and DUSTY models, for an
assumed age of 1 Myr, supports the identification of many of the sources with
brown dwarfs, and enables the estimation of effective temperature, Teff. The
cluster members are then readily distinguishable from background stars by their
locations on a plot of flux density versus Teff. The range of estimated Teff
extends down to ~ 750 K, suggesting the presence of objects of sub-Jupiter
mass. The results also suggest that the mass function for the rho Oph cloud
resembles that of the sigma Orionis cluster based on a recent study, with both
rising towards lower masses. The other main result from our study is the
apparent presence of a progressive blueward skew in the distribution of J-H and
H-Ks colors, such that the blue end of the range becomes increasingly bluer
with increasing magnitude. We suggest that this behavior might be understood in
terms of the 'ejected stellar embryo' hypothesis, whereby some of the
lowest-mass brown dwarfs could escape to locations close to the front edge of
the cloud, and thereby be seen with less extinction.Comment: 37 pages, 10 figures; to be published in Ap
A Study of the Diverse T Dwarf Population Revealed by WISE
We report the discovery of 87 new T dwarfs uncovered with the Wide-field
Infrared Survey Explorer (WISE) and three brown dwarfs with extremely red
near-infrared colors that exhibit characteristics of both L and T dwarfs. Two
of the new T dwarfs are likely binaries with L7+/-1 primaries and mid-type T
secondaries. In addition, our follow-up program has confirmed 10 previously
identified T dwarfs and four photometrically-selected L and T dwarf candidates
in the literature. This sample, along with the previous WISE discoveries,
triples the number of known brown dwarfs with spectral types later than T5.
Using the WISE All-Sky Source Catalog we present updated color-color and
color-type diagrams for all the WISE-discovered T and Y dwarfs. Near-infrared
spectra of the new discoveries are presented, along with spectral
classifications. To accommodate later T dwarfs we have modified the integrated
flux method of determining spectral indices to instead use the median flux.
Furthermore, a newly defined J-narrow index differentiates the early-type Y
dwarfs from late-type T dwarfs based on the J-band continuum slope. The K/J
indices for this expanded sample show that 32% of late-type T dwarfs have
suppressed K-band flux and are blue relative to the spectral standards, while
only 11% are redder than the standards. Comparison of the Y/J and K/J index to
models suggests diverse atmospheric conditions and supports the possible
re-emergence of clouds after the L/T transition. We also discuss peculiar brown
dwarfs and candidates that were found not to be substellar, including two Young
Stellar Objects and two Active Galactic Nuclei. The coolest WISE-discovered
brown dwarfs are the closest of their type and will remain the only sample of
their kind for many years to come.Comment: Accepted to ApJS on 15 January 2013; 99 pages in preprint format, 30
figures, 12 table
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