51 research outputs found
Spectral Energy Distributions for Disk and Halo M--Dwarfs
We have obtained infrared (1 to 2.5 micron) spectroscopy for 42 halo and disk
dwarfs with spectral type M1 to M6.5. These data are compared to synthetic
spectra generated by the latest model atmospheres of Allard & Hauschildt.
Photospheric parameters metallicity, effective temperature and radius are
determined for the sample. We find good agreement between observation and
theory except for known problems due to incomplete molecular data for metal
hydrides and water. The metal-poor M subdwarfs are well matched by the models
as oxide opacity sources are less important in this case. The derived effective
temperatures for the sample range from 3600K to 2600K; at these temperatures
grain formation and extinction are not significant in the photosphere. The
derived metallicities range from solar to one-tenth solar. The radii and
effective temperatures derived agree well with recent models of low mass stars.Comment: 24 pages including 13 figures, 4 Tables; accepted by Ap
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
The Solar Neighborhood. XXXIX. Parallax Results from the CTIOPI and NOFS Programs: 50 New Members of the 25 Parsec White Dwarf Sample
We present 114 trigonometric parallaxes for 107 nearby white dwarf (WD)
systems from both the Cerro Tololo Inter-American Observatory Parallax
Investigation (CTIOPI) and the U. S. Naval Observatory Flagstaff Station (NOFS)
parallax programs. Of these, 76 parallaxes for 69 systems were measured by the
CTIOPI program and 38 parallaxes for as many systems were measured by the NOFS
program. A total of 50 systems are confirmed to be within the 25 pc horizon of
interest. Coupled with a spectroscopic confirmation of a common proper motion
companion to a Hipparcos star within 25 pc as well as confirmation parallax
determinations for two WD systems included in the recently released Tycho Gaia
Astrometric Solution (TGAS) catalog, we add 53 new systems to the 25 pc WD
sample a 42% increase. Our sample presented here includes four strong
candidate halo systems, a new metal-rich DAZ WD, a confirmation of a recently
discovered nearby short-period (P = 2.85 hr) double degenerate, a WD with a new
astrometric pertubation (long period, unconstrained with our data), and a new
triple system where the WD companion main-sequence star has an astrometric
perturbation (P 1.6 yr).Comment: 32 pages, 12 figures. Figure 4 in the manuscript is a representative
set of plots - plots for all WDs presented here are available
(allfits_photo.pdf, allfits_photo_DQ.pdf, and allfits_photo_DZ.pdf). Accepted
for publication in The Astronomical Journa
The runaway binary LP 400−22 is leaving the Galaxy
We present optical spectroscopy, astrometry, radio and X-ray observations of the runaway binary LP 400−22. We refine the orbital parameters of the system based on our new radial velocity observations. Our parallax data indicate that LP 400−22 is significantly more distant (3σ lower limit of 840 pc) than initially predicted. LP 400−22 has a tangential velocity in excess of 830 km s^−1; it is unbound to the Galaxy. Our radio and X-ray observations fail to detect a recycled millisecond pulsar companion, indicating that LP 400−22 is a double white dwarf system. This essentially rules out a supernova runaway ejection mechanism. Based on its orbit, a Galactic Centre origin is also unlikely. However, its orbit intersects the locations of several globular clusters; dynamical interactions between LP 400−22 and other binary stars or a central black hole in a dense cluster could explain the origin of this unusual binary
Evidence for Cloud Disruption in the L/T Dwarf Transition
Clouds of metal-bearing condensates play a critical role in shaping the
emergent spectral energy distributions of the coolest classes of low-mass stars
and brown dwarfs, L and T dwarfs. Because condensate clouds in planetary
atmospheres show distinct horizontal structure, we have explored a model for
partly cloudy atmospheres in brown dwarfs. Our model successfully reproduces
the colors and magnitudes of both L and T dwarfs for the first time, including
the unexpected brightning of the early- and mid-type T dwarfs at J-band,
provided that clouds are rapidly removed from the photosphere at T_eff ~ 1200
K. The clearing of cloud layers also explains the surprising persistence and
strengthening of gaseous FeH bands in early- and mid-type T dwarfs. The breakup
of cloud layers is likely driven by convection in the troposphere, analogous to
phenomena observed on Jupiter. Our results demonstrate that planetary-like
atmospheric dynamics must be considered when examining the evolution of
free-floating brown dwarfs.Comment: 12 pages, 3 figures, accepted to ApJ Letters for June 200
Observation of micropores in hard-carbon using Xe-129 NMR porosimetry
The existence of micropores and the change of surface structure in pitch-based hard-carbon in xenon atmosphere were demonstrated using Xe-129 NMR. For high-pressure (4.0 MPa) Xe-129 NMR measurements, the hard-carbon samples in Xe gas showed three peaks at 27, 34 and 210 ppm. The last was attributed to the xenon in micropores (<1 nm) in hard-carbon particles. The NMR spectrum of a sample evacuated at 773 K and exposed to 0.1 MPa Xe gas at 773 K for 24 h showed two peaks at 29 and 128 ppm, which were attributed, respectively, to the xenon atoms adsorbed in the large pores (probably mesopores) and micropores of hard-carbon. With increasing annealing time in Xe gas at 773 K, both peaks shifted and merged into one peak at 50 ppm. The diffusion of adsorbed xenon atoms is very slow, probably because the transfer of molecules or atoms among micropores in hard-carbon does not occur readily. Many micropores are isolated from the outer surface. For that reason, xenon atoms are thought to be adsorbed only by micropores near the surface, which are easily accessible from the surrounding space.</p
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Discovery of Four Field Methane (T-type) Dwarfs with the Two Micron All-Sky Survey
We report the discovery of four field methane ( T -type) brown dwarfs using Two Micron All-Sky Survey (2MASS) data. One additional methane dwarf, previously discovered by the Sloan Digital Sky Survey, was also identified. Near-infrared spectra clearly show the 1.6 and 2.2 m CH4 absorption bands characteristic of objects with Teff 1300 K as well as broadened H2O bands at 1.4 and 1.9 m. Comparing the spectra of these objects with that of Gl 229B, we propose that all new 2MASS T dwarfs are warmer than 950 K, in order from warmest to coolest: 2MASS J1217-03, 2MASS J1225-27, 2MASS J1047+21, and 2MASS J1237+65. Based on this preliminary sample, we find a warm T dwarf surface density of 0.0022 T dwarfs deg-2, or 90 warm T dwarfs over the whole sky detectable to J \u3c 16. The resulting space density upper limit, 0.01 T dwarfs pc-3, is comparable to that of the first L dwarf sample from Kirkpatrick et al
Discovery of a Brown Dwarf Companion to Gliese 570ABC: A 2MASS T Dwarf Significantly Cooler than Gliese 229B
We report the discovery of a widely separated (258\farcs3\pm0\farcs4) T
dwarf companion to the Gl 570ABC system. This new component, Gl 570D, was
initially identified from the Two Micron All Sky Survey (2MASS). Its
near-infrared spectrum shows the 1.6 and 2.2 \micron CH absorption bands
characteristic of T dwarfs, while its common proper motion with the Gl 570ABC
system confirms companionship. Gl 570D (M = 16.470.07) is nearly a
full magnitude dimmer than the only other known T dwarf companion, Gl 229B, and
estimates of L = (2.80.3)x10 L_{\sun} and T = 75050
K make it significantly cooler and less luminous than any other known brown
dwarf companion. Using evolutionary models by Burrows et al. and an adopted age
of 2-10 Gyr, we derive a mass estimate of 5020 M for this object.Comment: 13 pages, 2 figures, 2 tables, accepted by ApJ
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