38 research outputs found
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
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
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
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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
Trigonometric Parallaxes of Central Stars of Planetary Nebulae
Trigonometric parallaxes of 16 nearby planetary nebulae are presented,
including reduced errors for seven objects with previous initial results and
results for six new objects. The median error in the parallax is 0.42 mas, and
twelve nebulae have parallax errors less than 20 percent. The parallax for
PHL932 is found here to be smaller than was measured by Hipparcos, and this
peculiar object is discussed. Comparisons are made with other distance
estimates. The distances determined from these parallaxes tend to be
intermediate between some short distance estimates and other long estimates;
they are somewhat smaller than estimated from spectra of the central stars.
Proper motions and tangential velocities are presented. No astrometric
perturbations from unresolved close companions are detected.Comment: 24 pages, includes 4 figures. Accepted for A