693 research outputs found
Discovery of a Companion at the L/T Transition with the Wide-field Infrared Survey Explorer
We report the discovery of a substellar companion to the nearby solar-type
star HD 46588 (F7V, 17.9 pc, ~3 Gyr). HD 46588 B was found through a survey for
common proper motion companions to nearby stars using data from the Wide-field
Infrared Survey Explorer and the Two-Micron All-Sky Survey. It has an angular
separation of 79.2" from its primary, which corresponds to a projected physical
separation of 1420 AU. We have measured a spectral type of L9 for this object
based on near-infrared spectroscopy performed with TripleSpec at Palomar
Observatory. We estimate a mass of 0.064+0.008/-0.019 Msun from a comparison of
its luminosity to the values predicted by theoretical evolutionary models for
the age of the primary. Because of its companionship to a well-studied star, HD
46588 B is one of the few known brown dwarfs at the L/T transition for which
both age and distance estimates are available. Thus, it offers new constraints
on the properties of brown dwarfs during this brief evolutionary phase. The
discovery of HD 46588 B also illustrates the value of the Wide-field Infrared
Survey Explorer for identifying brown dwarfs in the solar neighborhood via
their proper motions.Comment: Astrophysical Journal, in pres
On the difference between type E and A OH/IR stars
The observed SEDs of a sample of 60 OH/IR stars are fitted using a radiative
transfer model of a dusty envelope. Among the whole sample, 21 stars have
reliable phase-lag distances while the others have less accurate distances.
L*-P,Mlr-P and Mlr-L* relations have been plotted for these stars. It is found
that type E (with emission feature at 10um and type A (with absorption feature
at 10um) OH/IR stars have different L*-P and Mlr-L* relations while both of
them follow a single Mlr-P relation. The type E stars are proven to be located
in the area without large scale dense interstellar medium while the type A
stars are located probably in dense interstellar medium. It is argued here that
this may indicate the two types of OH/IR stars have different chemical
composition or zero age main sequence mass and so evolve in different ways.
This conclusion has reinforced the argument by Chen et al.(2001) who reached a
similar conclusion from the galactic distribution of about 1000 OH/IR stars
with the IRAS low-resolution spectra (LRS).Comment: 6 pages, 9 figures, 2 table
An inner ring and the micro lensing toward the Bulge
All current Bulge-Disk models for the inner Galaxy fall short of reproducing
self-consistently the observed micro-lensing optical depth by a factor of two
(). We show that the least mass-consuming way to increase the
optical depth is to add density roughly half-way the observer and the highest
micro-lensing-source density. We present evidence for the existence of such a
density structure in the Galaxy: an inner ring, a standard feature of barred
galaxies. Judging from data on similar rings in external galaxies, an inner
ring can contribute more than 50% of a pure Bulge-Disk model to the
micro-lensing optical depth. We may thus eliminate the need for a small viewing
angle of the Bar. The influence of an inner ring on the event-duration
distribution, for realistic viewing angles, would be to increase the fraction
of long-duration events toward Baade's window. The longest events are expected
toward the negative-longitude tangent point at -22\degr . A properly
sampled event-duration distribution toward this tangent point would provide
essential information about viewing angle and elongation of the over-all
density distribution in the inner Galaxy.Comment: 9 pages, 7(15) figs, LaTeX, AJ (accepted
Mean-field theory of the spin-Peierls systems: Application to CuGeO3
A mean-field theory of the spin Peierls systems based on the two dimensional
dimerized Heisenberg model is proposed by introducing an alternating bond order
parameter. Improvements with respect to previous mean-field results are found
in the one-dimensional limit for the ground state and the gap energies. In two
dimensions, the analysis of the competition between antiferromagnetic long
range order and the spin-Peierls ordering is given as a function of the
coupling constants. We show that the lowest energy gap to be observed does not
have a singlet-triplet character in agreement with the low temperature
thermodynamic properties of CuGeO3.Comment: 3 Revtex pages. Submitted to Rapid Comm. Figures available upon
reques
On the Reported Death of the MACHO Era
We present radial velocity measurements of four wide halo binary candidates
from the sample in Chaname & Gould (2004; CG04) which, to date, is the only
sample containing a large number of such candidates. The four candidates that
we have observed have projected separations >0.1 pc, and include the two widest
binaries from the sample, with separations of 0.45 and 1.1 pc. We confirm that
three of the four CG04 candidates are genuine, including the one with the
largest separation. The fourth candidate, however, is spurious at the 5-sigma
level. In the light of these measurements we re-examine the implications for
MACHO models of the Galactic halo. Our analysis casts doubt on what MACHO
constraints can be drawn from the existing sample of wide halo binaries.Comment: 6 Pages, 4 Figures, Accepted for MNRAS Letter
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
Does the momentum flux generated by gravitational contraction drive AGB mass-loss?
Gravitational contraction always generates a radially directed momentum flux.
A particularly simple example occurs in the electron-degenerate cores of AGB
stars, which contract steadily under the addition of helium ashes from shell
hydrogen burning. The resulting momentum flux is quantified here. And since the
cores of AGB stars lack efficient momentum cancellation mechanisms, they can
maintain equilibrium by exporting their excess momentum flux to the stellar
envelope, which disposes of much of it in a low velocity wind. Gravitational
contraction easily accounts for the momentum flux in the solar wind, as well as
the flux required to lift mass into the dust formation zone of every AGB star,
whereon radiation pressure continues its ejection as a low velocity wind. This
mechanism explains the dependence of the AGB mass-loss rate on core mass; its
generalization to objects with angular momentum and/or strong magnetic fields
suggests a novel explanation of why most planetary nebulae and proto planetary
nebulae exhibit axial symmetry.
Quasistatic contraction is inherently biased to the generation of the maximum
possible momentum flux. Its formalism is therefore readily adapted to providing
an upper limit to the momentum flux needed to sustain mass loss when this
begins from a semi-continuous rather than impulsive process.Comment: 35 pages, including 1 fig and 2 tables, to appear in Astrophysical
Journal -- ps documen
Characterization of the Benchmark Binary NLTT 33370
We report the confirmation of the binary nature of the nearby, very low-mass
system NLTT 33370 with adaptive optics imaging and present resolved
near-infrared photometry and integrated light optical and near-infrared
spectroscopy to characterize the system. VLT-NaCo and LBTI-LMIRCam images show
significant orbital motion between 2013 February and 2013 April. Optical
spectra reveal weak, gravity sensitive alkali lines and strong lithium 6708
Angstrom absorption that indicate the system is younger than field age.
VLT-SINFONI near-IR spectra also show weak, gravity sensitive features and
spectral morphology that is consistent with other young, very low-mass dwarfs.
We combine the constraints from all age diagnostics to estimate a system age of
~30-200 Myr. The 1.2-4.7 micron spectral energy distribution of the components
point toward T_eff=3200 +/- 500 K and T_eff=3100 +/- 500 K for NLTT 33370 A and
B, respectively. The observed spectra, derived temperatures, and estimated age
combine to constrain the component spectral types to the range M6-M8.
Evolutionary models predict masses of 113 +/- 8 M_Jup and 106 +/- 7 M_Jup from
the estimated luminosities of the components. KPNO-Phoenix spectra allow us to
estimate the systemic radial velocity of the binary. The Galactic kinematics of
NLTT 33370AB are broadly consistent with other young stars in the Solar
neighborhood. However, definitive membership in a young, kinematic group cannot
be assigned at this time and further follow-up observations are necessary to
fully constrain the system's kinematics. The proximity, age, and late-spectral
type of this binary make it very novel and an ideal target for rapid, complete
orbit determination. The system is one of only a few model calibration
benchmarks at young ages and very low-masses.Comment: 25 pages, 3 tables, 13 figures, accepted for publication in The
Astrophysical Journa
Near-Infrared Photometric Survey of Proto-Planetary Nebula Candidates
We present JHK' photometric measurements of 78 objects mostly consisting of
proto-planetary nebula candidates. Photometric magnitudes are determined by
means of imaging and aperture photometry. Unlike the observations with a
photometer with a fixed-sized beam, the method of imaging photometry permits
accurate derivation of photometric values because the target sources can be
correctly identified and confusion with neighboring sources can be easily
avoided. Of the 78 sources observed, we report 10 cases in which the source
seems to have been misidentified or confused by nearby bright sources. We also
present nearly two dozen cases in which the source seems to have indicated a
variability which prompts a follow-up monitoring. There are also a few sources
that show previously unreported extendedness. In addition, we present H band
finding charts of the target sources.Comment: 3 tables, 1 figur
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