237 research outputs found
Discovery of a Second L Subdwarf in the Two Micron All Sky Survey
I report the discovery of the second L subdwarf identified in the Two Micron
All Sky Survey, 2MASS J16262034+3925190. This high proper motion object (mu =
1.27+/-0.03 "/yr) exhibits near-infrared spectral features indicative of a
subsolar metallicity L dwarf, including strong metal hydride and H2O absorption
bands, pressure-broadened alkali lines, and blue near-infrared colors caused by
enhanced collision-induced H2 absorption. This object is of later type than any
of the known M subdwarfs, but does not appear to be as cool as the apparently
late-type sdL 2MASS 0532+8246. The radial velocity (Vrad = -260+/-35 km/s) and
estimated tangential velocity (Vtan ~ 90-210 km/s) of 2MASS 1626+3925 indicate
membership in the Galactic halo, and this source is likely near or below the
hydrogen burning minimum mass for a metal-poor star. L subdwarfs such as 2MASS
1626+3925 are useful probes of gas and condensate chemistry in low-temperature
stellar and brown dwarf atmospheres, but more examples are needed to study
these objects as a population as well as to define a rigorous classification
scheme.Comment: 11 pages, 3 figures, accepted for publication ApJ Letters, v. 614
October 200
The Schizophrenic Spectrum of LSR 1610-0040: a Peculiar M Dwarf/Subdwarf
We present a moderate resolution (R=2000), 0.8-4.1 micron spectrum of LSR
1610-0040, a high proper motion star classified as an early-type L subdwarf by
Lepine and collaborators based on its red-optical spectrum. The near-infrared
spectrum of LSR 1610-0040 does not fit into the (tentative) M/L subdwarf
sequence but rather exhibits a mix of characteristics found in the spectra of
both M dwarfs and M subdwarfs. In particular, the near-infrared spectrum
exhibits a Na I doublet and CO overtone bandheads in the K band, and Al I and K
I lines and an FeH bandhead in the H band, all of which have strengths more
typical of field M dwarfs. Furthermore the spectrum of Gl 406 (M6 V) provides a
reasonably good match to the 0.6-4.1 micron spectral energy distribution of LSR
1610. Nevertheless the near-infrared spectrum of LSR 1610 also exhibits
features common to the spectra of M subdwarfs including a strong Ti I multiplet
centered at ~0.97 microns, a weak VO band at ~1.06 microns, and possible
collision-induced H_2 absorption in the H and K bands. We discuss a number of
possible explanations for the appearance of the red-optical and near-infrared
spectrum of LSR 1610-0040. Although we are unable to definitively classify LSR
1610-0040, the preponderance of evidence suggests that it is a mildly
metal-poor M dwarf. Finally, we tentatively identify a new band of TiO at ~0.93
microns in the spectra of M dwarfs.Comment: Accepted for publication in the Astronomical Journa
The Inner Galaxy resolved at IJK using DENIS data
We present the analysis of three colour optical/near-infrared images, in IJK,
taken for the DENIS project. The region considered covers 17.4 square deg and
lies within |l|<5 deg, |b|<1.5 deg. The adopted methods for deriving photometry
and astrometry in these crowded images, together with an analysis of the
deficiencies nevertheless remaining, are presented. The numbers of objects
extracted in I,J and K are 748000, 851000 and 659000 respectively, to magnitude
limits of 17,15 and 13. 80% completeness levels typically fall at magnitudes
16, 13 and 10 respectively, fainter by about 2 magnitudes than the usual DENIS
limits due to the crowded nature of these fields. A simple model to describe
the disk contribution to the number counts is constructed, and parameters for
the dust layer derived. We find that a formal fit of parameters for the dust
plane, from these data in limited directions, gives a scalelength and
scaleheight of 3.4+-1.0 kpc and 40+-5 pc respectively, and a solar position
14.0+-2.5 pc below the plane. This latter value is likely to be affected by
localised dust asymmetries. We convolve a detailed model of the systematic and
random errors in the photometry with a simple model of the Galactic disk and
dust distribution, to simulate expected colour-magnitude diagrams. These are in
good agreement with the observed diagrams, allowing us to isolate those stars
from the inner disk and bulge. After correcting for local dust-induced
asymmetries, we find evidence for longitude-dependent asymmetries in the
distant J and K sources, consistent with the general predictions of some
Galactic bar models. We consider complementary L-band observations in a second
paper.Comment: 14 pages, 33 figures, LaTeX, MNRAS accepte
The 2MASS Wide-Field T Dwarf Search. II. Discovery of Three T Dwarfs in the Southern Hemisphere
We present the discovery of three new Southern Hemisphere T dwarfs identified
in the Two Micron All Sky Survey. These objects, 2MASS 0348-6022, 2MASS
0516-0445, and 2MASS 2228-4310, have classifications T7, T5.5, and T6.5,
respectively. Using linear absolute magnitude/spectral type relations derived
from T dwarfs with measured parallaxes, we estimate spectrophotometric
distances for these discoveries; the closest, 2MASS 0348-6022, is likely within
10 pc of the Sun. Proper motions and estimated tangential velocities are
consistent with membership in the Galactic disk population. We also list
Southern Hemisphere T dwarf candidates that were either not found in subsequent
near-infrared imaging observations and are most likely uncatalogued minor
planets, or have near-infrared spectra consistent with background stars.Comment: 12 pages including 4 figures (one as jpeg), accepted to A
Photometric Properties of Long-period Variables in the Large Magellanic Cloud
Approximately four thousand light curves of red variable stars in the LMC
were selected from the 2.3-years duration MOA database by a period analysis
using the Phase Dispersion Minimization method. Their optical features
(amplitudes, periodicities, position in CMD) were investigated. Stars with
large amplitues and high periodicities were distributed on the only one strip
amongst multiple structure on the LMC period-luminosity relation. In the CMD,
the five strips were located in the order of the period. The stars with
characterized light curves were also discussed.Comment: 8 pages, 5 figures, Proceeding of WS on Mass-Losing Pulsating Stars
and Their Circumstellar Matter, Sendai, Japa
Star Formation via the Little Guy: A Bayesian Study of Ultracool Dwarf Imaging Surveys for Companions
I have undertaken a comprehensive statistical investigation of the ultracool
dwarf companion distribution (spectral type M6 and later). Utilizing a Bayesian
algorithm, I tested models of the companion distribution against data from an
extensive set of space and ground-based imaging observations of nearby
ultracool dwarfs. My main conclusions are fivefold: 1) Confirm that the
concentration of high mass ratio ultracool binary systems is a fundamental
feature of the companion distribution, not an observational or selection bias;
2) Determine that the wide (>~20 AU) binary frequency can be no more the 1-2%;
3) Show that the decreasing binary frequency with later spectral types is a
real trend; 4) Demonstrate that a large population of currently undetected low
mass ratio systems are not consistent with the current data; 5) Find that the
population of spectroscopic binaries must be be at least 30% that of currently
known ultracool binaries. The best fit value for the overall M6 and later
binary frequency is ~20%-22%, of which only ~6% consists of currently
undetected companions with separations less than 1 AU. If this is correct, then
the upper limit of the ultracool binary population discovered to date is ~75%.
I find that the numerical simulation results of the ejection formation method
are inconsistent with the outcome of this analysis. However, dynamics do seem
to play an important role as simulations of small-N clusters and triple system
decays produce results similar to those of this work. The observational efforts
required to improve these constraints are shown to be primarily large
spectroscopic binary surveys and improved high-resolution imaging techniques.Comment: 42 pages, 17 figures, to be published in October 2007 Ap
Discovery of an M9.5 Candidate Brown Dwarf in the TW Hydrae Association - DENIS J124514.1-442907
We report the discovery of a fifth candidate substellar system in the ~5-10
Myr TW Hydrae Association - DENIS J124514.1-442907. This object has a NIR
spectrum remarkably similar to that of 2MASS J1139511-315921, a known TW Hydrae
brown dwarf, with low surface gravity features such as a triangular-shaped
H-band, deep H2O absorption, weak alkali lines, and weak hydride bands. We find
an optical spectral type of M9.5 and estimate a mass of <24 M_Jup, assuming an
age of ~5-10 Myr. While the measured proper motion for DENIS J124514.1-442907
is inconclusive as a test for membership, its position in the sky is coincident
with the TW Hydrae Association. A more accurate proper motion measurement,
higher resolution spectroscopy for radial velocity, and a parallax measurement
are needed to derive the true space motion and to confirm its membership.Comment: 8 pages - emulateapj style, 2 figures, 3 tables. Accepted to ApJL.
Fixed typos, added reference, added footnot
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