790 research outputs found
Discovery of very nearby ultracool dwarfs from DENIS
We report new spectroscopic results, obtained with UKIRT/CGS4, of a sample of
14 candidate ultracool dwarfs selected from the DENIS (Deep Near-Infrared
Survey of the Southern Sky) database. A further object, selected from the 2MASS
Second Incremental Release, was observed at a later epoch with the same
instrument. Six objects are already known in the literature; we re-derive their
properties. A further four prove to be very nearby (~10 pc) mid-to-late
L-dwarfs, three unknown hitherto, two of which are almost certainly substellar.
These findings increase the number of L-dwarfs known within ~10 pc by ~25%. The
remainder of the objects discussed here are early L or very late M-type dwarfs
lying between ~45 and 15 pc and are also new to the literature. Spectral types
have been derived by direct comparison with J-,H- and K- band spectra of known
template ultracool dwarfs given by Leggett et al.
(ftp://ftp.jach.hawaii.edu/pub/ukirt/skl/dL.spectra/) For the known objects, we
generally find agreement to within ~1 subclass with previously derived spectral
types. Distances are determined from the most recent M_J vs. spectral type
calibrations, and together with our derived proper motions yield kinematics for
most targets consistent with that expected for the disk population; for three
probable late M-dwarfs, membership of a dynamically older population is
postulated. The very nearby L-type objects discussed here are of great interest
for future studies of binarity and parallaxes.Comment: 4 pages, 2 figures, accepted in A&A Letter
A possible third component in the L dwarf binary system DENIS-P J020529.0-115925 discovered with the Hubble Space Telescope
We present results showing that the multiple system DENIS-P J020529.0-115925
is likely to be a triple system of brown dwarfs. The secondary of this
previously known binary system shows a clear elongation on six images obtained
at six different epochs. Significant residuals remain after PSF subtraction on
these images, characteristic of multiplicity, and indicating that the secondary
is probably a double itself. Dual-PSF fitting shows that the shape of the
secondary is consistent with that of a binary system. These measurements show
that the probability that DENIS-P J020529.0-115925 is a triple system is very
high. The photometric and spectroscopic properties of the system are consistent
with the presence of three components with spectral types L5, L8 and T0.Comment: 15 pages, 3 tables, 6 figures, accepted for publication in AJ. High
resolution version available at
ftp://ftp.mpe.mpg.de/people/hbouy/publications/denis0205.ps.g
Metallicities of M Dwarf Planet Hosts from Spectral Synthesis
We present the first spectroscopic metallicities of three M dwarfs with known
or candidate planetary mass companions. We have analyzed high resolution, high
signal-to-noise spectra of these stars which we obtained at McDonald
Observatory. Our analysis technique is based on spectral synthesis of atomic
and molecular features using recently revised cool-star model atmospheres and
spectrum synthesis code. The technique has been shown to yield results
consistent with the analyses of solar-type stars and allows measurements of M
dwarf [M/H] values to 0.12 dex precision. From our analysis, we find [M/H] =
-0.12, -0.32, and -0.33 for GJ 876, GJ 436, and GJ 581 respectively. These
three M dwarf planet hosts have sub-solar metallicities, a surprising departure
from the trend observed in FGK-type stars. This study is the first part of our
ongoing work to determine the metallicities of the M dwarfs included in the
McDonald Observatory planet search program.Comment: 13 pages, 2 figures, accepted for publication in ApJ
The HARPS search for southern extra-solar planets. VI. A Neptune-mass planet around the nearby M dwarf Gl 581
We report the discovery of a Neptune-mass planet around Gl 581 (M3V, M = 0.31
Msol), based on precise Doppler measurements with the HARPS spectrograph at La
Silla Observatory. The radial velocities reveal a circular orbit of period P =
5.366 days and semi-amplitude K1 = 13.2 m/s. The resulting minimum mass of the
planet (m2 sin i) is only 0.052 Mjup = 0.97 Mnep = 16.6 Mearth making Gl 581b
one of the lightest extra-solar planet known to date. The Gl 581 planetary
system is only the third centered on an M dwarf, joining the Gl 876
three-planet system and the lone planet around Gl 436. Its discovery reinforces
the emerging tendency of such planets to be of low mass, and found at short
orbital periods. The statistical properties of the planets orbiting M dwarfs do
not seem to match a simple mass scaling of their counterparts around solar-type
stars.Comment: letter submitted to A&
Multiplicity of Nearby Free-floating Ultra-cool Dwarfs: a HST-WFPC2 search for companions
We present HST/WFPC2 observations of a sample of 134 ultra-cool objects
(spectral types later than M7) coming from the DENIS, 2MASS and SDSS surveys,
with distances estimated to range from 7 pc to 105 pc. Fifteen new ultra-cool
binary candidates are reported here. Eleven known binaries are confirmed and
orbital motion is detected in some of them. We estimate that the closest binary
systems in this sample have periods between 5 and 20 years, and thus dynamical
masses will be derived in the near future. For the calculation of binary
frequency we restrict ourselves to systems with distances less than 20 pc.
After correction of the binaries bias, we find a ratio of visual binaries (at
the HST limit of detection) of around 10%, and that ~15% of the 26 objects
within 20 parsecs are binary systems with separations between 1 and 8 A.U. The
observed frequency of ultra-cool binaries is similar than that of binaries with
G-type primaries in the separation range from 2.1 A.U. to 140 A.U. There is
also a clear deficit of ultra-cool binaries with separations greater than 15
A.U., and a possible tendency for the binaries to have mass ratios near unity.
Most systems have indeed visual and near-infrared brightness ratios between 1
and 0.3. We discuss our results in the framework of current scenarios for the
formation and evolution of free-floating brown dwarfs.Comment: 67 pages, 14 figures, Accepted for publication in AJ, September 2003.
First submission to AJ: august 2002, 5 submission
Spectroscopic identification of DENIS-selected brown dwarf candidates in the Upper Scorpius OB association
We present low-resolution (R=900) optical (576.1--1,051.1 nm) spectroscopic
observations of 40 candidate very low-mass members in the Upper Scorpius OB
association. These objects were selected using the , and photometry
available in the DENIS database. We have derived spectral types and we have
measured H and NaI doublet (at 818.3 and 819.5 nm) equivalent widths.
We assess the youth of the objects by comparing them to their older
counterparts of similar spectral type in the Pleiades cluster and the field.
Our analysis indicates that 28 of our targets are young very low-mass objects,
and thus they are strong candidate members of the OB association. The other 12
DENIS sources are foreground M dwarfs or background red giants. Our sample of
spectroscopic candidate members includes 18 objects with spectral types in the
range M6.5 and M9, which are likely young brown dwarfs. We classify these
candidates as accreting/non accreting using the scheme proposed by Barrado y
Navascu\'es & Mart\'\i n (2003). We find 5 substellar-mass candidate cluster
members that are still undergoing mass accretion, indicating that the timescale
for accretion onto brown dwarfs can be as long as 5 Myr in some cases.Comment: Accepted for publication in The Astronomical Journal, January 200
Keck Imaging of Binary L Dwarfs
We present Keck near-infrared imaging of three binary L dwarf systems, all of
which are likely to be sub-stellar. Two are lithium dwarfs, and a third
exhibits an L7 spectral type, making it the coolest binary known to date. All
have component flux ratios near 1 and projected physical separations between 5
and 10 AU, assuming distances of 18 to 26 pc from recent measurements of
trigonometric parallax. These surprisingly similar binaries represent the sole
detections of companions in ten L dwarf systems which were analyzed in the
preliminary phase of a much larger dual-epoch imaging survey. The detection
rate prompts us to speculate that binary companions to L dwarfs are common,
that similar-mass systems predominate, and that their distribution peaks at
radial distances in accord both with M dwarf binaries and with the radial
location of Jovian planets in our own solar system. To fully establish these
conjectures against doubts raised by biases inherent in this small preliminary
survey, however, will require quantitative analysis of a larger volume-limited
sample which has been observed with high resolution and dynamic range.Comment: LaTex manuscript in 13 pages, 3 postscript figures, Accepted for
publication in the Letters of the Astrophysical Journal; Postscript pre-print
version available at: http://www.hep.upenn.edu/PORG/papers/koerner99a.p
An L0 dwarf companion in the brown dwarf desert, at 30 AU
We present the discovery of an L0 companion to the nearby M1.5 dwarf G
239-25, at a projected distance of 31 AU. It is the faintest companion
discovered so far in our adaptive optics survey of all known M dwarfs within 12
pc, and it lies at the stellar/substellar limit. Given the assumed age of the
primary star, the companion is likely an extremely low mass star. The long
orbital period of G 239-25 AB ( years) precludes a direct mass
determination, but the relatively wide angular separation will allow detailed
analyses of its near infrared and visible spectra.Comment: accepted by AA Letter
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