342 research outputs found
A Spectroscopic Binary at the M/L Transition
We report the discovery of a single-lined spectroscopic binary with an Ultra
Cool Dwarf (UCD) primary with a spectral type between M8 and L0.5. This system
was discovered during the course of an ongoing survey to monitor L dwarfs for
radial velocity variations and is the first known small separation (a<1 AU)
spectroscopic binary among dwarfs at the M/L transition. Based on
radial-velocity measurements with a typical precision of 300 m/s we estimate
the orbital parameters of this system to be P=246.73+/-0.49 d, a1
sin(i)=0.159+/-0.003 AU, M2 sin(i)=0.2062 (M1+M2)^(2/3)+/-0.0034 M_{\sun}.
Assuming a primary mass of M1=0.08M_{\sun} (based on spectral type), we
estimate the secondary minimum mass to be M2 sin(i)=0.054 M_{\sun}. With future
photometric, spectroscopic, and interferometric observations it may be possible
to determine the dynamical masses of both components directly, making this
system one of the best characterized UCD binaries known.Comment: 11 pages, 2 figures. Accepted for publication in ApJ Letter
Seventy-One New L and T Dwarfs from the Sloan Digital Sky Survey
We present near-infrared observations of 71 newly discovered L and T dwarfs,
selected from imaging data of the Sloan Digital Sky Survey (SDSS) using the
i-dropout technique. Sixty-five of these dwarfs have been classified
spectroscopically according to the near-infrared L dwarf classification scheme
of Geballe et al. and the unified T dwarf classification scheme of Burgasser et
al. The spectral types of these dwarfs range from L3 to T7, and include the
latest types yet found in the SDSS. Six of the newly identified dwarfs are
classified as early- to mid-L dwarfs according to their photometric
near-infrared colors, and two others are classified photometrically as M
dwarfs. We also present new near-infrared spectra for five previously published
SDSS L and T dwarfs, and one L dwarf and one T dwarf discovered by Burgasser et
al. from the Two Micron All Sky Survey. The new SDSS sample includes 27 T
dwarfs and 30 dwarfs with spectral types spanning the complex L-T transition
(L7-T3). We continue to see a large (~0.5 mag) spread in J-H for L3 to T1
types, and a similar spread in H-K for all dwarfs later than L3. This color
dispersion is probably due to a range of grain sedimentation properties,
metallicity, and gravity. We also find L and T dwarfs with unusual colors and
spectral properties that may eventually help to disentangle these effects.Comment: accepted by AJ, 18 pages, 10 figures, 5 tables, emulateapj layou
Quiescent X-ray emission from the M9 dwarf LHS 2065
We investigate an archival XMM-Newton observation of LHS 2065, an ultracool
dwarf with spectral type M9. We clearly detect LHS 2065 at soft X-ray energies
in less than 1 h effective exposure time above the 3 sigma level with the PN
and MOS1 detector. No flare signatures are present and we attribute the X-ray
detection to quasi-quiescent activity. From the PN data we derive an X-ray
luminosity of L_x = 2.2 +/- 0.7 x 10^26 erg/s in the 0.3-0.8 keV band, the
corresponding activity level of log L_x/L_bol = -3.7 points to a rather active
star. Indications for minor variability and possible accompanying spectral
changes are present, however the short exposure time and poor data quality
prevents a more detailed analysis. LHS 2065 is one of the coolest and least
massive stars that emits X-rays at detectable levels in quasi-quiescence,
implying the existence of a corona.Comment: accepted by A&A, 3 pages, 2 figures (research note
Companions of Stars: From Other Stars to Brown Dwarfs to Planets: The Discovery of the First Methane Brown Dwarf
The discovery of the first methane brown dwarf provides a framework for
describing the important advances in both fundamental physics and astrophysics
that are due to the study of companions of stars. I present a few highlights of
the history of this subject along with details of the discovery of the brown
dwarf Gliese 229B. The nature of companions of stars is discussed with an
attempt to avoid biases induced by anthropocentric nomenclature. With the newer
types of remote reconnaissance of nearby stars and their systems of companions,
an exciting and perhaps even more profound set of contributions to science is
within reach in the near future. This includes an exploration of the diversity
of planets in the universe and perhaps soon the first solid evidence for
biological activity outside our Solar System.Comment: 31 pages, 13 figure
Color Gradients Detected in the HD 15115 Circumstellar Disk
We report HST/NICMOS coronagraphic images of the HD 15115 circumstellar disk
at 1.1\micron. We find a similar morphology to that seen in the visible and at
H band--an edge-on disk that is asymmetric in surface brightness. Several
aspects of the 1.1\micron data are different, highlighting the need for
multi-wavelength images of each circumstellar disk. We find a flattening to the
western surface brightness profile at 1.1\micron interior to 2\arcsec (90 AU)
and a warp in the western half of the disk. We measure the surface brightness
profiles of the two disk lobes and create a measure of the dust scattering
efficiency between 0.55-1.65\micron at 1\arcsec, 2\arcsec, and 3\arcsec. At
2\arcsec the western lobe has a neutral spectrum up to 1.1\micron and a strong
absorption or blue spectrum 1.1\micron, while a blue trend is seen in the
eastern lobe. At 1\arcsec the disk has a red F110W-H color in both lobes.Comment: 4 pages, 4 figures, emulateapj; accepted to ApJ
Ground-Based Coronagraphy with High Order Adaptive Optics
We summarize the theory of coronagraphic optics, and identify a dimensionless
fine-tuning parameter, F, which we use to describe the Lyot stop size in the
natural units of the coronagraphic optical train and the observing wavelength.
We then present simulations of coronagraphs matched to adaptive optics (AO)
systems on the Calypso 1.2m, Palomar Hale 5m and Gemini 8m telescopes under
various atmospheric conditions, and identify useful parameter ranges for AO
coronagraphy on these telescopes. Our simulations employ a tapered, high-pass
filter in spatial frequency space to mimic the action of adaptive wavefront
correction. We test the validity of this representation of AO correction by
comparing our simulations with recent K-band data from the 241-channel Palomar
Hale AO system and its dedicated PHARO science camera in coronagraphic mode.Comment: To appear in ApJ, May 2001 (28 pages, 10 figs
HST/ACS Images of the GG Tauri Circumbinary Disk
Hubble Space Telescope Advanced Camera for Surveys images of the young binary
GG Tauri and its circumbinary disk in V and I bandpasses were obtained in 2002
and are the most detailed of this system to date. The confirm features
previously seen in the disk including: a "gap" apparently caused by shadowing
from circumstellar material; an asymmetrical distribution of light about the
line of sight on the near edge of the disk; enhanced brightness along the near
edge of the disk due to forward scattering; and a compact reflection nebula
near the secondary star. New features are seen in the ACS images: two short
filaments along the disk; localized but strong variations in disk intensity
("gaplets"); and a "spur" or filament extending from the reflection nebulosity
near the secondary. The back side of the disk is detected in the V band for the
first time. The disk appears redder than the combined light from the stars,
which may be explained by a varied distribution of grain sizes. The brightness
asymmetries along the disk suggest that it is asymmetrically illuminated by the
stars due to extinction by nonuniform circumstellar material or the illuminated
surface of the disk is warped by tidal effects (or perhaps both). Localized,
time-dependent brightness variations in the disk are also seen.Comment: 28 pages, 7 figures, accepted for publication in the Astronomical
Journa
Physical and Spectral Characteristics of the T8 and Later-Type Dwarfs
We use new and published near-IR spectra, with synthetic spectra, to derive
physical properties of three of the latest-type T dwarfs. A new R~1700 spectrum
of the T7.5 dwarf HD 3651B, with existing data, allows a detailed comparison to
the well-studied and very similar dwarf, Gl 570D. We find that HD 3651B has
both higher gravity and metallicity than Gl 570D, with Teff=820-830K, log g=
5.4-5.5, [m/H]= +0.2 and Kzz=10^4cm^2/s. Its age is 8-12 Gyr and its implied
mass is 60-70 M_Jup. We perform a similar analyis of the T8 and T7.5 dwarfs
2MASS J09393548-2448279 and 2MASS J11145133-2618235 using published data,
comparing them to the well-studied T8, 2MASS J04151954-0935066. We find that
the two dwarfs have the same Teff as the reference dwarf, and similar
gravities, but lower metallicities. The parameters are Teff=725-775K and [m/H]=
-0.3; log g=5.3-5.45 for 2MASS J09393548-2448279 and log g=5.0-5.3 for 2MASS
J11145133- 261823. The age and mass are ~10Gyr and 60M_Jup for 2MASS
J09393548-2448279, and ~5 Gyr and 40M_Jup for 2MASS J11145133-261823. A serious
limitation is the incompleteness of the line lists of CH4 and NH3 at lambda
<1.7um. Spectra of Saturn and Jupiter, and of laboratory CH4 and NH3 gas,
suggest that NH3 features in the Y- and J-bands may be useful as indicators of
the next cooler spectral type, and not features in the H- and K-bands as
previously thought. However large uncertainties remain, as the abundance of NH3
is likely to be significantly below the chemical equilibrium value, and
inclusion of laboratory NH3 opacities predicts band shapes that are discrepant
with existing data. It is possible that the T spectral class will have to be
extended to low temperatures around 400K, when water clouds condense in the
atmosphere [abridged].Comment: 34 pages including 10 figures and two tables; accepted for
publication in the Astrophysical Journa
Hubble and Spitzer Space Telescope Observations of the Debris Disk around the Nearby K Dwarf HD 92945
[ABRIDGED] We present the first resolved images of the debris disk around the
nearby K dwarf HD 92945. Our F606W (V) and F814W (I) HST/ACS coronagraphic
images reveal an inclined, axisymmetric disk consisting of an inner ring
2".0-3".0 (43-65 AU) from the star and an extended outer disk whose surface
brightness declines slowly with increasing radius 3".0-5".1 (65-110 AU) from
the star. A precipitous drop in the surface brightness beyond 110 AU suggests
that the outer disk is truncated at that distance. The radial surface-density
profile is peaked at both the inner ring and the outer edge of the disk. The
dust in the outer disk scatters neutrally but isotropically, and it has a low
V-band albedo of 0.1. We also present new Spitzer MIPS photometry and IRS
spectra of HD 92945. These data reveal no infrared excess from the disk
shortward of 30 micron and constrain the width of the 70 micron source to < 180
AU. Assuming the dust comprises compact grains of astronomical silicate with a
surface-density profile described by our scattered-light model of the disk, we
successfully model the 24-350 micron emission with a minimum grain size of
a_min = 4.5 micron and a size distribution proportional to a^-3.7 throughout
the disk, but with a maximum grain size of 900 micron in the inner ring and 50
micron in the outer disk. Our observations indicate a total dust mass of ~0.001
M_earth. However, they provide contradictory evidence of the dust's physical
characteristics: its neutral V-I color and lack of 24 micron emission imply
grains larger than a few microns, but its isotropic scattering and low albedo
suggest a large population of submicron-sized grains. The dynamical causes of
the disk's morphology are unclear, but recent models of dust creation and
transport in the presence of migrating planets indicate an advanced state of
planet formation around HD 92945.Comment: 29 pages, 10 figures; to be published in The Astronomical Journa
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