303 research outputs found
A New Brown Dwarf Desert? A Scarcity of Wide Ultracool Binaries
We present the results of a deep-imaging search for wide companions to
low-mass stars and brown dwarfs using NSFCam on IRTF. We searched a sample of
132 M7-L8 dwarfs to magnitude limits of and ,
corresponding to secondary-primary mass ratios of . No companions
were found with separations between 2{\arcsec} to 31{\arcsec} (40 AU
to 1000 AU). This null result implies a wide companion frequency below
2.3% at the 95% confidence level within the sensitivity limits of the survey.
Preliminary modeling efforts indicate that we could have detected 85% of
companions more massive than and 50% above .Comment: 27 pages, 8 figures, 3 tables: accepted to the Astronomical Journa
Discovery of a 66 mas Ultracool Binary with Laser Guide Star Adaptive Optics
We present the discovery of 2MASS J21321145+1341584AB as a closely separated
(0.066") very low-mass field dwarf binary resolved in the near-infrared by the
Keck II Telescope using laser guide star adaptive optics. Physical association
is deduced from the angular proximity of the components and constraints on
their common proper motion. We have obtained a near-infrared spectrum of the
binary and find that it is best described by an L5+/-0.5 primary and an
L7.5+/-0.5 secondary. Model-dependent masses predict that the two components
straddle the hydrogen burning limit threshold with the primary likely stellar
and the secondary likely substellar. The properties of this sytem - close
projected separation (1.8+/-0.3 AU) and near unity mass ratio - are consistent
with previous results for very low-mass field binaries. The relatively short
estimated orbital period of this system (~7-12 yr) makes it a good target for
dynamical mass measurements. Interestingly, the system's angular separation is
the tightest yet for any very low-mass binary published from a ground-based
telescope and is the tightest binary discovered with laser guide star adaptive
optics to date.Comment: 10 pages, 3 figures; accepted for publication to A
Lessons Learned for Cx PRACA. Constellation Program Problem Reporting, Analysis and Corrective Action Process and System
This slide presentation reviews the Constellation Program Problem Reporting, Analysis and Corrective Action Process and System (Cx PRACA). The goal of the Cx PRACA is to incorporate Lessons learned from the Shuttle, ISS, and Orbiter programs by creating a single tool for managing the PRACA process, that clearly defines the scope of PRACA applicability and what must be reported, and defines the ownership and responsibility for managing the PRACA process including disposition approval authority. CxP PRACA is a process, supported by a single information gathering data module which will be integrated with a single CxP Information System, providing interoperability, import and export capability making the CxP PRACA a more effective and user friendly technical and management tool
2MASS J05185995-2828372: Discovery of an Unresolved L/T Binary
We present the peculiar near-infrared spectrum of the newly discovered brown
dwarf 2MASS J05185995-2828372, identified in the Two Micron All-Sky Survey.
Features characteristic of both L and T dwarfs are present, namely strong
carbon monoxide absorption in K-band, strong methane absorption in J- and
H-bands, and red near-infrared colors. We consider several scenarios that could
produce these features and conclude that the object is most likely to be an
unresolved L/T binary system. We discuss how the estimated photometric
properties of this object are consistent with the observed J-band brightening
of brown dwarfs between late-L and early-T dwarfs, making detailed study of
this system an important probe of the L/T transition.Comment: 4 pages, 4 figures (2 color), accepted to ApJ Letter
Hubble Space Telescope NICMOS Observations of T Dwarfs: Brown Dwarf Multiplicity and New Probes of the L/T Transition
We present the results of a Hubble Space Telescope NICMOS imaging survey of
22 T-type field brown dwarfs. Five are resolved as binary systems with angular
separations of 0"05-0"35, and companionship is established on the basis of
component F110W-F170M colors (indicative of CH4 absorption) and low
probabilities of background contamination. Prior ground-based observations show
2MASS 1553+1532AB to be a common proper motion binary. The properties of these
systems - low multiplicity fraction (11[+7][-3]% resolved, as corrected for
sample selection baises), close projected separations (a = 1.8-5.0 AU) and
near-unity mass ratios - are consistent with previous results for field brown
dwarf binaries. Three of the binaries have components that span the
poorly-understood transition between L dwarfs and T dwarfs. Spectral
decomposition analysis of one of these, SDSS 1021-0304AB, reveals a peculiar
flux reversal between its components, as its T5 secondary is ~30% brighter at
1.05 and 1.27 micron than its T1 primary. This system, 2MASS 0518-2828AB and
SDSS 1534+1615AB all demonstrate that the J-band brightening observed between
late-type L to mid-type T dwarfs is an intrinsic feature of this spectral
transition, albeit less pronounced than previously surmised. We also find that
the resolved binary fraction of L7 to T3.5 dwarfs is twice that of other L and
T dwarfs, an anomaly that can be explained by a relatively rapid evolution of
brown dwarfs through the L/T transition, perhaps driven by dynamic
(nonequilibrium) depletion of photospheric condensates.Comment: ~40 pages, 17 figures, accepted for publication to ApJ. Note that
emulateapj style file cuts off part of Table
A search for binary systems among the nearest L dwarfs
We have used the NICMOS NIC1 camera on the Hubble Space Telescope to obtain
high angular resolution images of 51 ultracool dwarfs in the immediate Solar
Neighbourhood. Nine systems are resolved as binary, with component separations
from 1.5 and 15 AU. All of the systems have components with similar
luminosities, and, consequently, high mass ratios, q > 0.8. Limiting analysis
to L dwarfs within 20 parsecs, the observed binary fraction is 12(+7/-3).
Applying Bayesian analysis to our dataset, we derive a mass-ratio distribution
that peaks strongly at unity. Modelling the semi-major axis distribution as a
logarithmic Gaussian, the best fit is centered at log(a_0) = 0.8 AU (~6.3 AU),
with a (logarithmic) width of 0.3. The current data are consistent with an
overall binary frequency of ~24%.Comment: 29 pages, 4 tables, 8 figures; accepted for publication in A
A Sample of Very Young Field L Dwarfs and Implications for the Brown Dwarf "Lithium Test" at Early Ages
Using a large sample of optical spectra of late-type dwarfs, we identify a
subset of late-M through L field dwarfs that, because of the presence of
low-gravity features in their spectra, are believed to be unusually young. From
a combined sample of 303 field L dwarfs, we find observationally that
7.6+/-1.6% are younger than 100 Myr. This percentage is in agreement with
theoretical predictions once observing biases are taken into account. We find
that these young L dwarfs tend to fall in the southern hemisphere (Dec < 0 deg)
and may be previously unrecognized, low-mass members of nearby, young
associations like Tucana-Horologium, TW Hydrae, beta Pictoris, and AB Doradus.
We use a homogeneously observed sample of roughly one hundred and fifty
6300-10000 Angstrom spectra of L and T dwarfs taken with the Low-Resolution
Imaging Spectrometer at the W. M. Keck Observatory to examine the strength of
the 6708-A Li I line as a function of spectral type and further corroborate the
trends noted by Kirkpatrick et al. (2000). We use our low-gravity spectra to
investigate the strength of the Li I line as a function of age. The data weakly
suggest that for early- to mid-L dwarfs the line strength reaches a maximum for
a few 100 Myr, whereas for much older (few Gyr) and much younger (<100 Myr) L
dwarfs the line is weaker or undetectable. We show that a weakening of lithium
at lower gravities is predicted by model atmosphere calculations, an effect
partially corroborated by existing observational data. Larger samples
containing L dwarfs of well determined ages are needed to further test this
empirically. If verified, this result would reinforce the caveat first cited in
Kirkpatrick et al. (2006) that the lithium test should be used with caution
when attempting to confirm the substellar nature of the youngest brown dwarfs.Comment: 73 pages with 22 figures; to appear in ApJ (Dec 20, 2008, v689n2
issue
Discovery of Two Very Low-Mass Binaries: Final Results of an Adaptive Optics Survey of Nearby M6.0-M7.5 Stars
We present updated results of a high-resolution, magnitude limited (Ks<12
mag) imaging survey of nearby low-mass M6.0-M7.5 field stars. The observations
were carried out using adaptive optics at the Gemini North, VLT, Keck II, and
Subaru telescopes. Our sample of 36 stars consists predominantly of nearby (<30
pc) field stars, 5 of which we have resolved as binaries. Two of the binary
systems, 2MASSI J0429184-312356 and 2MASSI J1847034+552243, are presented here
for the first time. All 5 discovered binary systems have separations between
0.08"-0.53" (2-9 AU) with similar mass ratios (q>0.8, delta Ks < 1 mag). This
result supports the hypothesis that wide (a>20 AU) very low-mass (M_tot<0.19
M_sun) binary systems are rare. The projected semimajor axis distribution of
these systems peak at ~ 5 AU and we report a sensitivity-corrected binary
fraction of 9% (-3%, +4%) for stars with primaries of spectral type M6.0-M7.5
with separations >3 AU and mass ratios q>0.6. Within these instrumental
sensitivities, these results support the overall trend that both the semimajor
axis distribution and binary fraction are a function of the mass of the primary
star and decrease with decreasing primary mass. These observations provide
important constraints for low-mass binary star formation theories.Comment: 20 pages, 8 figures, accepted to Ap
Activity and Kinematics of Ultracool Dwarfs Including An Amazing Flare Observation
We present the activity and kinematics of a representative volume-limited (20
pc) sample of 152 late-M and L dwarfs (M7--L8) photometrically selected from
the Two Micron All-Sky Survey (2MASS). Using new proper motion measurements and
spectrophotometric distance estimates, we calculate tangential velocities. The
sample has a mean tangential velocity of = 31.5 km/s, a velocity
dispersion of sigma_tan = 20.7 km/s, and a maximum tangential velocity of V_tan
= 138.8 km/s. These kinematic results are in excellent agreement with previous
studies of ultracool dwarfs in the local solar neighborhood. H_alpha emission,
an indicator of chromospheric activity, was detected in 63 of 81 late-M dwarfs
and 16 of 69 L dwarfs examined. We find a lack of correlation between activity
strength, measured by log(F_H_alpha/F_bol), and V_tan, though velocity
distributions suggest that the active dwarfs in our sample are slightly younger
than the inactive dwarfs. Consistent with previous studies of activity in
ultracool dwarfs, we find that the fraction of H emitting objects per
spectral type peaks at spectral type M7 and declines through mid-L dwarfs.
Activity strength is similarly correlated with spectral type for spectral types
later than M7. Eleven dwarfs out of 150 show evidence of variability, ranging
from small fluctuations to large flare events. We estimate a flare cycle of ~5%
for late-M dwarfs and ~2% for L dwarfs. Observations of strong, variable
activity on the L1 dwarf 2MASS J10224821+5825453 and an amazing flare event on
the the M7 dwarf 2MASS J1028404143843 are discussed.Comment: 30 pages, 12 figures, accepted to A
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