72 research outputs found
HST Spectral Mapping of L/T Transition Brown Dwarfs Reveals Cloud Thickness Variations
Most directly imaged giant exoplanets are fainter than brown dwarfs with
similar spectra. To explain their relative underluminosity unusually cloudy
atmospheres have been proposed. However, with multiple parameters varying
between any two objects, it remained difficult to observationally test this
idea. We present a new method, sensitive time-resolved Hubble Space Telescope
near-infrared spectroscopy, to study two rotating L/T transition brown dwarfs
(2M2139 and SIMP0136). The observations provide spatially and spectrally
resolved mapping of the cloud decks of the brown dwarfs. The data allow the
study of cloud structure variations while other parameters are unchanged. We
find that both brown dwarfs display variations of identical nature: J- and
H-band brightness variations with minimal color and spectral changes. Our light
curve models show that even the simplest surface brightness distributions
require at least three elliptical spots. We show that for each source the
spectral changes can be reproduced with a linear combination of only two
different spectra, i.e. the entire surface is covered by two distinct types of
regions. Modeling the color changes and spectral variations together reveal
patchy cloud covers consisting of a spatially heterogenous mix of
low-brightness, low-temperature thick clouds and brighter, thin and warm
clouds. We show that the same thick cloud patches seen in our varying brown
dwarf targets, if extended to the entire photosphere, predict near-infrared
colors/magnitudes matching the range occupied by the directly imaged exoplanets
that are cooler and less luminous than brown dwarfs with similar spectral
types. This supports the models in which thick clouds are responsible for the
near infrared properties of these underluminous exoplanets.Comment: Astrophysical Journal, in pres
Discovery of a Visual T-Dwarf Triple System and Binarity at the L/T Transition
We present new high contrast imaging of 8 L/T transition brown dwarfs using
the NIRC2 camera on the Keck II telescope. One of our targets, the T3.5 dwarf
2MASS J08381155 + 1511155, was resolved into a hierarchal triple with projected
separations of 2.5+/-0.5 AU and 27+/-5 AU for the BC and A(BC) components
respectively. Resolved OSIRIS spectroscopy of the A(BC) components confirm that
all system members are T dwarfs. The system therefore constitutes the first
triple T-dwarf system ever reported. Using resolved photometry to model the
integrated-light spectrum, we infer spectral types of T3, T3, and T4.5 for the
A, B, and C components respectively. The uniformly brighter primary has a bluer
J-Ks color than the next faintest component, which may reflect a sensitive
dependence of the L/T transition temperature on gravity, or alternatively
divergent cloud properties amongst components. Relying on empirical trends and
evolutionary models we infer a total system mass of 0.034-0.104 Msun for the BC
components at ages of 0.3-3 Gyr, which would imply a period of 12-21 yr
assuming the system semi-major axis to be similar to its projection. We also
infer differences in effective temperatures and surface gravities between
components of no more than ~150 K and ~0.1 dex. Given the similar physical
properties of the components, the 2M0838+15 system provides a controlled sample
for constraining the relative roles of effective temperature, surface gravity,
and dust clouds in the poorly understood L/T transition regime. Combining our
imaging survey results with previous work we find an observed binary fraction
of 4/18 or 22_{-8}^{+10}% for unresolved spectral types of L9-T4 at separations
>~0.1 arcsec. This translates into a volume-corrected frequency of
13^{-6}_{+7}%, which is similar to values of ~9-12% reported outside the
transition. (ABRIDGED)Comment: Accepted for publication in the Astrophysical Journal. 23 pages, 12
figure
Searching for Exosatellites Orbiting L and T Dwarfs: Connecting Planet Formation to Moon Formation and Finding New Temperate Worlds
L-type and T-type dwarfs span the boundaries between main-sequence stars,
brown dwarfs, and planetary-mass objects. For these reasons, L and T dwarfs are
the perfect laboratories for exploring the relationship between planet
formation and moon formation, and evidence suggests they may be swarming with
close-in rocky satellites, though none have been found to date. The discovery
of satellites orbiting L or T dwarfs will have transformative implications for
the nature of planets, moons and even life in the Universe. These transiting
satellites will be prime targets for characterization with NASA's James Webb
Space Telescope. In this white paper, we discuss the scientific motivations
behind searching for transiting satellites orbiting L and T dwarfs and argue
that robotizing current 1-to-2-meter US optical/infrared (O/IR) facilities and
equipping them with recently developed low-cost infrared imagers will enable
these discoveries in the next decade. Furthermore, robotizing the 1-to-2-meter
O/IR fleet is highly synergistic with rapid follow-up of transient and
multi-messenger events.Comment: Science white paper submitted to the Astro 2020 Decadal Survey on
Astronomy and Astrophysic
A Monitoring Campaign for Luhman 16AB. I. Detection of Resolved Near-Infrared Spectroscopic Variability
[abbreviated] We report resolved near-infrared spectroscopic monitoring of
the nearby L dwarf/T dwarf binary WISE J104915.57-531906.1AB (Luhman 16AB), as
part of a broader campaign to characterize the spectral energy distribution and
temporal variability of this system. A continuous 45-minute sequence of
low-resolution IRTF/SpeX data spanning 0.8-2.4 micron were obtained, concurrent
with combined-light optical photometry with ESO/TRAPPIST. Our spectral
observations confirm the flux reversal of this binary, and we detect a
wavelength-dependent decline in the relative spectral fluxes of the two
components coincident with a decline in the combined-light optical brightness
of the system over the course of the observation. These data are successfully
modeled as a combination of brightness and color variability in the T0.5 Luhman
16B, consistent cloud variations; and no significant variability in L7.5 Luhman
16A. We estimate a peak-to-peak amplitude of 13.5% at 1.25 micron over the full
lightcurve. Using a two-spot brightness temperature model, we infer an average
cloud covering fraction of ~30-55% for Luhman 16B, varying by 15-30% over a
rotation period. A Rhines scale interpretation for the size of the variable
features explains an apparent correlation between period and amplitude for
three highly variable T dwarfs, and predicts relatively fast winds (1-3 km/s)
for Luhman 16B consistent with lightcurve evolution on an advective time scale
(1-3 rotation periods). Our observations support the model of a patchy
disruption of the mineral cloud layer as a universal feature of the L dwarf/T
dwarf transition.Comment: 11 pages, 7 figures; accepted for publication in Astrophysical
Journa
Weather on Other Worlds. II. Survey Results: Spots Are Ubiquitous on L and T Dwarfs
We present results from the "Weather on Other Worlds" Spitzer Exploration
Science program to investigate photometric variability in L and T dwarfs,
usually attributed to patchy clouds. We surveyed 44 L3-T8 dwarfs, spanning a
range of colors and surface gravities. We find that 14/23 (61%; 95%
confidence interval: 41%-78%) of our single L3-L9.5 dwarfs are variable with
peak-to-peak amplitudes between 0.2% and 1.5%, and 5/16 (31%; 95% confidence
interval: 14%-56%) of our single T0-T8 dwarfs are variable with amplitudes
between 0.8% and 4.6%. After correcting for sensitivity, we find that 80% (95%
confidence interval: 53%-100%) of L dwarfs vary by >0.2%, and 36% (95%
confidence interval: 19%-52%) of T dwarfs vary by >0.4%. Given viewing geometry
considerations, we conclude that photospheric heterogeneities causing >0.2%
3-5-micron flux variations are present on virtually all L dwarfs, and probably
on most T dwarfs. A third of L dwarf variables show irregular light curves,
indicating that L dwarfs may have multiple spots that evolve over a single
rotation. Also, approximately a third of the periodicities are on time scales
>10 h, suggesting that slowly-rotating brown dwarfs may be common. We observe
an increase in the maximum amplitudes over the entire spectral type range,
revealing a potential for greater temperature contrasts in T dwarfs than in L
dwarfs. We find a tentative association (92% confidence) between low surface
gravity and high-amplitude variability among L3-L5.5 dwarfs. Although we can
not confirm whether lower gravity is also correlated with a higher incidence of
variables, the result is promising for the characterization of directly imaged
young extrasolar planets through variability.Comment: 42 pages, 11 figures, 2 tables, accepted by Ap
Discovery of a Wide Substellar Companion to a Nearby Low-Mass Star
We report the discovery of a wide (135+/-25 AU), unusually blue L5 companion
2MASS J17114559+4028578 to the nearby M4.5 dwarf G 203-50 as a result of a
targeted search for common proper motion pairs in the Sloan Digital Sky Survey
and the Two Micron All Sky Survey. Adaptive Optics imaging with Subaru
indicates that neither component is a nearly equal mass binary with separation
> 0.18", and places limits on the existence of additional faint companions. An
examination of TiO and CaH features in the primary's spectrum is consistent
with solar metallicity and provides no evidence that G 203-50 is metal poor. We
estimate an age for the primary of 1-5 Gyr based on activity. Assuming
coevality of the companion, its age, gravity and metallicity can be constrained
from properties of the primary, making it a suitable benchmark object for the
calibration of evolutionary models and for determining the atmospheric
properties of peculiar blue L dwarfs. The low total mass (M_tot=0.21+/-0.03
M_sun), intermediate mass ratio (q=0.45+/-0.14), and wide separation of this
system demonstrate that the star formation process is capable of forming wide,
weakly bound binary systems with low mass and BD components. Based on the
sensitivity of our search we find that no more than 2.2% of early-to-mid M
dwarfs (9.0 0.06 M_sun.Comment: 24 pages, 5 figures, accepted for publication in Ap
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