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 J − K s colors and surface gravities. We find that 14/23 (61% +17% −20% , 95% confidence) of our single L3-L9.5 dwarfs are variable with peak-to-peak amplitudes between 0.2% and 1.5%, and 5/16 (31% +25% −17% ) of our single T0-T8 dwarfs are variable with amplitudes between 0.8% and 4.6%. After correcting for sensitivity, we find that 80% +20% −27% of L dwarfs vary by 0.2%, and 36% +26% −17% of T dwarfs vary by 0.4%. Given viewing geometry considerations, we conclude that photospheric heterogeneities causing >0.2% 3-5 μm 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 timescales >10 hr, 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.

Directly detecting thermal emission from young extrasolar planets allows measurement of their atmospheric compositions and luminosities, which are influenced by their formation mechanisms. Using the Gemini Planet Imager, we discovered a planet orbiting the ~20-million-year-old star 51 Eridani at a projected separation of 13 astronomical units. Near-infrared observations show a spectrum with strong methane and water-vapor absorption. Modeling of the spectra and photometry yields a luminosity (normalized by the luminosity of the Sun) of 1.6 to 4.0 × 10(-6) and an effective temperature of 600 to 750 kelvin. For this age and luminosity, "hot-start" formation models indicate a mass twice that of Jupiter. This planet also has a sufficiently low luminosity to be consistent with the "cold-start" core-accretion process that may have formed Jupiter.

We present results from an adaptive optics survey for substellar and stellar companions to Sun-like stars. The survey targeted 266 F5-K5 stars in the 3 Myr-3 Gyr age range with distances of 10-190 pc. Results from the survey include the discovery of two brown dwarf companions (HD 49197B and HD 203030B), 24 new stellar binaries, and a triple system. We infer that the frequency of 0.012-0.072 M brown dwarfs in 28-1590 AU orbits around young solar analogs is 3.2 +3.1 −2.7 % (2σ limits). The result demonstrates that the deficiency of substellar companions at wide orbital separations from Sun-like stars is less pronounced than in the radial velocity "brown dwarf desert." We infer that the mass distribution of companions in 28-1590 AU orbits around solar-mass stars follows a continuous dN/dM 2 ∝ M −0.4 2 relation over the 0.01-1.0 M secondary mass range. While this functional form is similar to that for isolated objects less than 0.1 M , over the entire 0.01-1.0 M range, the mass functions of companions and of isolated objects differ significantly. Based on this conclusion and on similar results from other direct imaging and radial velocity companion surveys in the literature, we argue that the companion mass function follows the same universal form over the entire range between 0 and 1590 AU in orbital semimajor axis and ≈ 0.01-20 M in companion mass. In this context, the relative dearth of substellar versus stellar secondaries at all orbital separations arises naturally from the inferred form of the companion mass function.

Brown dwarfs are massive analogs of extrasolar giant planets and may host types of atmospheric circulation not seen in the solar system. We analyzed a long-term Spitzer Space Telescope infrared monitoring campaign of brown dwarfs to constrain cloud cover variations over a total of 192 rotations. The infrared brightness evolution is dominated by beat patterns caused by planetary-scale wave pairs and by a small number of bright spots. The beating waves have similar amplitudes but slightly different apparent periods because of differing velocities or directions. The power spectrum of intermediate-temperature brown dwarfs resembles that of Neptune, indicating the presence of zonal temperature and wind speed variations. Our findings explain three previously puzzling behaviors seen in brown dwarf brightness variations.

We present Spitzer /IRAC Ch1 and Ch2 monitoring of six brown dwarfs during 8 different epochs over the course of 20 months. For four brown dwarfs, we also obtained simulataneous HST /WFC3 G141 Grism spectra during two epochs and derived light curves in five narrow-band filters. Probing different pressure levels in the atmospheres, the multi-wavelength light curves of our six targets all exhibit variations, and the shape of the light curves evolves over the timescale of a rotation period, ranging from 1.4 h to 13 h. We compare the shapes of the light curves and estimate the phase shifts between the light curves observed at different wavelengths by comparing the phase of the primary Fourier components. We use state-of-the-art atmosphere models to determine the flux contribution of different pressure layers to the observed flux in each filter. We find that the light curves that probe higher pressures are similar and in phase, but are offset and often different from the light curves that

We report new L and T dwarfs found in a cross-match of the SDSS Data Release 1 and 2MASS. Our simultaneous search of the two databases effectively allows us to relax the criteria for object detection in either survey and to explore the combined databases to a greater completeness level. We find two new T dwarfs in addition to the 13 already known in the SDSS DR1 footprint. We also identify 22 new candidate and bona fide L dwarfs, including a new young L2 dwarf and a peculiar potentially metal-poor L2 dwarf with unusually blue near-IR colors. These discoveries underscore the utility of simultaneous database cross-correlation in searching for rare objects. Our cross-match completes the census of T dwarfs within the joint SDSS and 2MASS flux limits to the %97% level. Hence, we are able to accurately infer the space density of T dwarfs. We employ Monte Carlo tools to simulate the observed population of SDSS DR1 T dwarfs with 2MASS counterparts and find that the space density of T0YT8 dwarf systems is 0:0070 þ0:0032 À0:0030 pc À3 (95% confidence interval), i.e., about one per 140 pc 3 . Compared to predictions for the T dwarf space density that depend on various assumptions for the substellar mass function, this result is most consistent with models that assume a flat substellar mass function dN /dM / M 0:0 . No >T8 dwarfs were discovered in the present cross-match, although less than one was expected in the limited area (2099 deg 2 ) of SDSS DR1.

We present the discovery of a brown-dwarf companion to the 130-400 Myr-old G8 V star HD 203030. Separated by 11. ′′ 9 (487 AU in projection) from its host star, HD 203030B has an estimated mass of 0.023 +0.008 −0.011 M ⊙ . The K-band spectral type of L7.5±0.5 places HD 203030B near the critical L/T transition in brown dwarfs, which is characterized by the rapid disappearance of dust in sub-stellar photospheres. From a comparative analysis with well-characterized field L/T transition dwarfs, we find that, despite its young age, HD 203030B has a bolometric luminosity similar to the >1 Gyr-old field dwarfs. Adopting a radius from current models of sub-stellar evolution, we hence obtain that the effective temperature of HD 203030B is only 1206 +74 −116 K, markedly lower than the ≈1440 K effective temperatures of field L/T transition dwarfs. The temperature discrepancy can be resolved if either:(1) the ages of field brown dwarfs have been over-estimated by a factor of ≈1.5, leading to underestimated radii, or (2) the lower effective temperature of HD 203030B is related to its young age, implying that the effective temperature at the L/T transition is gravity-dependent.

Measurements of accretion rates onto planetary mass objects may distinguish between different planet formation mechanisms, which predict different accretion histories. In this Letter, we use HST /WFC3 UVIS optical photometry to measure accretion rates onto three accreting objects, GSC06214-00210 b, GQ Lup b, and DH Tau b, that are at the planet/brown dwarf boundary and are companions to solar mass stars. The excess optical emission in the excess accretion continuum yields mass accretion rates of 10 −9 to 10 −11 M ⊙ /yr for these three objects. Their accretion rates are an order of magnitude higher than expected from the correlation between mass and accretion rates measured from the UV excess, which is applicable if these wide planetary mass companions formed by protostellar core fragmentation. The high accretion rates and large seperation from the central star demonstrate the presence of massive disks around these objects. Models for the formation and evolution of wide planetary mass companions should account for their large accretion rates. High ratios of Hα luminosity over accretion luminosity for objects with low accretion rates suggest that searches for Hα emission may be an efficient way to find accreting planets.