Jacqueline Radigan

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 heterogeneous 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.

STRONG BRIGHTNESS VARIATIONS SIGNAL CLOUDY-TO-CLEAR TRANSITION OF BROWN DWARFS

We report the results of a J-band search for cloud-related variability in the atmospheres of 62 L4-T9 dwarfs using the Du Pont 2.5 m telescope at Las Campanas Observatory and the Canada-France-Hawaii Telescope on Mauna Kea. We find 9 of 57 objects included in our final analysis to be significantly variable with >99% confidence, 5 of which are new discoveries. In our study, strong signals (peak-to-peak amplitudes >2%) are confined to the L/T transition (4/16 objects with L9-T3.5 spectral types and 0/41 objects for all other spectral types). The probability that the observed occurrence rates for strong variability inside and outside the L/T transition originate from the same underlying true occurrence rate is excluded at >99.7% confidence. Based on a careful assessment of our sensitivity to astrophysical signals, we infer that 39% of L9-T3.5 dwarfs are strong variables on rotational timescales. If we consider only L9-T3.5 dwarfs with 0.8 < J – K s < 1.5, and assume an isotropic distribution of spin axes for our targets, we find that % would be strong variables if viewed edge-on; azimuthal symmetry and/or binarity may account for non-variable objects in this group. These observations suggest that the settling of condensate clouds below the photosphere in brown dwarf (BD) atmospheres does not occur in a spatially uniform manner. Rather, the formation and sedimentation of dust grains at the L/T transition is coupled to atmospheric dynamics, resulting in highly contrasting regions of thick and thin clouds and/or clearings. Outside the L/T transition we identify five weak variables (peak-to-peak amplitudes of 0.6%-1.6%). Excluding L9-T3.5 spectral types, we infer that % of targets vary with amplitudes of 0.5%-1.6%, suggesting that surface heterogeneities are common among L and T dwarfs. Our survey establishes a significant link between strong variability and L/T transition spectral types, providing evidence in support of the hypothesis that cloud holes contribute to the abrupt decline in condensate opacity and 1 μm brightening observed in this regime. More generally, fractional cloud coverage is an important model parameter for BDs and giant planets, especially those with L/T transition spectral types and colors.

Brown Dwarf Photospheres are Patchy: A Hubble Space Telescope Near-infrared Spectroscopic Survey Finds Frequent Low-level Variability

Condensate clouds strongly impact the spectra of brown dwarfs and exoplanets. Recent discoveries of variable L/T transition dwarfs argued for patchy clouds in at least some ultracool atmospheres. This study aims to measure the frequency and level of spectral variability in brown dwarfs and to search for correlations with spectral type. We used Hubble Space Telescope/Wide Field Camera 3 to obtain spectroscopic time series for 22 brown dwarfs of spectral types ranging from L5 to T6 at 1.1-1.7 ?m for 40?minutes per object. Using Bayesian analysis, we find six brown dwarfs with confident (p > 95%) variability in the relative flux in at least one wavelength region at sub-percent precision, and five brown dwarfs with tentative (p > 68%) variability. We derive a minimum variability fraction over all covered spectral types. The fraction of variables is equal within errors for mid-L, late-L, and mid-T spectral types; for early-T dwarfs we do not find any confident variable but the sample is too small to derive meaningful limits. For some objects, the variability occurs primarily in the flux peak in the J or H band, others are variable throughout the spectrum or only in specific absorption regions. Four sources may have broadband peak-to-peak amplitudes exceeding 1%. Our measurements are not sensitive to very long periods, inclinations near pole-on and rotationally symmetric heterogeneity. The detection statistics are consistent with most brown dwarf photospheres being patchy. While multiple-percent near-infrared variability may be rare and confined to the L/T transition, low-level heterogeneities are a frequent characteristic of brown dwarf atmospheres.

Weather on Other Worlds I: Detection of Periodic Variability in the L3 Dwarf DENIS-P J1058.7-1548 with Precise Multi-Wavelength Photometry

Photometric monitoring from warm Spitzer reveals that the L3 dwarf DENIS-P J1058.7-1548 varies sinusoidally in brightness with a period of 4.25^(+0.26)_(-0.16) hr and an amplitude of 0.388% ± 0.043% (peak-to-valley) in the 3.6 μm band, confirming the reality of a 4.31 ± 0.31 hr periodicity detected in J-band photometry from the SOAR telescope. The J-band variations are a factor of 2.17 ± 0.35 larger in amplitude than those at 3.6 μm, while 4.5 μm Spitzer observations yield a 4.5 μm/3.6 μm amplitude ratio of only 0.23 ± 0.15, consistent with zero 4.5 μm variability. This wide range in amplitudes indicates rotationally modulated variability due to magnetic phenomena and/or inhomogeneous cloud cover. Weak Hα emission indicates some magnetic activity, but it is difficult to explain the observed amplitudes by magnetic phenomena unless they are combined with cloud inhomogeneities (which might have a magnetic cause). However, inhomogeneous cloud cover alone can explain all our observations, and our data align with theory in requiring that the regions with the thickest clouds also have the lowest effective temperature. Combined with published vsin (i) results, our rotation period yields a 95% confidence lower limit of R_* ≥ 0.111 R_☉, suggesting upper limits of 320 Myr and 0.055 M_☉ on the age and mass. These limits should be regarded cautiously because of ~3σ inconsistencies with other data; however, a lower limit of 45° on the inclination is more secure. DENIS-P J1058.7-1548 is only the first of nearly two dozen low-amplitude variables discovered and analyzed by the Weather on Other Worlds project.

DENIS J081730.0−615520: AN OVERLOOKED MID-T DWARF IN THE SOLAR NEIGHBORHOOD

Recent wide-field near-infrared surveys have uncovered a large number of cool brown dwarfs, extending the temperature sequence down to less than 500 K and constraining the faint end of the luminosity function. One interesting implication of the derived luminosity function is that the brown dwarf census in the immediate (<10 pc) solar neighborhood is still largely incomplete, and some bright (J<16) brown dwarfs remain to be identified in existing surveys. These objects are especially interesting as they are the ones that can be studied in most detail, especially with techniques that require large fluxes (e.g. time-variability, polarimetry, high-resolution spectroscopy) that cannot realistically be applied to objects uncovered by deep surveys. By cross-matching the DENIS and the 2MASS point-source catalogs, we have identified an overlooked brown dwarf -DENIS J081730.0-615520- that is the brightest field mid-T dwarf in the sky (J = 13.6). We present astrometry and spectroscopy follow-up observations of this brown dwarf. Our data indicate a spectral type T6 and a distance -from parallax measurement- of 4.9\pm0.3 pc, placing this mid-T dwarf among the 3 closest isolated brown dwarfs to the Sun.

DISCOVERY OF THE WIDEST VERY LOW MASS FIELD BINARY

We present the discovery of the widest (~6700 AU) very low mass (VLM) field binary to date, found in a proper motion cross-match of the Sloan Digital Sky Survey and the Two Micron All Sky Survey. Our follow-up J-band imaging provides a 10 year baseline for measuring proper motions. Consequently, we are able to confirm the common proper motion of the pair to within 10 mas, implying a 99.5% probability of their physical association. Near-infrared spectra of the components indicate spectral types of M6 ± 1 and M7 ± 1. The system resides at a spectroscopic distance of 105 ± 13 pc and has an angular separation of 6338 ± 005. We have used evolutionary models to infer component masses of 0.105+0.029 –0.017 M ☉ and 0.091+0.010 –0.007 M ☉. The large separation and low binding energy of this system can provide constraints for formation models of VLM stars.

AN INDEPENDENT ANALYSIS OF THE BROWN DWARF ATMOSPHERE MONITORING (BAM) DATA: LARGE-AMPLITUDE VARIABILITY IS RARE OUTSIDE THE L/T TRANSITION

Observations of variability can provide valuable information about the processes of cloud formation and dissipation in brown dwarf atmospheres. Here we report the results of an independent analysis of archival data from the Brown dwarf Atmosphere Monitoring (BAM) program. Time series data for 14 L and T dwarfs reported to be significantly variable over timescales of hours were analyzed. We confirm large-amplitude variability (amplitudes

Extrasolar storms: Pressure-dependent changes in light-curve phase in brown dwarfs from simultaneous HST and SPITZER observations

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HST ROTATIONAL SPECTRAL MAPPING OF TWO L-TYPE BROWN DWARFS: VARIABILITY IN AND OUT OF WATER BANDS INDICATES HIGH-ALTITUDE HAZE LAYERS

2\%) for 4/13 targets and place upper limits of 0.7\%-1.6\% on variability in the remaining sample. For two targets we find evidence of weak variability at amplitudes of 1.3\% and 1.6\%. Based on our revised classification of variable objects in the BAM study, we find strong variability outside the L/T transition to be rare at near infrared wavelengths. From a combined sample of 81 L0-T9 dwarfs from the revised BAM sample and the variability survey of Radigan et al. we infer an overall observed frequency for large-amplitude variability outside the L/T transition of 3.2

CLOUD ATLAS: DISCOVERY OF PATCHY CLOUDS AND HIGH-AMPLITUDE ROTATIONAL MODULATIONS IN A YOUNG, EXTREMELY RED L-TYPE BROWN DWARF

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