Ultracool L and T dwarf bridge the gap between cool stars and giant planets and therefore provide an important laboratory for both probing stellar atmospheres and devising future observations of cool, large-separation giant exoplanets. A key challenge of ultracool atmospheres is the understanding of the transition from dusty L-type atmospheres to clearer T-type atmospheres, dominated by methane and water absorption bands. It is clear that understanding cloud formation and evolution is crucial to understanding the L/T transition, and hence giant exoplanet atmospheres. Numerous diverse models have been proposed to explain the atmospheric evolution, from multi-condensate clouds to growing holes and sudden collapse of the cloud layers with decresing temperature. As yet, those models remain weakly constrained by observations. We propose to use the unparalleled sensitivity and stability of WFC3 on HST to tackle this question. We will obtain time series of G141 grism spectra of six L/T dwarfs, including two resolved binaries and two unresvolved L/T dwarfs. We will measure the level of rotational variability as a function of wavelength to derive the one-dimensional spectral maps of their cloud covers. Those maps will address the following questions: How heterogenous is the cloud cloud cover as a function of spectral type? and, What are the spectral properties and diversity of clouds across the photosphere of the targets? The proposed observations will provide a unique, statistically constraining data set on cloud distribution, properties, and evolution as a function of spectral type. These new constraints will allow direct comparison to the models and will drive the development of new models with realistic treatment of the cloud layers. With typical rotation periods of 3 hours, we can cover two rotation periods within 6 HST orbits. We propose to obtain simultaneous IRAC [4.5] observations of the two unresolved T dwarfs. The Spitzer data will enhance our ability to identify absorbers in individual clouds within their complex atmospheres and will probe non-equilibrium CO chemistry and vertical mixing. Combined, these observations will provide the first spatially and spectrally resolved maps of ultracool dwarf photospheres across the L/T transition.

This proposal will obtain time-resolved WFC3 grism spectra of brown dwarfs. Two of the targets are well-separated binary sources; we plan to obtain spectra of both components, which places constraints on the orientation. It is critical that the observations of individual targets are scheduled back-to-back. In addition, two of the targets will also be observed with Spitzer and attempts should be made to coordinate the observations. The coordinated observations (or observations as close in time as possible) will make the data set much more valuable. COOL STARS 12 false 0.0 12.0 0.0 false false false false AVAILABLE STAR BROWN DWARF J2000 J2000 J=13.45 STAR BROWN DWARF This object was generated by the targetselector and retrieved from the SIMBAD database. J2000 J2000 J=15.30 STAR BROWN DWARF Selected for Joint Spitzer/HST observations Sp Type=T6.5 Proper Motion: 0.31"/yr (175degree) Burgasser et al. 2003 J2000 J2000 J=15.6 STAR BROWN DWARF J2000 J2000 J=15.26 H=14.16 K=13.58 WFC3/IR Spectroscopic No No G141 WFC3/IR Spectroscopic No No G141 WFC3/IR Spectroscopic No No G141 WFC3/IR Spectroscopic No No G141 The scheduling data was generated by CASM 18.3.1. CASM was run in Phase II mode. 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Using the same POS TARG as for the G141 exposures. Must be coordinated with Spitzer/IRAC observations Must be coordinated with Spitzer/IRAC observations Direct image in F132N to help wavelength calibration. Using the same POS TARG as for the G141 exposures. Must be coordinated with Spitzer/IRAC observations Must be coordinated with Spitzer/IRAC observations Direct image in F132N to help wavelength calibration. Using the same POS TARG as for the G141 exposures. Must be coordinated with Spitzer/IRAC observations Must be coordinated with Spitzer/IRAC observations Direct image in F132N to help wavelength calibration. Using the same POS TARG as for the G141 exposures. Must be coordinated with Spitzer/IRAC observations Must be coordinated with Spitzer/IRAC observations Direct image in F132N to help wavelength calibration. Using the same POS TARG as for the G141 exposures. Must be coordinated with Spitzer/IRAC observations Must be coordinated with Spitzer/IRAC observations Direct image in F132N to help wavelength calibration. Using the same POS TARG as for the G141 exposures. Must be coordinated with Spitzer/IRAC observations Must be coordinated with Spitzer/IRAC observations The scheduling data was generated by CASM 18.3.1. CASM was run in Phase II mode. 2M2228 - Joint Spitzer/HST Observations Joint visibility window: Jun 30-July 29, 2011. 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Using the same POS TARG as for the G141 exposures. Must be coordinated with Spitzer/IRAC observations Direct image in F132N to help wavelength calibration. Using the same POS TARG as for the G141 exposures. Must be coordinated with Spitzer/IRAC observations Direct image in F132N to help wavelength calibration. Using the same POS TARG as for the G141 exposures. Must be coordinated with Spitzer/IRAC observations Direct image in F132N to help wavelength calibration. Using the same POS TARG as for the G141 exposures. Must be coordinated with Spitzer/IRAC observations Direct image in F132N to help wavelength calibration. Using the same POS TARG as for the G141 exposures. Must be coordinated with Spitzer/IRAC observations Direct image in F132N to help wavelength calibration. Using the same POS TARG as for the G141 exposures. Must be coordinated with Spitzer/IRAC observations The scheduling data was generated by CASM 18.3.1. 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Using the same POS TARG as for the G141 exposures. Must be coordinated with Spitzer/IRAC observations Direct image in F132N to help wavelength calibration. Using the same POS TARG as for the G141 exposures. Must be coordinated with Spitzer/IRAC observations Direct image in F132N to help wavelength calibration. Using the same POS TARG as for the G141 exposures. Must be coordinated with Spitzer/IRAC observations Direct image in F132N to help wavelength calibration. Using the same POS TARG as for the G141 exposures. Must be coordinated with Spitzer/IRAC observations Direct image in F132N to help wavelength calibration. Using the same POS TARG as for the G141 exposures. Must be coordinated with Spitzer/IRAC observations Direct image in F132N to help wavelength calibration. Using the same POS TARG as for the G141 exposures. Must be coordinated with Spitzer/IRAC observations The scheduling data was generated by CASM 18.3.1. 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Using the same POS TARG as for the G141 exposures. SIMP0136: Direct FN132 Image + G141 Spectra with Coordinated Spitzer/IRAC Observations Copy of visit 01, which was lost in the SI C&DH hang on day 2011.270. 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Using the same POS TARG as for the G141 exposures. Must be coordinated with Spitzer/IRAC observations Must be coordinated with Spitzer/IRAC observations Direct image in F132N to help wavelength calibration. Using the same POS TARG as for the G141 exposures. Must be coordinated with Spitzer/IRAC observations Must be coordinated with Spitzer/IRAC observations Direct image in F132N to help wavelength calibration. Using the same POS TARG as for the G141 exposures. Must be coordinated with Spitzer/IRAC observations Must be coordinated with Spitzer/IRAC observations Direct image in F132N to help wavelength calibration. Using the same POS TARG as for the G141 exposures. Must be coordinated with Spitzer/IRAC observations Must be coordinated with Spitzer/IRAC observations Direct image in F132N to help wavelength calibration. Using the same POS TARG as for the G141 exposures. Must be coordinated with Spitzer/IRAC observations Must be coordinated with Spitzer/IRAC observations Direct image in F132N to help wavelength calibration. Using the same POS TARG as for the G141 exposures. Must be coordinated with Spitzer/IRAC observations Must be coordinated with Spitzer/IRAC observations