0 Assigned ID: 183 ----- Attempting Submission 1 (Thu Apr 10 13:41:12 GMT 2014) ----- HST Phase I Proposal 183 (luhman16_cycle22.apt) successfully submitted. ----- Attempting Submission 2 (Fri Apr 11 15:37:47 GMT 2014) ----- If the orbital visibility should end up being slightly longer or shorter, the number of exposures with the G102 grism in each orbit could be adjusted accordingly to optimize the available observing time. 1970004099 ----- Attempting Submission 1 (Mon Jul 07 13:24:23 GMT 2014) ----- Understanding cloud formation, evolution and dispersal is one of the biggest challenges in the field of substellar atmospheres. At the transition from L to T type brown dwarfs, thick silicate and iron clouds disappear below the visible photosphere and the spectra are altered dramatically despite little change in effective temperature. Observed strengthening of the FeH band through the transition has indicated an opening of cloud holes rather than gradual sinking of the clouds. This view has been supported by the discovery of brown dwarfs with photometric variability attributed to patchy cloud cover. However, our recent spectral variability observations have complicated the picture because patchy cloud models with cloud holes currently cannot reproduce the color variations and spectral variability signatures. Whether this implies an absence of cloud holes (in contradiction to the interpretation of the observed strengthening of FeH) or is because models neglect other mechanisms that can affect the colors (eg temperature anomalies) remains unclear. Here, we propose WFC3/G102 spectral variability observations of the very nearby L/T transition benchmark binary Luhman 16AB, the only known variable brown dwarfs in a binary system. These observations target the FeH band. As the FeH molecule contains the cloud forming atom Fe, it is a direct tracer of the cloud holes. By comparing the variability amplitude in the FeH band with the surrounding continuum, as well as between the L and T component at different stages of cloud evolution, we will obtain direct evidence whether the formation of cloud holes is indeed the driving mechanism behind cloud dispersal at the L/T transition. We are observing a binary Brown Dwarfs with WFC3 IR slitless spectroscopy with grism G102 to search for time variability. The target is observed for 5 consecutive orbits in a single visit. At the beginning of each orbit, a direct image is taken for wavelength calibration. Then, a sequence of exposures in G102 mode without dithering is taken for the rest of each orbit. We are using the 256 subarray mode to avoid buffer dumps during the useful portion of the orbit. The orientation of the space craft has to be such that the two spectra on the detector are well separated. COOL STARS Default false false false false false SUPPORTED STAR BROWN DWARF L T Coordinates from Luhman et al. 2013, last WISE measurement, Jan 6 2011 2MASS-J10491891-5319100 J2000 J2000 J=11.2, H=10.4 WFC3/IR Spectroscopic No No G102 1404739236975 /Users/buenzli/Documents/work/HST/cycle22/OP-cache/HST_13640_phase2-v01.xml The scheduling data was generated by CASM 22.2. CASM was run in Phase II mode. (Tic-Visit :proposal-id 13640 :visit-id "01" :proposal-version -1 :trans-version "20140605" :date "07-Jul-2014 15:20:34.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 5 :saa-model "30" :si-bright-earth-limb-angle 20.0 :data-volume 1363280 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 13640 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 13640 :visit-id "01" :orbit-number 1 :pcs-time 333 :si-vis-utilization 3015 :al-overhead-at-end 0 :internal-dur-at-end 400 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3409 :pcs-gap 0 ) (Tic-Orbit :proposal-id 13640 :visit-id "01" :orbit-number 2 :pcs-time 222 :si-vis-utilization 3159 :al-overhead-at-end 0 :internal-dur-at-end 412 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3409 :pcs-gap 0 ) (Tic-Orbit :proposal-id 13640 :visit-id "01" :orbit-number 3 :pcs-time 222 :si-vis-utilization 3159 :al-overhead-at-end 0 :internal-dur-at-end 412 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3409 :pcs-gap 0 ) (Tic-Orbit :proposal-id 13640 :visit-id "01" :orbit-number 4 :pcs-time 222 :si-vis-utilization 3159 :al-overhead-at-end 0 :internal-dur-at-end 412 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3409 :pcs-gap 0 ) (Tic-Orbit :proposal-id 13640 :visit-id "01" :orbit-number 5 :pcs-time 222 :si-vis-utilization 3159 :al-overhead-at-end 0 :internal-dur-at-end 382 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3409 :pcs-gap 0 ) true 5 0