We request DD observations of the SDWFS J1433+35, which is a strong candidate for the coldest brown dwarf yet discovered and the first example of the elusive "Y-dwarf" spectral class. This source was discovered from a deep, wide-field survey of 10 square degrees with the IRAC instrument on Spitzer, and is redder in both [3.6]-[4.5] and H-[4.5] than any published brown dwarfs. The spectrum falls beyond 5 microns, strongly implying that the red mid-infrared colors are due to methane absorption in the IRAC 3.6 micron channel rather than dust obscuration. The source is undetected in deep, ground-based near-IR images, with a H>24.2 (Vega) limit derived from Keck AO imaging. The proposed WFC3 imaging should provide detections in two near-infrared bands. By showing the target is morphologically unresolved and that it has the non-uniform spectral energy distribution of an ultracool brown dwarf, the proposed observations will definitely show that (i) SDWFS J1433+35 is the coldest brown dwarf known, (ii) test models of ultracool, planet-like, brown dwarf atmospheres, and (iii) refine predictions and strategies for the Wide-Field Infrared Survey Explorer (WISE; planned to launch this December).

We propose multi-band near-IR imaging observations of SDWFS J143356.62+351849.2, the Y-dwarf candidate, with WFC3. Such data will confirm the nature of this source (i) by showing that it is unresolved (as expected for a brown dwarf, but not for a dusty galaxy, the most likely --- but still unlikely --- alternate explanation for the unusual SED), and (ii) by showing that the near-IR portion of the SED has the characteristic and distinguishing SED of an ultracool brown dwarf. Given the faintness of the target, these observations are beyond the capabilities of ground-based observatories, but well within the reach of a modestly sized (7 orbit) HST program. We have used the WFC3 exposure time calculator to estimate the time needed under default background and detection assumptions to detect a 400 K Burrows et al. (2003) non-equilibrium brown dwarf model, normalized to H=26. This is an order of magnitude deeper than our Keck AO derived limit (Figure~3). We investigated models with temperatures ranging from 400 to 600 K assuming both equilibrium or non-equilibrium conditions, and the 400 K non-equilibrium model was found to be the most conservative (e.g., the most challenging) case. The most sensitive filter for this model is F140W. To achieve a signal-to-noise (SNR) of 5 in F140W will require 1.9 hours of exposure time and should provide adequate sensitivity to determine whether the source is morphologically unresolved. Assuming 40 usable minutes of integration per orbit, this is achievable in 3 orbits. To further distinguish between the Y-dwarf and DOG hypotheses, we request 2.6 hours (4 orbits) with F127M which will achieve SNR of 5 for the brown dwarf spectrum, but substantially less on a DOG, since the brown dwarf spectra peak up in F127M. COOL STARS 12 false 0.0 0.0 false false false false false false SUPPORTED STAR BROWN DWARF J>21.9, H>24.2, Ks>18.8, [3.6]=20.71, [4.5]=18.47 J2000 00 00 0.0000 +00 00 0.00 J2000 WFC3/IR Imaging No No F140W WFC3/IR Imaging No No F127M POS-TARG DITHER 5.725 LINE 3.655 false false 74.653 4 18.528 WFC3-IR-DITHER-BOX-MIN 1 false The scheduling data was generated by CASM 17.5. CASM was run in Phase II mode. F140W imaging (Tic-Visit :proposal-id 12044 :visit-id "01" :proposal-version -1 :trans-version "20090915" :date "17-Nov-2009 10:55:14.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 3 :saa-model "30" :si-bright-earth-limb-angle 20.0 :data-volume 3342512 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 12044 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 12044 :visit-id "01" :orbit-number 1 :pcs-time 241 :si-vis-utilization 3059 :al-overhead-at-end 0 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3310 :pcs-gap 0 ) (Tic-Orbit :proposal-id 12044 :visit-id "01" :orbit-number 2 :pcs-time 176 :si-vis-utilization 3059 :al-overhead-at-end 0 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3310 :pcs-gap 0 ) (Tic-Orbit :proposal-id 12044 :visit-id "01" :orbit-number 3 :pcs-time 176 :si-vis-utilization 3059 :al-overhead-at-end 0 :internal-dur-at-end 535 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3310 :pcs-gap 0 ) true 3 0 The scheduling data was generated by CASM 17.2. CASM was run in Phase II mode. F127M imaging (Tic-Visit :proposal-id 12044 :visit-id "02" :proposal-version -1 :trans-version "20090702" :date "09-Nov-2009 15:37:50.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 4 :saa-model "30" :si-bright-earth-limb-angle 20.0 :data-volume 2228304 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 12044 :visit-id "02" :obset-id "02" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 12044 :visit-id "02" :orbit-number 1 :pcs-time 333 :si-vis-utilization 2917 :al-overhead-at-end 0 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3310 :pcs-gap 0 ) (Tic-Orbit :proposal-id 12044 :visit-id "02" :orbit-number 2 :pcs-time 222 :si-vis-utilization 2917 :al-overhead-at-end 0 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3310 :pcs-gap 0 ) (Tic-Orbit :proposal-id 12044 :visit-id "02" :orbit-number 3 :pcs-time 222 :si-vis-utilization 2917 :al-overhead-at-end 0 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3310 :pcs-gap 0 ) (Tic-Orbit :proposal-id 12044 :visit-id "02" :orbit-number 4 :pcs-time 222 :si-vis-utilization 2917 :al-overhead-at-end 0 :internal-dur-at-end 497 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3310 :pcs-gap 0 ) false 4 0