Phase II proposal for program accepted ad part of Spitzer cycle 4 program 40486. No HST Phase I proposal was submitted. ----- Attempting Submission 1 (Thu Jun 14 18:02:18 GMT 2007) ----- We propose 3.6-8.0 micron IRAC photometry and 5.5?22 micron low-resolution IRS spectroscopy and imaging of the sub-stellar companions to the ~0.3 Gyr old stars HN Peg and HD 203030. The spectral types of the two secondaries span the critical transition between L and T dwarfs, which is characterized by a rapid sedimentation of dust and appearance of methane in sub-stellar photospheres. HN Peg B (T2.5) and HD 203030 B (L7.5) are the youngest known brown dwarfs at this transition, and present a unique opportunity to examine the role of surface gravity in the process. Both objects stand out from 1-10 Gyr L/T transition dwarfs in the field because they are underluminous in the near-IR compared to the expected luminosities for their ages. Probable reasons include: (1) a decrease in the effective temperature at the onset of methane formation at lower surface gravities in sub-stellar photospheres, or (2) a shift in the emitted flux from the near-IR to the mid-IR region of the SED of young brown dwarfs. The mid-IR is key for distinguishing between these two hypotheses because it contains several fundamental molecular transitions that create deep absorption bands in the SEDs of L and T dwarfs, and that are inaccessible for study from the ground. Our existing IRAC photometry of HN Peg B does reveal a 0.3-0.5 mag excess in its 3.6-8.0 micron SED. However, this excess is insufficient to account for the lower luminosity of HN Peg B, and indicates that both of the above hypotheses may hold true to certain degrees. With the present proposal we aim to independently confirm the gravity-dependent behavior of L/T transition photospheres in the mid-IR through IRAC photometry of HD 203030 B. We will also seek the culprit for the mid-IR excess of HN Peg B through low resolution spectroscopy and peak-up imaging with IRS. To check for possible duplicity of HN Peg B as the reason for its excess, we request high angular resolution imaging with HST to complement our lower resolution Spitzer imaging.

NIC 1 images of target HN Peg B in F090M, F110W and F170 filters. The target is dithered between 5 positions within the NIC1 field of view. At each dither position we obtain 1 MULTIACCUM image in each of the three filters. We have adopted the observing strategy of HST Program 10247. COOL STARS 12 false 18.7 false false false false false false SUPPORTED STAR BROWN DWARF J = 15.8; H = 15.4; K = 15.1 J2000 21 44 28.5500 +14 46 6.80 J2000 NIC1 Imaging No No F090M NIC1 Imaging No No F110W NIC1 Imaging No No F170M POS-TARG DITHER DITHER 1.3 NIC-XSTRIP-DITH false true 5 0 POS-TARG NIC-SPIRAL-DITH 0 1 false /Users/mmarengo/Work/Folds/proposals/Spitzer/Cycle-04/irs_young_lt/HST/OP-cache/phaseII-v01.xml 1181835142073 (Tic-Visit :proposal-id 11304 :visit-id "01" :proposal-version -1 :trans-version "20070325" :date "14-Jun-2007 11:32:21.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 1 :saa-model "23" :si-bright-earth-limb-angle 20.0 :data-volume 154592 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 11304 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 11304 :visit-id "01" :orbit-number 1 :pcs-time 355 :si-vis-utilization 2776 :al-overhead-at-end 0 :internal-dur-at-end 114 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3137 :pcs-gap 0 ) true 1 0 /private/tmp/OPVisit42085.xml 1181835142073