Computer Science
Scientific paper
May 2007
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007sptz.prop40489m&link_type=abstract
Spitzer Proposal ID #40489
Computer Science
Scientific paper
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 under-luminous 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.
Luhman Kevin
Marengo Massimo
Metchev Stanimir
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