Discovery of Two T Dwarf Companions with theSpitzer Space Telescope

We report the discovery of T dwarf companions to the nearby stars HN Peg(G0V, 18.4 pc, ~0.3 Gyr) and HD 3651 (K0V, 11.1 pc, ~7 Gyr). During an ongoingsurvey of 5'x5' fields surrounding stars in the solar neighborhood with IRACaboard the Spitzer Space Telescope, we identified these companions as candidateT dwarfs based on their mid-IR colors. Using near-IR spectra obtained with SpeXat the NASA IRTF, we confirm the presence of methane absorption thatcharacterizes T dwarfs and measure spectral types of T2.5+/-0.5 and T7.5+/-0.5for HN Peg B and HD 3651 B, respectively. By comparing our Spitzer data toimages from 2MASS obtained several years earlier, we find that the propermotions of HN Peg B and HD 3651 B are consistent with those of the primaries,confirming their companionship. HN Peg B and HD 3651 B have angular separationsof 43.2" and 42.9" from their primaries, which correspond to projected physicalseparations of 795 and 476 AU, respectively. A comparison of their luminositiesto the values predicted by theoretical evolutionary models implies masses of0.021+/-0.009 and 0.051+/-0.014 Msun for HN Peg B and HD 3651 B. In addition,the models imply an effective temperature for HN Peg B that is significantlylower than the values derived for other T dwarfs at similar spectral types,which is the same behavior reported by Metchev & Hillenbrand for the younglate-L dwarf HD 203030 B. Thus, the temperature of the L/T transition appearsto depend on surface gravity. Meanwhile, HD 3651 B is the first substellarcompanion directly imaged around a star that is known to harbor a close-inplanet from RV surveys. The discovery of this companion supports the notionthat the high eccentricities of close-in planets like the one near HD 3651 maybe the result of perturbations by low-mass companions at wide separations.Comment: Astrophysical Journal, in pres