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We present $Spitzer$ IRS 5--14 $mu$m spectra and 16 $mu$m and 22 $mu$m photometry of the T2.5 companion to the $sim$300 Myr-old G0V star HN Peg. We incorporate previous 0.8--5 $mu$m observations to obtain the most comprehensive spectral energy distribution of an intermediate-gravity L/T-transition dwarf which, together with an accurate Gaia EDR3 parallax of the primary, enable us to derive precise fundamental parameters. We find that young ($approx$0.1--0.3 Gyr) early-T dwarfs on average have $approx$140 K lower effective temperatures, $approx$20% larger radii, and similar bolometric luminosities compared to $gtrsim$1 Gyr-old field dwarfs with similar spectral types. Our accurate infrared spectrophotometry offers new detail at wavelengths where the dominant carbon-bearing molecules have their strongest transitions: at 3.4 $mu$m for methane and at 4.6 $mu$m for carbon monoxide. We assess the performance of various widely available photospheric models and find that models with condensates and/or clouds better reproduce the full SED of this moderately young early-T dwarf. However, cloud-free models incorporating a more general convective instability treatment reproduce at least the low-resolution near-IR spectrum similarly well. Our analysis of $Rapprox2300$ $J$-band spectra shows that the near-infrared potassium absorption lines in HN Peg B have similar strengths to those seen in both younger and older T2-T3 dwarfs. We conclude that while alkali lines are well-established as surface gravity indicators for L-type or warmer stars, they are insensitive to surface gravity in early-T dwarfs
We present the first results from our high-precision infrared (IR) astrometry program at the Canada-France-Hawaii Telescope. We measure parallaxes for 83 ultracool dwarfs (spectral types M6--T9) in 49 systems, with a median uncertainty of 1.1 mas (2.
We present a low-resolution (R = 90), 5.5-38 micron spectral sequence of a sample of M, L, and T dwarfs obtained with the Infrared Spectrograph (IRS) onboard the Spitzer Space Telescope. The spectra exhibit prominent absorption bands of H_2O at 6.27
Gas-giant planets emit a large fraction of their light in the mid-infrared ($gtrsim$3$mu$m), where photometry and spectroscopy are critical to our understanding of the bulk properties of extrasolar planets. Of particular importance are the L and M-ba
Mid-infrared data, including Spitzer warm-IRAC [3.6] and [4.5] photometry, is critical for understanding the cold population of brown dwarfs now being found, objects which have more in common with planets than stars. As effective temperature (T_eff)
We present an interferometric study of the continuum surface of the red supergiant star Betelgeuse at 11.15 microns wavelength, using data obtained with the Berkeley Infrared Spatial Interferometer each year between 2006 and 2010. These data allow an