The Late‐T Dwarf Companion to the Exoplanet Host Star HD 3651: A New Benchmark for Gravity and Metallicity Effects in Ultracool Spectra

Abstract

We present near-IR (1.0-2.5 {μ}m) photometry and spectroscopy of HD3651B, the low-luminosity, wide-separation (480 AU) companion to the K0V exoplanet host star HD 3651A. We measure a spectral type of T7.5+/-0.5for HD 3651B, confirming both its substellar nature and the fact thatwide-separation brown dwarfs and giant planets can co-exist around thesame star. We estimate an age of 3-12 Gyr for the primary star HD 3651Aand find that it is ~3 times older than the K4 V star Gl 570A (~1-5Gyr), the host star of the T7.5 dwarf Gl 570D. We derive a bolometricluminosity of log(L_{bol}/L_{solar})=-5.58+/-0.05 for HD3651B and infer an effective temperature of 780-840 K and a mass of40-72 M_{Jup}; the luminosity and temperature are among thelowest measured for any brown dwarf. Furthermore, HD 3651B belongs tothe rare class of substellar objects that are companions tomain-sequence stars and thus provides a new benchmark for studying verylow temperature objects. Given their similar temperatures({Δ}T_{eff}~30 K) and metallicities ({Δ}[Fe/H]~0.1 dex)but different ages, a comparison of HD 3651B and Gl 570D allows us toexamine gravity-sensitive diagnostics in ultracool spectra. We find thatthe expected signature of HD 3651B's higher surface gravity due to itsolder age, namely a suppressed K-band flux relative to Gl 570D, is notseen. Instead, the K-band flux of HD 3651B is enhanced compared to Gl570D, indicative of a younger age. Thus, the relative ages derived frominterpretation of the T dwarf spectra and from stellar activityindicators appear to be in discord. One likely explanation is that theK-band fluxes are also very sensitive to metallicity differences.Metallicity variations may be as important as surface gravity variationsin causing spectral differences among field late-T dwarfs.