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 (1.0Y2.5 m) photometry and spectroscopy of HD 3651B, the low-luminosity, wide-separation (480 AU ) companion to the K0 V exoplanet host star HD 3651A. We measure a spectral type of T7.5AE0.5 for HD 3651B, confirming both its substellar nature and the fact that wide-separation brown dwarfs and giant planets can coexist around the same star. We estimate an age of 3Y12 Gyr for the primary star HD 3651A and find that it is %3 times older than the K4 V star Gl 570A (%1Y5 Gyr), the host star of the T7.5 dwarf Gl 570D. We derive a bolometric luminosity of log (L bol /L ) ¼ À5:58 AE 0:05 for HD 3651B and infer an effective temperature of 780Y840 K and a mass of 40Y72 M Jup ; the luminosity and temperature are among the lowest measured for any brown dwarf. Furthermore, HD 3651B belongs to the rare class of substellar objects that are companions to main-sequence stars and thus provides a new benchmark for studying very low temperature objects. Given their similar temperatures (ÁT eA % 30 K ) and metallicities (Á½Fe/H % 0:1 dex) but different ages, a comparison of HD 3651B and Gl 570D allows us to examine gravity-sensitive diagnostics in ultracool spectra. We find that the expected signature of HD 3651B's higher surface gravity due to its older age, namely a suppressed K-band flux relative to Gl 570D, is not seen. Instead, the K-band flux of HD 3651B is enhanced compared to Gl 570D, indicative of a younger age. Thus, the relative ages derived from interpretation of the T dwarf spectra and from stellar activity indicators appear to be in discord. One likely explanation is that the K-band fluxes are also very sensitive to metallicity differences. Metallicity variations may be as important as surface gravity variations in causing spectral differences among field late-T dwarfs.

Atmospheric Parameters of Field L and T Dwarfs1

“…Our method of SED fitting should be applied to those ultracool dwarfs with well-constrained T eA (AE20 K) and log g (AE0.05 dex) values (derived from other methods) in order to better assess its internal uncertainties. Currently, such a sample is limited to a few late-type T dwarfs Liu et al 2007) that are companions to main-sequence stars with known distances, metallicities, and well-constrained ages. Such studies should be extended to include earlier spectral types in order to firmly associate the observable properties of the L and T spectral sequence with fundamental astrophysical parameters.…”

Atmospheres From Very Low-Mass Stars to Extrasolar Planets

“…For this reason, and because T dwarfs are more readily modeled than the dustier L dwarfs (see section 6 below), several studies such as those by Liu et al (2007), Leggett et al (2010), and Pinfield et al (2012) have been done, despite the extreme difficulty tied to the poor spectral resolution of the observations and the fact that the SED of T dwarfs changes very slightly with the atmospheric parameters compared to those of L and M dwarfs. The submetallicity of L and T brown dwarfs is not extreme ([Fe/H] > −0.5).…”

Binary frequency of planet-host stars at wide separations

“…York et al 2000;Leggett et al 2000b;Geballe et al 2002;Chiu et al 2006), the UKIRT Infrared Deep Sky Survey (UKIDSS; Lawrence et al 2007;Lodieu et al 2007;Pinfield et al 2008;Burningham et al 2010Burningham et al , 2013, and the Wide-field Infrared Survey Explorer (WISE; Wright et al 2010;Kirkpatrick et al 2011;Mace et al 2013;Thompson et al 2013). However, only one wide brown dwarf companion co-moving with a planet-host star has been reported to date, HD 3651B (T7.5; d = 11.1 ± 0.1 pc; Mugrauer et al 2006b;Liu et al 2007; Leggett et al 2007;Burgasser 2007).…”