The Prototypical Young L/T-Transition Dwarf HD 203030B Likely Has Planetary Mass

Abstract

Upon its discovery in 2006, the young L7.5 companion to the solar analog HD 203030 was found to be K cooler than older late-L dwarfs, which is quite unusual. HD 203030B offered the first clear indication that the effective temperature at the L-to-T spectral type transition depends on surface gravity: now a well-known characteristic of low-gravity ultra-cool dwarfs. An initial age analysis of the G8V primary star indicated that the system was 130–400 Myr old, and so the companion would be between 12 and 31  . Using moderate-resolution near-infrared spectra of HD 203030B, we now find features of very low gravity comparable to those of 10–150 Myr old L7–L8 dwarfs. We also obtained more accurate near-infrared and Spitzer /IRAC photometry, and we find a color of 2.56 ± 0.13 mag—comparable to those observed in other young planetary-mass objects—and a luminosity of log( ) = −4.75 ± 0.04 dex. We further re-assess the evidence for the young age of the host star, HD 203030, with a more comprehensive analysis of the photometry and updated stellar activity measurements and age calibrations. Summarizing the age diagnostics for both components of the binary, we adopt an age of 100 Myr for HD 203030B and an age range of 30–150 Myr. Using cloudy evolutionary models, the new companion age range and luminosity result in a mass of 11  with a range of 8–15  , and an effective temperature of 1040 ± 50 K.