We present new 0.9–2.45 \mu m spectroscopy ( R \sim 1000 ) , and Y , J , H , K _ { s } , L ^ { \prime } photometry , obtained at Gemini North , of three low-mass brown dwarf companions on wide orbits around young stars of the Upper Scorpius OB association : HIP 78530 B , [ PGZ2001 ] J161031.9–191305 B , and GSC 06214-00210 B .
We use these data to assess the companions ’ spectral type , temperature , surface gravity and mass , as well as the ability of the BT-SETTL and Drift-Phoenix atmosphere models to reproduce the spectral features of young substellar objects .
For completeness , we also analyze the archival spectroscopy and photometry of the Upper Scorpius planetary mass companion 1RXS J160929.1–210524 b .
Based on a comparison with model spectra we find that the companions , in the above order , have effective temperatures of 2700 \pm 100 K , 2500 \pm 200 K , 2300 \pm 100 K and 1700 \pm 100 K. These temperatures are consistent with our inferred spectral types , respectively M7 \beta , M9 \gamma , M9 \gamma , and L4 \gamma , obtained from spectral indices and comparisons with templates .
From bolometric luminosities estimated from atmosphere model spectra adjusted to our photometry , and using evolution models at 5–10 Myr , we estimate masses of 21–25 M _ { Jup } , 28–70 M _ { Jup } , 14–17 M _ { Jup } , and 7–12 M _ { Jup } , respectively .
[ PGZ2001 ] J161031.9–191305 B appears significantly over-luminous for its inferred temperature , which explains its higher mass estimate .
Synthetic spectra based on the BT-Settl and Drift-Phoenix atmosphere models generally offer a good fit to our observed spectra , although our analysis has highlighted a few problems .
For example , the best fits in the individual near-infrared bands occur at different model temperatures .
Also , temperature estimates based on a comparison of the broadband magnitudes and colors of the companions to synthetic magnitudes from the models are systematically lower than the temperature estimates based on a comparison with synthetic spectra .