We report the discovery of an L-type companion to the young M3.5V star 2MASS J01225093–2439505 at a projected separation of 1.45 ^ { \prime \prime } ( \approx 52 AU ) as part of our adaptive optics imaging search for extrasolar giant planets around young low-mass stars .
2MASS 0122–2439 B has very red near-infrared colors similar to the HR 8799 planets and the reddest known young/dusty L dwarfs in the field .
Moderate-resolution ( R \approx 3800 ) 1.5–2.4 \mu m spectroscopy reveals a near-infrared spectral type of L4–L6 and an angular H -band shape , confirming its cool temperature and young age .
The kinematics of 2MASS 0122–2439 AB are marginally consistent with members of the \sim 120 Myr AB Dor young moving group based on the photometric distance to the primary ( 36 \pm 4 pc ) and our radial velocity measurement of 2MASS 0122–2439 A from Keck/HIRES .
We adopt the AB Dor group age for the system , but the high energy emission , lack of Li i \lambda 6707 absorption , and spectral shape of 2MASS 0122–2439 B suggest a range of \sim 10–120 Myr is possible .
The age and luminosity of 2MASS 0122–2439 B fall in a strip where “ hot-start ” evolutionary model mass tracks overlap as a result of deuterium burning .
Several known substellar companions also fall in this region ( 2MASS J0103–5515 ABb , AB Pic b , \kappa And b , G196-3 B , SDSS 2249+0044 B , LP 261-75 B , HD 203030 B , and HN Peg B ) , but their dual-valued mass predictions have largely been unrecognized .
The implied mass of 2MASS 0122–2439 B is \approx 12–13 M _ { \mathrm { Jup } } or \approx 22–27 M _ { \mathrm { Jup } } if it is an AB Dor member , or possibly as low as 11 M _ { \mathrm { Jup } } if the wider age range is adopted .
Evolutionary models predict an effective temperature for 2MASS 0122–2439 B that corresponds to spectral types near the L/T transition ( \approx 1300–1500 K ) for field objects .
However , we find a mid-L near-infrared spectral type , indicating that 2MASS 0122–2439 B represents another case of photospheric dust being retained to cooler temperatures at low surface gravities , as seen in the spectra of young ( 8–30 Myr ) planetary companions .
Altogether , the low mass , low temperature , and red colors of 2MASS 0122–2439 B make it a bridge between warm planets like \beta Pic b and cool , very dusty ones like HR 8799 bcde .