The Extremely Red, Young L Dwarf PSO J318.5338-22.8603: A Free-floating Planetary-mass Analog to Directly Imaged Young Gas-giant Planets
Abstract
We have discovered using Pan-STARRS1 an extremely red late-L dwarf, which has (J - K)MKO = 2.78 and (J - K)2MASS = 2.84, making it the reddest known field dwarf and second only to 2MASS J1207-39b among substellar companions. Near-IR spectroscopy shows a spectral type of L7 ± 1 and reveals a triangular H-band continuum and weak alkali (K I and Na I) lines, hallmarks of low surface gravity. Near-IR astrometry from the Hawaii Infrared Parallax Program gives a distance of 24.6 ± 1.4 pc and indicates a much fainter J-band absolute magnitude than field L dwarfs. The position and kinematics of PSO J318.5-22 point to membership in the β Pic moving group. Evolutionary models give a temperature of 1160^{+30}_{-40} K and a mass of 6.5^{+1.3}_{-1.0} M Jup, making PSO J318.5-22 one of the lowest mass free-floating objects in the solar neighborhood. This object adds to the growing list of low-gravity field L dwarfs and is the first to be strongly deficient in methane relative to its estimated temperature. Comparing their spectra suggests that young L dwarfs with similar ages and temperatures can have different spectral signatures of youth. For the two objects with well constrained ages (PSO J318.5-22 and 2MASS J0355+11), we find their temperatures are ≈400 K cooler than field objects of similar spectral type but their luminosities are similar, i.e., these young L dwarfs are very red and unusually cool but not "underluminous." Altogether, PSO J318.5-22 is the first free-floating object with the colors, magnitudes, spectrum, luminosity, and mass that overlap the young dusty planets around HR 8799 and 2MASS J1207-39.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- November 2013
- DOI:
- 10.1088/2041-8205/777/2/L20
- arXiv:
- arXiv:1310.0457
- Bibcode:
- 2013ApJ...777L..20L
- Keywords:
-
- brown dwarfs;
- parallaxes;
- planets and satellites: atmospheres;
- proper motions;
- solar neighborhood;
- surveys;
- Astrophysics - Earth and Planetary Astrophysics;
- Astrophysics - Solar and Stellar Astrophysics
- E-Print:
- ApJ Letters, in press. Version 2 incorporates tiny cosmetic changes to match final published version, including a revised Figure 1 that uses the most recent AB Pic b photometry from Biller et al (2013)
