Stellar Systems Group

Ivanov, V. D.; Vaisanen, P.; Kniazev, A. Y.; Beletsky, Y.; Mamajek, E. E.; Muzic, K.; Beamin, J. C.; Boffin, H. M. J.; Pourbaix, D.; Gandhi, P.; Gulbis, A.; Monaco, L.; Saviane, I.; Kurtev, R.; Mawet, D.; Borissova, J.; Minniti, D.
Astronomy & Astrophysics, Volume 574, id.A64, 8 pp. (2015).
02/2015

ABSTRACT

Context. The severe crowding towards the Galactic plane suggests that the census of nearby stars in that direction may be incomplete. Recently, Scholz reported a new M9 object at an estimated distance d ≃ 7 pc (WISE J072003.20-084651.2; hereafter WISE J0720) at Galactic latitude b = 2.3°. 

Aims: Our goals are to determine the physical characteristics of WISE J0720, its kinematic properties, and to address the question of whether it is a binary object, as was suggested in the discovery paper. 
Methods: Optical and infrared spectroscopy from the Southern African Large Telescope and Magellan, respectively, and spectral energy distribution fitting were used to determine the spectral type of WISE J0720. The measured radial velocity, proper motion, and parallax yielded its Galactic velocities. We also investigated if WISE J0720 may show X-ray activity based on archival data. 
Results: Our spectra are consistent with spectral type L0±1. We find no evidence for binarity, apart from a minor 2σ level difference in the radial velocities taken at two different epochs. The spatial velocity of WISE J0720 does not connect it to any known moving group; instead, it places the object with high probability in the old thin disk or in the thick disk. The spectral energy distribution fit hints at excess in the 12 μm and 22 μm WISE bands which may be due to a redder companion, but the same excess is visible in other late-type objects, and it more likely implies a shortcoming of the models (e.g., problems with the effective wavelengths of the filters for these extremely cool objects, etc.) rather than a disk or redder companion. The optical spectrum shows some Hα emission, indicative of stellar activity. Archival X-ray observations yield no detection. 
Conclusions: WISE J0720 is a new member of the solar neighborhood, the third nearest L dwarf. Our data do not support the hypothesis of its binary nature.

Based on observations made with the Southern African Large Telescope (SALT), the DDT Proposal Code 2013-2-RSA-016.