A new T2 dwarf within 15 pc, WISE J2121-6239

2M2121Images: NASA/IPAC Infrared Science Archive. These near-infrared (2MASS) images clearly show the object, now identified as WISE J212100.87-623921.6. It was discovered as part of a new study of high proper motion sources from the WISE survey, published today (preprint) and accepted to MNRAS. It is interesting to note that the object is clearly defined in 2MASS with magnitudes J = 15.43, H = 14.54 and K = 14.27, ± 0.05 – 0.07 (2MASS PSC). Both because it is faint – there is no optical counterpart in the SuperCosmos catalogue, preventing a proper motion measurement over a suitably long timeline – and because T dwarfs have quite blue near-infrared colours, it was missed in the previous epoch of surveys. (Moreover, the object is in a region of the sky not observed by the Sloan Digital Sky Survey). Despite the fact that the T dwarfs populate a locus somewhat blueward of the main sequence, there is perhaps more potential for contamination by background stars than in the very red L dwarf locus, as can be seen in the figures below:jhkl1 Above: Example colour selection criteria for ultracool dwarfs in the (J-H)/(H-K) two-colour diagram: blue triangles are nearby main-sequence stars; green points ultracool M and L dwarfs; red stars are T dwarfs; deep blue circles are M subdwarfs; and purple crosses are giants. The box outlines the (J-H)/(H-K) selection limits for much redder L dwarfs. Gliese 229B is the prototypical and first to be discovered brown dwarf, with spectral type T6.

Tcomp2Above: Illustrative comparison of T dwarf JHK colours (right) compared to a selection of main sequence and other cluster, association or field dwarfs of various ages. Data for main sequence stars are from Straižys & Lazauskaitė (Baltic Astronomy, 18, 19, 2009) and are on the 2MASS system. Data for T dwarfs from S. Leggett (link here) are on the MKO system. However for these roughly comparative purposes it is sufficient to see that T dwarfs can potentially occupy the same JHK colour space as any number of background interlopers. Conversion between the two systems (and many others) are here. T dwarf data were plotted using TopCat. Note the position of WISE  J2121 and the direction of reddening.

From the preprint abstract:

The census of the solar neighborhood is almost complete for stars and becoming more complete in the brown dwarf regime. Spectroscopic, photometric and kinematic characterization of nearby objects helps us to understand the local mass function, the binary fraction, and provides new targets for sensitive planet searches. We aim to derive spectral types and spectro-photometric distances of a sample of new high proper motion sources found with the WISE satellite, and obtain parallaxes for those objects that fall within the area observed by the Vista Variables in the Via Lactea survey (VVV). We used low resolution spectroscopy and template fitting to derive spectral types, multiwavelength photometry to characterize the companion candidates and obtain photometric distances. Multi-epoch imaging from the VVV survey was used to measure the parallaxes and proper motions for three sources. We confirm a new T2 brown dwarf within 15 pc. We derived optical spectral types for twenty four sources, mostly M dwarfs within 50 pc. We addressed the wide binary nature of sixteen objects found by the WISE mission and previously known high proper motion sources. Six of these are probably members of wide binaries, two of those are new, and present evidence against the physical binary nature of two candidate binary stars found in the literature, and eight that we selected as possible binary systems.

The spectrum of the object is compared to T1, T2 and T3 spectral standards below (taken from the same paper).

WISE 2121-62 spec.


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