Cool Stars 14 - Submitted Abstract # 6
This version created on 05 October 2006

Discovery of a Brown Dwarf Very Close to the Sun:  A Methane-rich
Brown Dwarf Companion to the Low-Mass Star SCR 1845-6357


Beth Biller, University of Arizona
Markus Kasper, ESO 
Laird Close, University of Arizona
Wolfgang Brandner, MPIA-Heidelberg
Stephan Kellner, W.M. Keck Observatory

We present VLT/NACO SDI images of the very nearby star SCR 1845-6357
(hereafter SCR 1845).  SCR 1845 is a recently discovered (Hambly et
al.  2004) M8.5 star just 3.85 pc from the sun (Henry et al.  2006).
Using the capabilities of the unique SDI device, we discovered a
substellar companion to SCR 1845 at a separation of 4.5 AU (1.170" +-
0.003" on the sky) and fainter by 3.57 +- 0.057 mag in the 1.575 um
SDI filter.  This substellar companion has an H magnitude of
13.16+0.31-0.26 (absolute H magnitude of 15.30+0.31-0.26), making it
likely the brightest mid-T dwarf known.  The unique Simultaneous
Differential Imager (SDI) consists of 3 narrowband filters placed
around the 1.6 um methane absorption fetaure characteristic of
T-dwarfs (Teff < 1200  K).  The flux of the substellar companion drops
by a factor of 2.7 +- 0.1 between the SDI F1(1.575 um) filter and the
SDI F3(1.625 um) filter, consistent with strong methane absorption in
a substellar companion.  We estimate a spectral type of T5.5 +- 1 for
the companion based on the strength of this methane break.  the
chances that this object is a background T dwarf are vanishingly small
-- and there is no isolated background T-dwarf in this part of the sky
according to 2MASS.  Thus, it is a bound companion, hereafter SCR
1845-6357B.  For an age range of 100  Myr - 10  Gyr and spectral type
range of T4.5-T6.5, we find a mass range of 9-65 MJup for SCR 1845B
from the Baraffe et al.  2003 COND models.  SCR 1845AB is the 24th
closest stellar system to the Sun (at 3.85 pc);  the only brown dwarf
system closer to the Sun is Eps Indi Ba-Bb (at 3.626 pc).  In
addition, this is the first T-dwarf companion discovered around a low
mass star.  

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