1. Adam J. Burgasser Massachusetts Institute of Technology
Astrometric Observations
of the Lowest Luminosity Brown Dwarfs
Adam J. Burgasser
Massachusetts Institute of Technology

2. Teff ≈ 3000 K 1700 K 700 K 125 K Marley & Leggett (2008)
With the discovery of objects cooler than the lowest-mass stars has arisen two new spectral classes whose spectral properties derive from complex atmospheres
Marley & Leggett (2008)
Data from Cushing et al. (2005,2007)

3. luminosities & effective temperatures (Teff)
Parallax astrometric observations essential for fundamental characterization of luminosity and temperature scales.
Current brown dwarf population extends down to Teff ≈ 600 K
Golimowski et al. (2004)
See also Dahn et al. (2002); Tinney et al. (2003); Vrba et al. (2004); Cushing et al. (2006); Burgasser (2007)

4. new lows in substellar astronomy
2MASS
d = 5.74 pc
log Lbol/Lsun = -5.67
Teff ≈ 700 K
(Vrba et al. 2004)
2MASS
d = 5.34 pc
Log Lbol/Lsun = -5.69
Teff ≈ 600 K
10-6 Lsun binary pair!
(Burgasser et al. 2008)

5. “J-band bump”
Tinney et al. (2003)
Brightening at 1 µm from late- type L to mid-type T despite declining Teff and Lbol  non- thermal mechanism
Resolved binaries verify brightening, but reduce overall effect (excess of binaries)
Most likely the dynamic evolution (sinking) of clouds in brown dwarf photospheres on a short timescale.
J ≈ 1 mag
see also Dahn et al. (2002); Burgasser et al. (2002); Tinney et al. (2003); Vrba et al. (2004); Tsuji (2004); Burrows et al. (2005); Burgasser et al. (2006); Liu et al. (2006); Looper et al. (2008)
Burgasser et al. (2006)
see also Dahn et al. (2002); Tinney et al. (2003), Vrba et al. (2004); Liu et al. (2006)

6. brown dwarfs in motion Astrometry + radial velocity = UVW velocities
Age/activity/metallicity relationships (Mohanty et al. 2003; Schmidt et al. 2007; West et al , 2008)
Are local brown dwarfs very young (Zapatero Osorio et al. 2006; Jameson et al. 2007) or “normal”- aged? (Reid et al. 2002; Faherty et al. 2008)
Activity lifetimes of M dwarfs
(West et al. 2007,2008)
Population ages
(Faherty et al. 2008)

7. Burgasser et al. (in prep.)
Galactic orbits
Astrometric & radial velocity measurements provide UVW measurements, and hence the potential to look at Galactic orbits of very low mass halo objects - first pointed out to me by Sebastien Lepine many of these ultracool subdwarfs have plunging orbits, well within 1 kpc of Galactic center (so calculations are suspect) - origins? Selection biases? A nice dynamics problem. CF DANA CASSETTI-DINESCU
Dahn et al. (2008)
d/sd?M6pec LSR 1610
Burgasser et al. (in prep.)
sdL3.5 SDSS 1256

8. red/NIR proper motion surveys: nearby ultracool dwarfs & subwarfs
e.g. LSPM (Lepine et al. 2004,2008), SSSPM (Scholz et al. 2004), SCR (Hambly et al. 2004; Henry et al. 2004), SIMP (Artigau et al. 2006), 2MASS (Kirkpatrick et al. 2008), SDSS (Lang et al. 2008), etc…
20"
t = 3.2 yr
Brown dwarfs discovered largely through availability of wide-field red and NIR detectors. One of the new players in the astrometry game are NIR PM surveys now possible with 2kx2k, 4kx4k NIR detectors - USNO leading the way on this but several groups developing programs including my own graduate student J. Faherty. At level of mas/yr, Good PMs take time, important to start now!
Burgasser et al. (2003, 2008)
L subdwarf 2MASS
halo kinematics (V = -350 km/s), Teff = 1730±90 K

9. binaries: astrometric measures of mass
GJ 569Bab (e.g., Lane et al. 2001)
Resolved M dwarf binary and companion to nearby young star
Several binaries now have measured dynamical masses, including one spectroscopic binary with radii determinations
Roughly ~ 20% of brown dwarfs are resolved binaries, most are quite tight so orbit measurements are viable on graduate student timescales. New techniques including aperture mask inteferometry being brought to bear. CF ELLIOTT HORCH BINARY ASTROMETRY
2MASS AB (e.g., Liu et al. 2008)
Resolved T dwarf binary
LSR AB (e.g., Dahn et al.2008)
Astrometric M/L subdwarf binary

10. Adam J. Burgasser Massachusetts Institute of Technology
Astrometric Observations
of the Lowest Luminosity Brown Dwarfs
Adam J. Burgasser
Massachusetts Institute of Technology