Astrostatistics, and Brown Dwarfs Chris Koen, Dept. Statistics, University of the Western Cape

The GMC mass distribution in M33, as determined from radio flux measurements

HartRAO observations of G

HartRAO observations of G : detail

What are brown dwarfs? Stars : stable fusion of Hydrogen, M>70 Planets: no nuclear reactions, M<13 Intermediate objects with can sustain burning of Deuterium

A few properties Radius ~ 0.1 R₀ Surface temperature < 3000 K Luminosity ~ 10⁻⁴ L₀ Central temperature ~ 10⁶ K Deuterium burning lifetime a few Myr Spectral classes M, L, T

A conventional Hertzsprung-Russell diagram

The ultracool bottom end of the HR diagram

The Sun; M, L, T dwarfs; the planet Jupiter

“Only a few hundred stellar radio sources are now known” (SKA website). Because of their low temperatures and small radii, radiation levels of brown dwarfs are lower than those of stars.

Nonetheless, radio emission has been detected from a few late M (i.e. spectral types M7-M9) and L dwarfs. The emission is non-thermal, i.e. it is due to the action of magnetic fields.

What do astronomers hope to learn from the radio emission of brown dwarfs? Only direct access to magnetic fields Field configuration

Extent of the study to data About 90 objects have been studied (more than 500 L and T dwarfs known) Only 9 detections Quiescent, flaring and burst emission have been seen Quiescent emission not always at the same level Mostly single frequency

Quiescent radio emission from UCDs Name Spectral type Flux (microJy) LHS3003 M7 270±40 2M M8 140±40 TVLM M ±15 LSR M ±15 LP M9 74±13 BRI M9.5 83±18 2M L ±24 2M L ±14 2M L ±16

Flaring radio emission from UCDs Name Spectral type Flux (microJy) 2M M8 (29.6±1)X10⁴ TVLM M ±40 LP M9 2600±200 2M L ±40

DENIS flaring emission at 4.80 (gray) and 8.64 GHz (black)

8.44 GHz observations of TVLM

8.44 GHz observations of 2M

Acknowledgments Comparison of the sun, MLT dwarfs, and Jupiter: Dr. Robert Hurt of the Infrared Processing and Analysis Center First HR diagram: Wikipedia Second HR diagram : Space Telescope Science Institute Maser data: Dr. Sharmila Goedhart TVLM : Hallinan et al. (ApJ 663, L25; 2007) 2M : Hallinan et al. (ApJ 684, 644; 2008) Denis : Burgasser & Putman (ApJ 626, 486; 2005)