stars, brown dwarfs, superplanets, and planets
may '05
object
energy source
structure
formation process
spectrum
star

75 MJ <  Mstar

.075 Msun <  Mstar
stabilizes its luminosity with 
H fusion
(and Li and D fusion)
intermediate mass are sun-like:
nuclear-burning region  surr by photon-diffusion zone

low mass:
nuclear-burning region surr by convective mixing zone

supported by ideal-gas pressure
gravitational collapse & fragnmentation
no Li visible

T > 2600 K

TiO lines present
brown dwarf

13 MJ < MBD < 75 MJ
gravitational contraction provides energy in young phase

D fusion 

Li fusion only if
M > 65 MJ

but not H fusion

fully convective

supported by free-electron degeneracy
gravitational collapse & fragnmentation
(evidence:
a brown dwarf  alone, or in binary with another brown dwarf (another image), or surrounded by a disk (animated gif), or with a planet  (image)

and NOT circumstellar condensation/accretion
(as for planets)
Li visible

L dwarfs
(1400 K < T < 2200 K):
dust obliterates TiO lines; H2O, CO, Na, K lines present

 T dwarfs
(T < 1400 K):
CH4 lines present
superplanet

2 MJ < MSP < 13 MJ
no D/Li/H fusion

gravitational-
contraction luminosity
exceeds 
reflected light
supported by free-electron degeneracy
accretion in circumstellar disk and gravitational capture of H/He gas
methane visible
planet

MP  <  2 MJ
no D/Li/H fusion

reflected light exceeds gravitational-
contraction luminosity
supported by bound-electron degeneracy
  accretion in circumstellar disk [and gravitational capture of H/He gas if sufficiently cold and massive]
methane visible


how to identify a brown dwarf

a brown dwarf must have

a) M/ M ¤ < 0.075 (minimum mass for sustained H fusion)

b) lithium in spectrum and is old (M/ M ¤ > 0.065 burns Li)

    (tell age by cluster membership)

c) L/ L ¤ < 8 x 10 -4 (all stars are brighter than this)

d) CO/H2O (below 2200 K) or CH4 (below 1400 K) in the spectrum