SpS1-The evolution of brown dwarf infrared spectroscopic properties

Brown dwarfs (hereafter BDs) are formed, like stars, by interstellar cloud collapse, but attaining masses of less then 0.075 M_⊙ (Baraffe et al. 1998), i.e. too low core temperatures (< 3.5 × 10⁶ K) to stabilize the nuclear burning of the hydrogen PP chain. Therefore, even the most massive BDs begin cooling after some 10⁹ yrs. However, for masses above 0.06 M_⊙, core temperatures become hotter than the lithium burning temperature (2.4 x 10⁶ K). All BDs above 0.013 M_⊙ (13 M_Jup) reach core temperatures above the 1.0 x 10⁶ K necessary to burn deuterium from about 10⁷ yrs. The IAU has adopted the definition of the planetary regime as objects having masses below the deuterium burning conditions. But BDs are likely to form well below this limit into the planetary mass regime down to some 5 M_Jup. It is therefore convenient, in the absence of indices on their formation mechanisms, to call them planetary mass objects or planemos.