Brown Dwarf Stars By: Katie Leonard

What are brown dwarfs? Sub-stellar objects with mass below that necessary to maintain H- burning nuclear fusion reactions in their core, but which have fully convective surfaces and interiors, with no chemical differentiation by depth. Also called failed stars Brown dwarfs larger than 13M J burn deuterium and larger than 65M J burn lithium

Why are Brown Dwarfs Interesting? Star/planet formation The missing mass Pushing the observational limit Better understanding of degenerate bodies

Basic Facts mass < 80M J (.08 solar masses) ρ: gm/cm 3 Core density and pressure: Polytropic equation of state: P = K ρ 5/3 Main thermonuclear reactions: Core temperature:

Formation protostars less than.08 solar masses cannot trigger H thermonuclear fusion in core Contraction is stopped by electron degeneracy pressure Typical temperature: T c ~210 6 K

Structure Generally have same chemical composition at all depths Made predominantly of liquid metallic H and He Convective at all depths

Differentiation

Brown Dwarf or Low Mass Star? Lithium test: Low mass stars quickly deplete their lithium Methane: Older brown dwarfs are sometimes cool enough that over very long periods of time their atmospheres can gather observable quantities of methane Luminosity: brown dwarfs cool and darken steadily over their lifetimes

Brown Dwarf or planet? Fusion: high mass brown dwarfs can fuse deuterium Density: brown dwarfs will have much more mass in approximately the same radius X-ray and infrared: some emit x-rays and all emit infrared until they cool to temperatures of planets

Where do we look for brown dwarfs? Companions Open Clusters Wide Field

First Brown Dwarf Discoverd ~35M J 6.3pc away Confirmed using methane test

Searches for Single Objects Proper motion surveys Optical and infrared color surveys Gravitational microlensing

Searches in Young Clusters Objects will more luminous and close to the same age Easier to assign masses But distance increases

Searching for Companions Direct photography Astrometric perturbation analyses Spectroscopic measurements

Only Known Brown Dwarf to have a Planet Orbiting it 2M1207b orbits 2M1207 Discovered in 2004

Death Surfaces cool from 3,500k to 1,500k Methane builds up Fade and cool to become black dwarfs

References

Baillion, P., et al., 1993, A&A, 277, 1B Burrows, A., et al., 1993, ApJ, 406, 158B Burrows, A., and Leibert, J., 1993, RvMP, 65, 301B Dantona, F., and Mazzitelli, I., 1985, ApJ, 296, 502D Delfosse, X., et al., 1999, A&AS, 135, 41D Stevenson, D, 1991, ARA&A, 29, 163S