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: 10-1000 gm/cm 3 • Core density and pressure: • Main thermonuclear reactions: • Core temperature: • Polytropic equation of state: P = K ρ 5/3

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 ~2•10 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 http://www.wikipedia.org/ http://astro.berkeley.edu/~stars/bdwarfs/ http://www.astro.ex.ac.uk/people/mbate/Cluster/pr.html

http://www.news.wisc.edu/12290 http://news.nationalgeographic.com/news/2006/03/0323_060323_brown_dwarf.html

http://www.astro.ex.ac.uk/people/mbate/Cluster/pr.html

http://www.harmsy.freeuk.com/bld.html

http://abyss.uoregon.edu/~js/ast122/lectures/lec13.html

http://www.astronomy.com/asy/default.aspx?c=a&id=5665 http://www.keckobservatory.org/view_album.php?album_id=5 http://chandra.harvard.edu/photo/2003/twa5b/more.html

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