Signatures of Magnetic Activity in Very Low Mass Stars and

Brown Dwarfs

Rachel Osten

Hubble Fellows Symposium

Collaborators

Suzanne Hawley University of WashingtonTim Bastian NRAONeill Reid STScIRay Jayawardhana U. Toronto

Sun M dwarf L dwarf T dwarf Jupiter

Transition to completely convective interior happens at ~M3 Should expect a changeover in magnetic activity observables, sinceno dynamo can operate; not observed See same manifestations of magnetic activity (H, X-ray, radio emission) in some late M/L objects as in solar-like active starsCool, neutral, high density atmospheres have large resistivity, magnetic fields and matter are decoupled

Activity Signatures Are Seen (Sporadically) in Late-M/L Dwarfs

H emission2MASS 0144-0716 (L5V; Liebert et al. 2003)

Radio emission

Berger 2002; 2MASS 0036+18 (L3.5)

X-ray emission

Stelzer (2004)

BD pair:

Ba 55-87 Mjup

Bb 34-70 Mjup

steadyand variable (flaring)levels of emission

How common is radio emission in late M, L, T field dwarfs?

Not Very:only a handful of detections, all earlier than L4V

Consistent with H, X-ray decline in emission with decreasing Teff

Ave

rage

rad

io p

rope

rtie

s

Osten et al. 2006

Teff more important than mass, age in controlling X-ray properties

Preibisch et al. 2005

BDs in Orion t~1 MYr

Field objectst>500 MYr

No dramatic changes in magnetic activity observables, despite large

differences in stellar properties

West et al. 2004

Detailed Investigation of Radio Properties: TVLM513-46456 (M9 V)

Osten et al. 2006

Radio spectral properties consistent with behavior of earlier spectral type active M stars: magnetic activity present at levels comparable with extrapolations

Detailed Investigation of Radio Properties: Compare with EV Lac (dM3e)

Osten et al. 2006, ApJ, accepted

Radio Emission from Young Brown Dwarfs

TWA 5B; ~10 Myr old

Tsuboi et al. 2003

Fossil magnetic fields probably play a significant role in X-ray activity (Feigelson et al. 2002).

Radio Emission from Young Brown Dwarfs

Radio upper limits on all 4 substellar members of the nearest youngest association TW Hydrae (Burgasser & Putman 2005, Osten & Jayawardhana 2006)

Radio Emission from Young Brown Dwarfs

Lr/Lx of TWA 5B anomalous behavior compared to field brown dwarfs

TWA 5B

Berger 2005

Caution: Lx/Lr relationship for active stars still not well understood

age

rotation

Mass, temperature(Spectral type)

Some possibly importantmeasurable parameters

Radio

H

UV

X-ray

Steady

Flaring

Why is this so difficult?

Consistent behavior?

Future Work• What kind of magnetic activity survives in ultracool

objects, and how cool? transient H emission out to L5

to date, no X-ray detection of L dwarfs,

radio detections up to L4

is plasma heating to chromospheric temps. easier than to coronal temps?relative ease/efficiency of particle acceleration vs. plasma heating?

• What is the systematic behavior of magnetic activity as a function of age, mass, temperature, rotation?

Need large samples with known properties

• What do the different activity observables tell us about the nature of the activity?

In-depth investigation of magnetic activity properties to discern differences from active stars