Signatures of Magnetic Activity in Very Low Mass Stars and Brown Dwarfs Rachel Osten Hubble Fellows Symposium
Collaborators Suzanne Hawley University of Washington Tim Bastian NRAO Neill Reid STScI Ray 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, since • no aw dynamo can operate; not observed • See same manifestations of magnetic activity (Ha, X-ray, radio emission) in some late M/L objects as in solar-like active stars • Cool, neutral, high density atmospheres have large resistivity, magnetic fields and matter are decoupled
Activity Signatures Are Seen (Sporadically) in Late-M/L Dwarfs steady and variable (flaring) levels of emission X-ray emission Stelzer (2004) BD pair: Ba 55-87 Mjup Bb 34-70 Mjup Ha emission 2MASS 0144-0716 (L5V; Liebert et al. 2003) Radio emission Berger 2002; 2MASS 0036+18 (L3.5)
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 Ha, X-ray decline in emission with decreasing Teff Average radio properties Osten et al. 2006
Teff more important than mass, age in controlling X-ray properties Field objects t>500 MYr BDs in Orion t~1 MYr Preibisch et al. 2005
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 Tsuboi et al. 2003 TWA 5B; ~10 Myr old 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 Caution: Lx/Lr relationship for active stars still not well understood Berger 2005
Why is this so difficult? age Some possibly important measurable parameters Mass, temperature (Spectral type) rotation Radio Ha UV X-ray Steady Flaring Consistent behavior?
Future Work • What kind of magnetic activity survives in ultracool objects, and how cool? transient Ha 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