1. Rachel Osten Hubble Fellows Symposium
Signatures of Magnetic Activity in Very Low Mass Stars and Brown Dwarfs
Rachel Osten
Hubble Fellows Symposium

2. Collaborators Suzanne Hawley University of Washington Tim Bastian NRAO
Neill Reid STScI
Ray Jayawardhana U. Toronto

3. Transition to completely convective interior happens at ~M3
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

4. 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 Mjup
Bb Mjup
 Ha emission
2MASS (L5V; Liebert et al. 2003)
Radio emission
Berger 2002; 2MASS (L3.5)

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

6. 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

7. No dramatic changes in magnetic activity observables, despite large differences in stellar properties
West et al. 2004

8. 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

9. Detailed Investigation of Radio Properties: Compare with EV Lac (dM3e)
Osten et al. 2006, ApJ, accepted

10. 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).

11. 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)

12. 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

13. 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?

14. 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