Signatures of Magnetic Activity in Very Low Mass Stars and Brown Dwarfs

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Signatures of Magnetic Activity in Very Low Mass
Stars and Brown Dwarfs

• Rachel Osten
• Hubble Fellows Symposium

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Collaborators
Suzanne Hawley University of Washington Tim
Bastian NRAO Neill Reid STScI Ray
Jayawardhana U. Toronto
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Magnetic Activity manifestationsin Solar-like
Stars
Persistent transient mag. activity
Scaling laws constrain heating processes
Ha emission (104K)
Coronal emission (106K)
Radio radiation (nonthermal radiation)
sunspots
White 2002
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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

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The Party Line
West et al. (2004)

• Projected rotation
• velocities increase from
• mid-M to L field dwarfs
• Marked decrease in numbers of objects showing Ha
  in emission (active fraction)
• Breakdown in rotation-activity connection for
  ultracool stars brown dwarfs magnetic activity
  is dying

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Activity Signatures Are Seen (Sporadically) in
Late-M/L Dwarfs
X-ray emission Stelzer (2004) BD pair Ba 55-87
Mjup Bb 34-70 Mjup
steady and variable (flaring) levels of emission
? Ha emission 2MASS 0144-0716 (L5V Liebert et
al. 2003)
Radio emission Berger 2002 2MASS 003618 (L3.5)
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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
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Teff more important than mass, age in controlling
X-ray properties
Field objects tgt500 MYr
BDs in Orion t1 MYr
Preibisch et al. 2005
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No dramatic changes in magnetic activity
observables, despite large differences in stellar
properties
West et al. 2004
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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
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Detailed Investigation of Radio Properties
Compare with EV Lac (dM3e)
Osten et al. 2006, ApJ, accepted
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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).
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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)
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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
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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?
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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