Title: Signatures of Magnetic Activity in Very Low Mass Stars and Brown Dwarfs
1Signatures of Magnetic Activity in Very Low Mass
Stars and Brown Dwarfs
- Rachel Osten
- Hubble Fellows Symposium
2Collaborators
Suzanne Hawley University of Washington Tim
Bastian NRAO Neill Reid STScI Ray
Jayawardhana U. Toronto
3Magnetic 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
4Sun 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
5The 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
6Activity 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)
7How 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
8Teff more important than mass, age in controlling
X-ray properties
Field objects tgt500 MYr
BDs in Orion t1 MYr
Preibisch et al. 2005
9No dramatic changes in magnetic activity
observables, despite large differences in stellar
properties
West et al. 2004
10Detailed 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
11Detailed Investigation of Radio Properties
Compare with EV Lac (dM3e)
Osten et al. 2006, ApJ, accepted
12Radio 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).
13Radio 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)
14Radio 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
15Why is this so difficult?
age
Some possibly important measurable parameters
- Mass, temperature
- (Spectral type)
rotation
Radio Ha UV X-ray
Steady Flaring
Consistent behavior?
16Future 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