Download Accretion of brown dwarfs

Accretion of brown dwarfs
Clues from spectroscopic variability
Alexander Scholz
(University of Toronto)
Ray Jayawardhana, Alexis Brandeker,
Jaime Coffey, Marten van Kerkwijk
(University of Toronto)
Outline
1. Variability as a tool: Rotation, spots, activity
2. Accretion: Clues from emission line variations
Case studies: 2M1207, 2M1101, TWA5A
3. General implications:
Accretion from solar-mass stars to brown dwarfs
Photometric monitoring
Conclusions about rotation, spots, magnetic activity
Photometric rotation periods
solar-mass stars: ~2000
very low mass objects: ~500
Period vs. Mass
ONC: Herbst
et al. (2002)
Scholz & Eislöffel,
A&A, 2004, 2005
VLM objects rotate faster than solar-mass stars
average period correlated with mass
VLM rotation periods
Scholz & Eislöffel:
A&A, 2004, 419, 249
A&A, 2004, 421, 259
A&A, 2005, 429, 1007
PhD thesis A. Scholz
2003: 6 periods (squares)
2004: 80 periods (large dots)
Amplitudes vs. mass
Amplitudes in
young open
clusters
VLM objects: low amplitudes, low rate of active objects
 change in spot properties
Spot properties
Scholz, Eislöffel &
Froebrich, 2005, A&A,
438, 675
cool spots, either symmetric distribution or low spot coverage
 indication for a change in the magnetic field generation
High-amplitude variability
11 objects with large amplitudes, partly irregular variability
 `T Tauri lightcurves` - produced by accretion in hot spots
Scholz & Eislöffel, A&A, 2005
Accretion disk
Spectroscopic monitoring
How to get from flux(,t) to flux(x,y,z)?
degenerated problem: necessarily of speculative nature
Case study: 2M1207
Brown dwarf at 8 Myr with
wide, planetary-mass
companion
No NIR colour excess, but
clear signature of accretion
and wind
Final stage of accretion?
Profile Variability
4 hours
4 hours
broad emission plus redshifted absorption feature
cool, infalling material, co-rotating  accretion column
close to edge-on geometry, asymmetric flow geometry
Scholz, Jayawardhana, Brandeker, ApJL, 2005
Linewidth variations
variations in the linewidth by ~30% on a timescale of 6 weeks
Scholz, Jayawardhana, Brandeker, ApJL, 2005
Accretion rate variations
Natta et al. (2004)
Accretion rate changes
by ~one order of
magnitude in
2M1207 and 2M1101
Case study: 2M1101-7718
8 hours
10% width: 122
EW:
12
other lines:
24 hours
232
92
+HeI,CaII,Hβ
194 km/s
126 Å
+HeI,CaII,Hβ,Hγ
strong variations in the accretion rate, evidence for clumpy flow
Scholz & Jayawardhana, ApJ, 2006
Case study: TWA5A
Brandeker et al. 2003
close binary, at least one of the components is accreting
Aa + Ab (+ Ac?) = one solar mass
Hα variability of TWA5A
dashed: broad
dotted: narrow
profile decomposition:
broad and narrow
component
both components contribute to „flare“
event - delay of broad component?
Jayawardhana, et al., ApJL, in prep.
Velocity variations
broad: P = 19.6 h, FAP = 0.004%
narrow: P = 19.2 h, FAP = 0.8%
comparable periods in both components
either rotation period of Aa or Ab  hot and cool spots
or orbital period of a third body Ac
Jayawardhana, et al., ApJ, in prep.
Accretion flow geometry
Scholz & Jayawardhana,
ApJ, 2006
profile asymmetry AND profile variability
 nonspherical accretion
 indirect evidence for magnetically funneled flow
Young stars and variability
H linewidths for stars in young
associations (age 6-30 Myr)
`errorbars` show scatter over
multi-epoch observations
 variability common
phenomenon in young stars
Jayawardhana et al.,
ApJ, in prep.
Accretion rate vs. mass
Mohanty et al. (2005)
Natta et al. (2004)
accretion rate proportional to object mass
large scatter mainly due to variability
Most important
conclusion:
Keep an eye
on them...
... because you
never know
Conclusions
1. Photometric variability:
primary tool to study stellar rotation and activity
- positive correlation between rotation period and mass
- rotational evolution determined by contraction + winds
- change of dynamo in very low mass regime
2. Spectroscopic variability:
close-up view on accretion behaviour
- strong accretion rate variations in stars and brown dwarfs
- evidence for asymmetric flow geometry
Outlook: Spitzer
Spitzer provides means to
study the dust in the inner part
of the disk
GO program for 35 brown
dwarfs in UpSco:
- IRS spectra from 8-14 m
- MIPS photometry at 24 m
Dusty disks of brown dwarfs
without disk
with disk
more to come!
Period vs. Mass I
Pleiades (+ literature)
IC4665 (+ literature)
VLM objects rotate faster than solar-mass stars
Period vs. Mass II
Pleiades (+ Terndrup et al.)
IC4665
VLM regime: period decreases with mass
Period vs. Mass III
σOri + Herbst et al. (2001)
εOri + Herbst et al. (2001)
Median period decreases with mass, even at very young ages
The physics of VLM objects
0.35 MS objects are fully convective
0.15 MS degeneracy pressure dominates
(radius independent of mass)
0.075 MS no stable hydrogen burning
(substellar limit)
0.060 MS only deuterium burning
0.013 MS no deuterium burning
Interior structure
solar-type star
VLM object
radiative zone
fully convective
Consequences for magnetic fields, activity, rotation
Rotation and stellar evolution
´Disk locking´
Bouvier et al. 1997
Stellar winds
Stellar winds
SOHO
TRACE
The clusters
σOri, εOri
3-10 Myr
Pleiades
125 Myr
Scholz & Eislöffel,
A&A, 2004, 419, 249
Scholz & Eislöffel,
A&A, 2005, 429, 1007
Scholz & Eislöffel,
A&A, 2004, 421, 259
Praesepe
700 Myr
IC4665
36 Myr
Eislöffel & Scholz 2002,
ESO-Conf.
Time series imaging with TLS Schmidt, ESO/MPG WFI, Calar Alto
1Myr
10Myr
100Myr
1Gyr
Lightcurves
VLM star in the Pleiades
Brown Dwarf in εOri
90% of all variable objects: regular, periodic variability
Period vs. Mass II
Pleiades (+ Terndrup et al.)
IC4665
VLM regime: period decreases with mass
Models
Period evolution between 3 and 750 Myr determined by…
- hydrostatic contraction
- rotational braking by stellar winds
- disk-locking (not important)
 P(t) = α(t) (R(t)/Ri)2 Pi
A) α(t) = const. = 1
B) α(t) = (t / ti ) ½
C) α(t) = exp((t – ti) / )
only contraction
Skumanich law (dL/dt ~ω3)
exponential braking (dL/dt ~ ω)
Surface features: Magnetic spots
Amplitudes of variability determined by spot properties
Spot configuration
How do the surfaces of VLM objects look like?
b) Only polar spots
c) Low spot coverage
d) High symmetry
e) Low contrast
Lamm (2003)
Barnes & Collier Cameron (2001)
Disks around VLM objects
Colour-colour diagram
NIR colour excess
Optical spectroscopy
Strong emission lines
but: disk frequency only 5-15% in Ori cluster
Accretion vs. rotation
Basri, Mohanty &
Jayawardhana, in prep.
Scholz & Eislöffel 2004
Breakup period
models not adequate for fastest rotators
Rotational evolution
Only contraction
angular momentum loss necessary to explain slow rotators
Contraction + Skumanich
Skumanich braking is too strong
Contraction + exponential braking
best agreement of model and observations
Multi-filter monitoring
Calar Alto
Observatory,
1.2m and 2.2m
telescope
simultaneous monitoring with two telescopes in I, J, H
Magnetic field generation
Fully convective objects:
no interface layer
 solar-type ω-dynamo,
 only small-scale
magnetic fields?
inefficient wind braking
 fast rotation
symmetric spot distribution
 small amplitudes
Spectroscopic monitoring
accretion = strong emission line variability
Hα line: σ(EW) = 22-90%
σ(10%width) = 4-30%