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Accretion of brown dwarfs: Clues from spectroscopic variability

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'T Tauri has a magnitude range of from 9.4 to 13.5 or 14, but ... solar-type ?-dynamo, only small-scale magnetic fields? inefficient wind braking. fast rotation ... – PowerPoint PPT presentation

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Title: Accretion of brown dwarfs: Clues from spectroscopic variability


1
Accretion of brown dwarfs Clues from
spectroscopic variability
Alexander Scholz (University of Toronto)
Ray Jayawardhana (UoT) Jochen Eislöffel
(Tautenburg) Alexis Brandeker (UoT)
2
Brown dwarfs in T Tauri phase
Accretion flow ? variability
3
T Tauri stars are variable
T Tauri has a magnitude range of from 9.4 to
13.5 or 14, but no regular period has as yet been
detected. (Knott, 1891)
4
Example RW Aur 1945-2005
Alencar et al. 2005
Joy 1945
5
Monitoring brown dwarfs
Long-term project five years 1999-2004 of deep
photometric monitoring in young open clusters
with ages from 3 to 700 Myr Using data from ESO,
TLS, Calar Alto Main goal evolution of rotation
and activity for Mlt0.3Ms
6
T Tauri lightcurves
11 objects with large amplitudes, partly
irregular variability ? typical T Tauri
lightcurves
Scholz Eislöffel, AA, 2005
7
Brown dwarfs have accretion disks
Muzerolle et al. 2003
Natta et al. 2002
Scholz Eislöffel 2005
Jayawardhana et al. 2003
8
Origin of variability
shockfront (hot gas) rotation instabilities
strong, irregular variability (in photometry and
emission lines)
9
Spectroscopic monitoring
three observing runs with MIKE/Magellan,
Jan-March 2005 six targets accreting brown
dwarfs in star forming regions Ha line s(EW)
22-90 s(10width) 4-30
10
2M1207
Brown dwarf at 8 Myr with wide, planetary-mass
companion No NIR (but MIR) colour excess, clear
signature of accretion and wind Final stage of
accretion?
11
Profile Variability
4 hours
4 hours
broad emission plus redshifted absorption feature
absorption disappears and re-appears on
timescales of 1 day
Scholz, Jayawardhana, Brandeker, ApJL, 2005
12
Interpretation Accretion flow
cool, infalling, co-rotating material ?
accretion column ? close to edge-on geometry ?
asymmetric flow geometry
Scholz, Jayawardhana, Brandeker, ApJL, 2005
13
ISO217
Scholz Jayawardhana, ApJ, 2006
profile asymmetry AND profile variability ?
nonspherical accretion ? indirect evidence for
magnetically funneled flow
14
Linewidth variations for 2M1207
variations in the linewidth by 30 on a
timescale of 6 weeks
Scholz, Jayawardhana, Brandeker, ApJL, 2005
15
2M1101-7718
8 hours
24 hours
10 width 122
232
194 km/s EW 12
92
126 Å other lines
HeI,CaII,Hß
HeI,CaII,Hß,H?
strong variations of the accretion-related
emission lines
Scholz Jayawardhana, ApJ, 2006
16
Accretion rate variations
Accretion rate changes by 0.5-1 order of
magnitude in 2M1207 and 2M1101 ? clumpy,
unsteady accretion flow
Natta et al. (2004)
17
Accretion rate vs. mass
Mohanty et al. (2005)
Natta et al. (2004)
accretion rate vs. object mass M2 large
scatter influenced by variability? ?
variability studies essential to study accretion
fundamentals
18
Most important conclusion
Keep an eye on them...
19
... because you never know
20
Conclusions
1. Brown dwarfs show T Tauri like
variability 2. monitoring allows
close-up view on accretion behaviour
3. strong accretion rate variations up to one
order of magnitudes on timescales of
days to weeks 4. profile asymmetry and
variability evidence for asymmetric
flow (large-scale magnetic fields?)
21
Brown dwarfs in T Tauri phase
Accretion flow 1st part Variability
Dusty disk 2nd part mm/MIR SEDs
22
VLM rotation periods
Scholz Eislöffel AA, 2004, 419, 249 AA,
2004, 421, 259 AA, 2005, 429, 1007 PhD thesis
A. Scholz
2003 6 periods (squares) 2004 80 periods
(large dots)
23
Dusty disks
Mohanty et al. (2004)
Pascucci et al. (2003)
constraints from MIR-SEDs flared disk, flat
disks, grain growth but only 1 object with SED
from NIR to mm
24
Connection to disk accretion
near-infrared colour excess, emission line
spectrum ? variability related to accretion from
circumsubstellar disk
Scholz Eislöffel, AA, 2004
25
A 1.3mm survey in Taurus
IRAM 30m telescope with MAMBOII, Pico Veleta
(Spain)
20 sources with SpTgtM6, noise level lt1mJy for all
objects
26
Fluxes and disk masses
20 sources, 6 detections, flux levels lt0.7... 7
mJy transformation to disk masses lt0.4... 2.4
Jupiter masses
Scholz, Jayawardhana, Wood, ApJ, 2006
27
Disk mass vs. object mass
Relative disk masses comparable from 0.02 to 3
Ms No trend to lower disk masses in the brown
dwarf regime
28
Enter Spitzer
IRAC photometry 3-8 µm IRS spectroscopy 8-13
µm MIPS photometry 24 µm
IRACMIPS available for all Taurus sources NIR
(2MASS) MIR (Spitzer) mm (IRAM)
29
SED modeling
Minimum outer disk radius for objects with mm
detection 10 AU gt25 of the disks have radii
gt10AU NO evidence for truncated disks Ejection
excluded as dominant formation mechanism
Scholz, Jayawardhana, Wood 2006
30
Evolution of brown dwarf disks
Spitzer GO program to study 35 brown dwarf disks
in Up Sco IRS spectroscopy MIPS
photometry Inner disks and chemistry after 5
Myr All observations finished
31
Young and old
Taurus
UpSco
2Myr - strongly flared disk
5Myr - dust settling
more to come!
32
Origins of brown dwarfs
in situ formation - ultra-low-mass stars -
ejection as embryos - failed stars -
signature of formation binarity, kinematics,
accretion disks
33
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.
34
Case study TWA5A
Brandeker et al. 2003
close binary, at least one of the components is
accreting Aa Ab one solar mass
35
Ha variability of TWA5A
dashed broad dotted narrow
both components contribute to flare event -
delay of broad component?
profile decomposition broad and narrow component
Jayawardhana, et al., ApJL, in prep.
36
Velocity variations
broad P 19.6 h, FAP 0.004
narrow P 19.2 h, FAP 0.8
comparable periods in both components possible
scenario rotation period of Aa or Ab ? two
co-rotating spots from accretion and activity
Jayawardhana, et al., ApJ, in prep.
37
Spot properties
Scholz, Eislöffel Froebrich, 2005, AA, 438, 675
cool spots, either symmetric distribution or low
spot coverage ? indication for a change in the
magnetic field generation
38
Amplitudes vs. mass
Amplitudes in young open clusters
VLM objects low amplitudes, low rate of active
objects ? change in spot properties
39
Period vs. Mass
ONC Herbst et al. (2002)
Scholz Eislöffel, AA, 2004, 2005
VLM objects rotate faster than solar-mass
stars average period correlated with mass
40
Period vs. Mass I
Pleiades ( literature)
IC4665 ( literature)
VLM objects rotate faster than solar-mass stars
41
Period vs. Mass II
Pleiades ( Terndrup et al.)
IC4665
VLM regime period decreases with mass
42
Period vs. Mass III
sOri Herbst et al. (2001) eOri
Herbst et al. (2001)
Median period decreases with mass, even at very
young ages
43
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
44
Interior structure
VLM object
solar-type star
radiative zone
fully convective
Consequences for magnetic fields, activity,
rotation
45
Rotation and stellar evolution
Disk locking
Stellar winds
Bouvier et al. 1997
46
Stellar winds
SOHO
TRACE
47
The clusters
sOri, eOri 3-10 Myr Scholz Eislöffel, AA,
2004, 419, 249 Scholz Eislöffel, AA, 2005,
429, 1007
Pleiades 125 Myr Scholz Eislöffel,
AA, 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
48
Lightcurves
VLM star in the Pleiades
Brown Dwarf in eOri
90 of all variable objects regular, periodic
variability
49
Period vs. Mass II
Pleiades ( Terndrup et al.)
IC4665
VLM regime period decreases with mass
50
Models
  • Period evolution between 3 and 750 Myr determined
    by
  • hydrostatic contraction
  • rotational braking by stellar winds
  • disk-locking (not important)
  • P(t) a(t) (R(t)/Ri)2 Pi
  • A) a(t) const. 1 only contraction
  • B) a(t) (t / ti ) ½ Skumanich law (dL/dt
    ?3)
  • C) a(t) exp((t ti) / ?) exponential braking
    (dL/dt ?)

51
Surface features Magnetic spots
Amplitudes of variability determined by spot
properties
52
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)
53
Disks around VLM objects
Colour-colour diagram
Optical spectroscopy
NIR colour excess
Strong emission lines but
disk frequency only 5-15 in ?Ori cluster
54
Accretion vs. rotation
Basri, Mohanty Jayawardhana, in prep.
Scholz Eislöffel 2004
55
Breakup period
models not adequate for fastest rotators
56
Rotational evolution
57
Only contraction
angular momentum loss necessary to explain slow
rotators
58
Contraction Skumanich
Skumanich braking is too strong
59
Contraction exponential braking
best agreement of model and observations
60
Multi-filter monitoring
Calar Alto Observatory, 1.2m and 2.2m telescope
simultaneous monitoring with two telescopes in I,
J, H
61
Magnetic field generation
Fully convective objects no interface layer ?
solar-type ??-dynamo, ? only small-scale magnetic
fields? inefficient wind braking ? fast
rotation symmetric spot distribution ? small
amplitudes
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