Young Stars I,II

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Young Stars I,II
Lee Hartmann, Smithsonian Astrophysical
Observatory

• magnetic flux and primordial stellar fields
• infall and disk accretion
• magnetic fields and turbulence in disks
• winds/jets
• magnetospheric accretion and stellar spindown

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Stars form from the collapse of protostellar gas
clouds, r ? 104 AU
optical
infrared
Alves, Lada Lada 2001
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The magnetic flux problem
(Mestel Spitzer)
For gravity to overcome magnetic pressure
GM2 gt () B2 R4 () ?c
Flux-freezing ? ? const (plasma drift t 106
yr, free-fall t 105 yr)
no reason to expect ltlt ?c (equipartition)
R 1017 cm R 1011 cm conserve BR2 Bo
10-5 G, ? B 107 G!
Why? low ionization at high ? as collapse
proceeds, so flux-freezing is not a good
approximation (Umebayashi Nakano 1988)
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The angular momentum problem
R 1017 cm R 1011 cm conserve angular
momentum during (nearly) free-fall collapse ? R2?
? constant R(final)/R ? (?o/?K)2
Stars must form from disk accretion (magnetic
flux loss in low-ionization disks)
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• molecular cloud core undergoes free-fall
  collapse to protostar with disk and jet

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Why do disks accrete?
Hydrodynamic exchange? Doesnt seem to work
Gravitational instability? May work requires
massive disk
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Magnetorotational Instability?
(Balbus Hawley) Consistent with ? disk
formalism (B Papaloizou)
Disks with very low initial B ? dynamo activity ?
MRI!
But dusty protostellar disks have VERY LOW
ionization ?B doesnt couple to gas
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Stone, Balbus, Hawley, Gammie 1996
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BUT low ionization ? no magnetic viscosity ? no
accretion!
T Tauri disk (model)
Does any primordial magnetic flux survive infall
to disk? Even if it does, can it survive ohmic
diffusion in disk? What does the turbulence in
MRI do? Can there be any highly organized fossil
field in A(p) stars?
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Fleming Stone 2003 Simulation of shearing box
with dead zone MRI operates only in upper
layers, but Reynolds stress extends into
midplane ? Dead zone somewhat active, can
accrete?!
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Disk accretion can be highly time-variable, with
short bursts of very rapid accretion.
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FU Ori outburst of disk accretion

• Disk accretion ? 10-7 - 10-8M?/yr ? protostar
• ??
• Disk accretion ? 10-4M?/yr ? FU Ori object

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Why unsteady accretion?
Infall to disk high velocity disk accretion low
radial velocity ? no reason to balance!

• if dM/dt (infall) gt dM/dt (accretion)
• onto disk onto star
• mass buildup ? eventual rapid disk accretion

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Outburst sequence (Armitage et al. 2002 Gammie
Hartmann 200?)
matter builds up in dead zone
mass added at outer edge (infall)
Grav. Instability ? accretion heating ? thermal
ioniz. ? rapid accretion
rapid accretion triggers thermal instability in
innermost disk
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What happens to the star??

• During FU Ori outburst, L(acc) 100 L
• Likely advection of large amounts of thermal
  energy, (Popham et al 1996) ? star expands (but
  relaxes quickly if only 0.01 M? is added in each
  outburst?)
• Rapid episodic accretion may be typical of the
  earliest phases of protostellar formation

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Magnetic fields CAN couple to protostellar
disks Jets/Winds

• Thermal pressure too low to accelerate flows
• Radiation pressure negligible
• Collimation!

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bead on a wire analogy
collimation
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Accretion leads to ejection
dM/dt (wind) 0.1 dM/dt (acc)
Calvet 1997
Accretion power drives strong mass loss (NOT
stellar winds! Stars without disks do not show
detectable mass loss)
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FU Ori disk winds
disk rotation
Hartmann Calvet (1995) accelerating disk wind
results in shifts increasing with increasing
strength (upper levels)
Petrov Herbig 1992
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Winds and turbulence
FU Ori winds are extremely time-variable
consistent with complex disk magnetic field
geometry
Blandford Payne 1982
FU Ori winds must be heated to explain H?, etc
numerical simulations of MRI show waves
propagating upward and shocking
Miller Stone 2002
Atmospheric absorption line profiles show
evidence for sonic turbulence (Hartmann, Hinkle
Calvet 04)
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IMTTS predecessors of the HAeBe
HAe/Be
T Tauri stars CTTS accreting WTTSnot acc.
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T Tauri (FGKM) pre-main sequence stars with disks
Hartmann 1998
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V410 Tau
T Tauri star spots (cool) BIG! (large stellar B)
Stelzer et al. 2003
(stellar luminosity perturbed? Rosner
Hartmann - observational problems
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Proxies for magnetic fields (activity) enhanced
in pre-main sequence stars - saturated behavior
(i.e. not strongly rotation-dependent)
Chromospheric fluxes
X-ray fluxes
(accretion)
Walter et al. 1988
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Orion Nebula cluster stars (ages 1 Myr)
Flaccomio et al. 2003
Saturation B or heating efficiency?
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T Tauri magnetic fields
BP Tau Longitudinal (circular polarization)
photospheric B lt 200 G Mean Zeeman broadening
2.8kG ? cancellation! Circular polarization of
He I emission (magnetospheric) 2.5 kG
Johns-Krull et al. 1999, 2001
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Summary of magnetic properties of pre-main
sequence stars

• Spot areas gt 30 of stellar surface
  (non-axisymmetric part)
• Measured field strengths 2kG (average over
  visible surface!)
• Circular polarization low ? cancellation
  (complex structure)
• Magnetic activity strongly enhanced from solar,
  saturated

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• Why magnetospheric accretion?

• Hole in inner disk (Bertout, Basri, Bouvier
  1988)
• Periodic modulation of light from hot spots
  (BBB)
• High-velocity infall (Calvet, Edwards, Hartigan,
  Hartmann)
• Stellar spindown through disk locking (Königl
  1991) (?)
• Stellar magnetic fields several kG, strong
  enough to disrupt disks (e.g., Johns-Krull,
  Valenti, Koresko 1999)

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• Magnetospheric accretion line profiles

(Muzerolle et al. 1998) line width ? (2GM/R)1/2
Königl 1991
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• Models for magnetospheric emission

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• Circularly polarized He I emission

LCP
RCP
Johns-Krull et al. 1999
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• Accretion power in T Tauri Stars

Classical TTS
Bertout et al. 88 Kenyon Hartmann 87 Hartigan
et al. 90,91 Valenti et al. 93
Weak TTS
Blue excess (veiling) continuum can be gt L ?
not stellar magnetic activity, but accretion
powered inner disks (IR emission) ?? veiling ?
accretion
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Magnetospheric accretion and outflow
Numerical simulations show complex accretion
pattern, not always polar, even when pure aligned
dipole (Miller Stone 1997)
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Tilted dipole ? asymmetric streams of
accretion But we dont see implied strong
variations of line profiles. Geometry must be
more complicated.
Romanova et al. 2003, 2004
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• Complex magnetosphere?

• Continuum emission (Calvet Gullbring 1998)
• very small ( 1 ) covering factors
• high dM/dt ? larger covering factor on star
• Line emission (Muzerolle et al)
• high dM/dt ? larger magnetosphere area
• ? ? Flux tube accretion

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The angular momentum problem
If stars accrete most of their mass from disks,
why arent they rotating rapidly? dJ/dt loss in
wind? But then dont get spin-up to main
sequence (Pleiades) Solution transfer J to disk
with B (disk-locking) (??)
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Why do young stars rotate so slowly if they are
formed from disk accretion?
And why faster for lower-mass stars??
Clarke Bouvier 2000
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Disk-star magnetic coupling does it work?
accreting
non-accreting
Taurus accreting stars (stars with disks) rotate
more slowly (Bouvier et al., Edwards et al. 1993)
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Why do young stars rotate so slowly if they are
formed from disk accretion?
Bimodal? (Herbst et al. 2002)?? (should plot in
log P)
Note wide range
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The angular momentum problem
Accretion implies J(disk) ? J(star) how to get
rid of it?
Solution 1 different field lines problem field
lines wind up unless perfect slippage
(Collier Cameron Campbell)
Solution 2 exact co-rotation, no winding
problem unrealistic (axisymmetric,
etc.) detailed assumptions not very clear
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The angular momentum problem
Shu et al. funnel flow x-wind
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Lovelace, Romanova, Bisnovatyi-Kogan 1995
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Disk-star magnetic coupling

• Generally, field lines wind up
• ? accretion and spindown alternate?
• intermingled accreting flux tubes with spindown
  field lines?
• limits spindown too much? (Matt Pudritz 2004)

Reconnection? Flares? Not clear that accreting
TTS have more activity than non-accreting (weak)
TTS(n.b. Need to heat accreting loops somehow)
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Disk dynamo? Opposed field to star? Accretion,
spindown oscillatory
von Rekowski Brandenburg 2004 also Goodson,
Winglee, Matt
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Disk-star magnetic coupling does it work?
To spin down star, either wind or disk must carry
away the stellar J!
? k2 (M/dM/dt) (?/?K) (R/Rco)1/2 ? 0.2 ?
108 yr ? (?/?K) / 2 so either slow rotation or
need very high dM/dt to spin down in 106 yr
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Disk-star magnetic coupling does it work?
or?? coronal mass ejection-type loss, except
using disk material??
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Bouvier et al. 1997
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Questions about young stars

• How does the dynamo work in young, completely
  convective stars?

• How are magnetic fields distributed over
  surfaces of young stars? What happens to surface
  convection, etc. when PB Pg (photospheric)
  everywhere??

• Why is activity saturated?

• How is stellar angular momentum regulated?

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