Accretion onto
Magnetized Stars
Marina Romanova, Cornell University
Collaborators: R. Lovelace, A. Koldoba, G. Ustyugova , A. Kulkarni, M.
Long, R. Kurosawa, M. Bachetti, J.-F. Donati
14 Jul. 2010
Accreting Magnetized Stars:
Young stars: B=103 G, R=1011
cm
White dwarfs: cataclysmic
variables –Intermediate polars
B=106-108 G, R=5x108 cm
Neutron stars: accreting millisecond
pulsars, B=108-109 G, R=106 cm
X-ray pulsars: B=1012 G -larger
magnetospheres
Black holes: variable magnetospheres
Different problems and outline
1.Magnetospheric flow
2. Weak B-field, MBL
3. Outflows and jets
Different codes, 2D, 3D
Grids: spherical, cylindrical,
Cubed sphere
I. a- disks (40 years). avis and adif
Matt & Pudritz 2005
- Wind-driven accretion
- Spiral waves: a.m. transport
- Partially ionized disks
- Models describe MHD flow
- Different instabilities
- Variability, X-ray
II. MRI disks
3D simulations of accretion onto tilted dipoles
Two funnel streams
Two hot spots
Ordered light-curve
Hot spots – slice
across the funnel
stream
4
Romanova, Ustyugova, Koldoba & Lovelace 2003,2004
Spectral analysis: 3D radiative transfer, TORUS
φ=0
φ=0.25
φ=0.5
φ=0.75
Paβ emission map (upper) and profiles (lower)
Kurosawa, Romanova & Harries, MNRAS, 2008
Comparisons with accreting tilted BHs
Matter flow around tilted rotating
magnetized star Romanova et al. 2003
Matter flow around tilted rotating BH
Fragile, Blaes, Anninos, Salmonson 2007
In both cases matter chooses energetically favorable path
Matter has tendency to flow in magnetic/BH equator
(disk is warped)
3D simulations, larger scale, waves, QPOs
Stable or unstable regimes:
Chaotic light-curve, QPO oscillations
Next talk
Predicted: Arons & Lea 1976; Lamb 1976
Global simulations: Kulkarni & Romanova 2008; Romanova, Kulkarni, Lovelace 2008
Case of very small tilt: Q=2-5o
QPO oscillations at different frequencies
Spots rotate faster/slower than the star
RXTE, future IXO
Bachetti, Romanova, Kulkarni, Burderi & Di Salvo 2009
3D simulations of MRI-driven accretion, Q=30o
B=0
B=0
Large-scale turbulence is observed like in case of nonmagnetic star (e.g., Hawley 2000)
Low-m spiral modes
Romanova et al. (in prep)
3D view of MRI-driven Accretion
Matter accretes in funnel streams
Funnels form episodically
Variability is higher than in case of the laminar flow
Romanova et al. (in prep)
2D simulations of MRI-driven Accretion
Rcor=6
T=15 rotations at r=30
B
B
Different
configurations of the
seed poloidal field in the
disk:
- Parallel field
-Parallel with cut
- Different orientation
- Loops
animation
MRI-driven accretion: Balbus & Hawley 1991, 1998; Hawley & Balbus 1999;
Stone, Howley, Gammie, Balbus 1995; Hawley & Krolik 2001, and more
Romanova et al. (in prep.)
Episodic accretion in case of parallel fields
Observations of
accreting millisecond
pulsar
SAX J1808.4-3658
Simulations of the case
Bdisk
Bdip
Simulations of the case
Bdisk
Bdip
Romanova et al. (in prep)
Variability in CTTS
TW Hya
DF Tau
Rucinski et al. 2008
MOST: Microvariability & Oscillations of Stars (dT=100 min)
Variability is not very high, factor of x1.5
Usually CTTS vary 0.1m-0.5m on the dynamical time-scale
Reconnection – heating, particles acceleration
e, g
e-e+
e-i
Spectrum: power law (R&L 1992)
Large Flare (CTTS):
3.3x1031 erg/s
Particles are accelerated in the current
layer (Alfven 1968;
Romanova & Lovelace 1992)
2. Magnetic boundary layer
Tiny magnetosphere forms
Ordered spots, frequency n=nstar
Romanova & Kulkarni 2009
Unstable regime
Spots rotate in the equatorial plane
QPO, frequency of the inner disk
(millisecond pulsars, Dwarf Novae)
3. Winds and Jets
I.
avis = adif
avis >> adif
II.
outflows
Matter inflows with the same rate as the Matter inflows faster than the field
magnetic field diffuses out
diffuses out
Accretion, no outflows
Accretion and outflows
Field lines inflate
c
c
Wstar
Wdisk
animation
19
Conical Winds formation
avis=0.3 adif=0.1
Matter flux, velocity, field lines
Compression of the magnetosphere (Shu et al. 1994)
Magnetic force drives outflows
Formation of Conical Winds
Applications: EXORs, millisecond pulsars, CVs, BHs, more
Propeller regime
Poynting Jet
Two-component outflow forms
Conical winds carry most of matter outwards
Poynting jet carries energy and ang. momentum
Romanova et al. 2005; Ustyugova et al. 2006; Romanova et al. 2009
22
3D rendering
Lower speed
Higher speed
B-lines
GR simulations of outflows from BHs
a/M=0.5
a/M=0.998
Poynting flux jet increases with a/M
Krolik, Hawley, Hirose 2004
Poloidal current increases with a/M
Hirose, Krolik, De Villiers, Hawley 2004
Propeller Case
Simulations:
7 years
Major outbursts:
2 months
HST
Observations:
25
Cycle of inflation
Ustyugova et al. 2006
HH30
Flaring matter flux in the propeller regime:
MRI Simulations
(avis)max= 0.1
adif
= 0.01
(numerical)
a-disk simulations
Lmatter
avis=0.3
adif=0.1
field
Accretion onto Stars with Complex fields
Dipole+quadrupole
27
Realistic Magnetic Fields
MIN LONG, Poster # 13
The magnetic field of the young star
V2129 Oph is measured on the
Surface of the star with the
Doppler-Zeeman technique
and extrapolated to the larger
distances in force-free approximation
Donati et al. 2007
3D field of V2129 modeled
with 1.2 kG octupole and
0.35 kG dipole fields
Long et al. 2010
Romanova et al. 2010
Pure octupole field
Winds from Stars with Complex Fields
Different quadrupole moments
Lovelace, Romanova, Ustyugova, Koldoba , MNRAS 2010
29
Wind is Asymmetric:
Lovelace, Romanova, Ustyugova, Koldoba , MNRAS 2010
30
Matter accretes from the disk onto the star either in funnel streams or
trough instabilities.
In MRI regime matter accretes in funnel streams but the higher level
of variability is expected
The weak magnetic field modifies the boundary layer: in stable
regime - periodic signal, in the unstable regime – drifting QPOs.
Outflows can be connected with periods of enhanced accretion and
the bunching of the field lines. Persistent conical winds form.
In the propeller regime accretion is episodic (in both MRI and
laminar disks). Outflows have two components: slow heavy conical
winds and fast Poynting jet
Magnetic field of stars may be complex. May lead to formation of
one-sided jets. See poster by Min Long on complex fields.