Magnetic Field & Accretion
Structures around
Young Stars
Shinsuke Takasao (Nagoya Univ.)
Collaborators:
Kengo Tomida, Kazunari Iwasaki (Osaka Univ.),Takeru K. Suzuki (Univ. of Tokyo)

Importance of investigating accretion processes
onto young stars
Accretion structure onto stars➤ ang. mom. evolution of stars➤ estimation of mass accretion rate
➤ occultation of the star
(̶> impact on the disk evolution)
Angular momentum/mass extraction
from disks & stars➤ Jet, outflow, wind

Understanding the roles of a magnetic field aroundthe star is crucial
Note: In this talk,➤<< 1au scale is focused➤late protostars ~ early pre-main seq. stars consideredStructure of the inner region?

Structure of the inner region?

Classical picture➤ UV excess compared to the stellar emission➤ Hot spots at high latitudes

quiet disk accretion

Magnetospheric Accretionaccompanied by the accretion shockMag.
fielde.g. Konigl 1991

Magnetospheric accretion is successful?
UV excess due to the shock heating

Accretion shock

Hartmann+16, see also Calvet & Gullbring 1998
➤ UV excess (Valenti+93), hot spots at high-latitudes (Donati+11)➤ Indicating a fast accretion at high-latitudes➤

opt./UV excess ̶ [fitting by the shock model] ̶> Estimation ofMagnetospheric accretion scenario looks OK?

Occultation of the star Bouvier + 1999 and many
Changing stellar radiation to the disk: Important for the disk evolution
CoRoT white-light flux

periodic

aperiodic/stochastic

Cody+14

Cody+14

Occultation by a warped disk

Rayleigh-Taylor instability

caused by the magnetospherein the magnetosphere

Romanova+13

Kulkarni & Romanova08

Magnetospheric accretion is successful?
Assume thatthe inner disk is truncated at a radius
where Emag ~ Ekin

(Ghosh & Lamb 1978, Konigl 1991)

truncated

No clear correlation foundfrom observations,,,

Not clear if magnetosphericaccretion is successful or not.
Johns-Krull & Gafford 2002

Magnetospheric accretion even in weak B-ﬁeld stars?
Herbig Ae/Be: intermediate mass stars at the PMS stage. The fractionof magnetic (> ~100 G) stars is only ~10% (Wade+2007)
̶> too weak B-field for magnetospheric acc.

Red : CTTS
Blue: Herbig Ae/BeHerbig Ae stars also have alarge accretion speed(Cauley & Johns-Krull 14)??

Re-examine disk accretion process
Magnetospheric accretion

Disk accretion

Existence of magnetosphere is unclear
̶> Re-examine the disk accretion processusing 3D magnetohydrodynamic (MHD) simulations➤ Is fast accretion possible without the magnetosphere?
➤ Occultation process?
➤ Does a fast, magnetically-driven jet blow ?

Setting of 3D MHD simulation:
Accretion onto a star without a magnetosphere
Code : Athena++ (Stone, Tomida, White in prep)
Basic eqs: ideal MHD (OK for this inner region)

hourglass-shapeinitial mag. fieldDomain size: 60 Rstar ~ 0.6 au
Physical time span: ~300 rot ~ 0.4 yr

Model setting: Stellar surface & disk
Slowly rotating (r̲corot = 3 Rstar)
Stellar wind (thermally driven)➤ Cold (thin) disk: Hp/R = 0.14
➤ Weakly magnetized: β=10^4➤ A simplified radiation coolingweakly magnetizeda damping layer method used:
The disturbed stellar surface reverts
to a certain coronal state gradually.

is adopted to maintain theinitial disk temperature profile

B-ﬁeld and gas ﬂow structures: large view
StellarwindDisk wind(< v̲esc)

No magnetically-driven jets with v ~ v̲esc from the MRI disk found(consistent with previous simulations of disks with non-rotating BH, e.g. Beckwith+09)

B-ﬁeld and gas ﬂow structures: large view
Stellarwind

Disk wind(< v̲esc)

No magnetically-driven jets with v ~ v̲esc from the MRI disk found(consistent with previous simulations of disks with non-rotating BH, e.g. Beckwith+09)

Gas map around the starDensityplasma beta (Gas pres./ Mag. pres.)➤ The density above the disk increases➤ Highly ﬂuctuating/turbulent disk atmosphere(source of turbulence: MagnetoRotational Instability, MRI)MRI-driven windwind
MRI turbulence

Suzuki & Inutsuka 2009, 2014
Fromang et al. 2013, Bai & Stone 2013
➤ The wind supplies a large amountof mass to the upper atmosphere➤ Slow (<< escape velocity)➤ The wind is expected to become

magnetically-driven outflows(but unclear)
Suzuki & Inutsuka 2009

Gas map around the starradial velocity

plasma beta (Gas pres./ Mag. pres.)

Fast accretion at high-latitudes of the staroccurs even without a stellar magnetosphereB-ﬁeld and gas ﬂow structures: centeral region

Magnetic funnel

Outer region: wind is blowing outward (but slowly)
Inner region: MRI-driven wind is flowing to the star(“failed” wind) along the magnetic funnel
Funnel-wall accretion3D structure of funnel-wall accretion
Blue: fast accretion flow
arrows: velocity vectors

Patchy accretion streams flowing to high-latitudes➤ Coexistence of the disk accretion and funnel-wall accretion➤Maximum accretion speed
Blue: fast accretion flow
arrows: velocity vectors

Even without a magnetosphere,accretion with a speed of(>100 km/s) is possible(observed soft X-ray emission can be produced at acc. shocks)

Accretion rate
Blue: fast accretion flow

The disk opening angle: ~15

arrows: velocity vectors

Mid plane accretion is dominant:Rate of the funnel-wall acc. ~ 0.01-0.5 x rate of mid plane acc.

Accretion structure on the stellar surface (r=1)

Many localized
accretion spots

Large kinetic energy
flux regions ~ hot spots

3D magnetic ﬁeld structure

Blue:density isosurface

Toroidal field dominantContrast to the magnetospheric accretion model,accretion streams do not move along a field line in this caseWhy funnel-wall accretion is so fast (~free-fall)?
Significant ang. mom. loss by the Lorentz force
Lorentz force/centrifugal force

iso specific ang. mom. line
(centrifugal barrier)

The deceleration by mag. torque becomes important when
Lorentz force

centrifugal force

So-called avalanche flow (Matsumoto+ 1996)
but it occurs well above the disk surface

Angular momentum exchange mechanism
Blue:Field line

MRI-like ang. mom. exchange

fast accretion flowzRThis is confirmed in our sim.
Field lineOrigin of funnel-wall accretion: Relation to the disk dynamo-dominant disk

Parker instability
B-field

gas slides down (ρ decreases)
move upward due to buoyancy
As for reversal of the sign of Bphi,
see e.g. Machida et al. 2013

Origin of funnel-wall accretion: Relation to the disk dynamo

Magnetic funnel

As for reversal of the sign of Bphi,
see e.g. Machida et al. 2013

Rising magnetic field cannot penetrate
the magnetic funnel.
̶> move along the funnel
̶> supply B-field along the funnel

Movement of B-ﬁelds & accreting materials
Movement of magnetic fields

Movement of accreting materials

increase of Mag. TorqueGas is slowed down
by torque,falling onto the star

Magneticfunnel

MRI-driven windbuoyancy
B amplification

Disk dynamo
̶> strong B above the disk

MRI-driven wind
̶> Rapid ang. mom. loss
around the funnel
̶> funnel-wall accretion

Note: B-field and materials move in the opposite direction (decoupled)Why a fast jet does not blow?
magneticpressurePrediction (Kudoh & Shibata 1995, 1998):
Magnetically-driven jets can form
even when the disk B-field is weak

Our result:
No jet from a 3D weakly magnetized (β=10^4),
cold (Hp/R ~ 0.1, Eth << Eg) disk

Prediction
Amplify B-field
Our result

Emag ~ Eg
Emag ~ Eth << Eg(β ~ 1)Jet(probably OKfor thick/hot disks)

Parker instability
Growth time ~ Amp. time

Note: Emag << Eg even well above the disk because
the density is enhanced by the MRI-driven windParker instability
low-β (dark purple) = strong B
plasma βplasma β on Rz plane + 3D B-fieldbuoyant loops(low-β)Parker instability
plasma β on Rz plane + 3D B-fieldbuoyant loops(low-β)Angular momentum transport

Transport in R-direction >> Transport in z-direction(MRI)(outflow, wind)(consistent with other MRI disk sims.: Beckwith+09, Zhu & Stone 17)

Occultation due to dynamo

Large density (so optical depth) ﬂuctuation near the disk
(Disk dynamo is the main cause of the density ﬂuctuation)
̶> The star can be occulted at a wide range of wavelengths

Comparison with magnetospheric acc. model
MagnetosphericAccretion (MA) model

Our model

Romanova+12

Strong stellar B necesary?
fast accretion?
flow along field lines?
aperiodic accretion?
occultation

MA modelour model

yes
yes
yes
not cleardisk warp

no
yes
no
yes
dynamoSummary➤ Fast accretion at high-latitudes of the star (funnel-wallaccretion) is found to occur even without magnetosphere.
➤ Failed MRI-driven wind = Funnel-wall accretion➤ Funnel-wall accretion is a result of a complex coupling

among the disk wind, dynamo, and ang. mom. transport.
(not a local process!!)➤ A fast jet does not blow from our cold, MRI-turbulent disk.