MagnetosphericAccre0on:

Howdoyoungstarsaccretefromtheirdisks?

WillFischer

PHYS6820/7820March1,2011

Romanovaetal.MNRAS2011

Howdoyoungstarsaccretefromdisks?EarlyModel:BoundaryLayer

•  Lynden‐Bell&Pringle1974•  Assumediskextendstostellarsurface

•  Viscosityatthestar/diskboundarylayeraccountsfortheUV/op0calexcessofTTS

BoundaryLayer

•  Atthestellarsurface,diskKeplerianvelocityis220km/sforaTTauristar(R=2R,M=0.5M)

•  Youngstarsshouldbespunuptorota0onalveloci0esof~fewhundredkm/s

•  ButTTSrotateatafrac0onofthisspeed

€

vdisk =GMR*

(km/s)

(Hartmannetal.1986)

MagnetosphericAccre0on•  Ghosh&Lamb(1978)for

neutronstars•  Camenzind(1990),Königl

(1991),Shuetal.(1994),Wang(1995)forTTstars

•  Avoidthespin‐upproblem:Atseveralstellarradii,diskKeplerianvelocityissimilartoobservedstellarrota0onveloci0es(tens,nothundreds,ofkm/s)

•  Astellarmagne0cfieldcantruncatethediskattherequiredradius

€

Pmag ∝ B2

= B*2 R*r

6

€

Pacc = ˙ M vin

r2 = ˙ M 2GM* /r

r2

Magne0cpressure(assumeddipolar)

Accre0onpressure

MagnetosphericAccre0on

•  Theseareequalatthetrunca0onradiusrT

•  Fora1kGfield,anaccre0onrateof10‐7M/yr,amassof0.5M,andaradiusof2R,rT=7.2R*

€

Pmag ∝ B2

= B*2 R*r

6

€

Pacc = ˙ M vin

r2 = ˙ M 2GM* /r

r2

Magne0cpressure Accre0onpressure

€

rT /R* ∝B04 / 7 ˙ M −2 / 7M*

−1/ 7R*5 / 7

Trunca0onradiiareexpectedtobeabitlessthanthis;theaccre0onpressureaboveisforsphericalinfallandisalowerlimittothecaseofarota0ng,equatorialdisk.

MagnetosphericAccre0on

•  AtrT,the(ionized)gasstopsflowingradiallyinwardandfollowsmagne0cfieldlinestothestellarsurface

•  Maherisinfree‐fall•  Shockvelocity•  Dissipatedenergy

(Accre0onluminosity)

€

rT /R* ∝B04 / 7 ˙ M −2 / 7M*

−1/ 7R*5 / 7

€

vs =2GM*

R*1− R*

rT

€

Lacc =G ˙ M M*

R*

1− R*

rT

Free‐fallregion

Accre0onshock

rT

MagnetosphericAccre0on:Regula0onofStellarRota0on

•  Thecorota0onradiusiswheretheKeplerianangularvelocityofthediskequalstheangularvelocityofthestar:

•  Forastellarrota0onperiodof7daysandtheusualstellarparameters,rco=6.1R*

€

Ωdisk =Ω*

GM /rco3 =Ω*

rco = GM /Ω*2( )1/ 3

r>rco:Angularmomentumflowsfromthestartothediskr<rco:Angularmomentum&massflowfromthedisktothestar(accre0on)

MagnetosphericAccre0on:Regula0onofStellarRota0on

•  Accre0onclearlyaffectsstellarrota0on

•  Slowlyrota0ngstarsaremorelikelytohavedisks

Rebulletal.2008:Orion

FastRota0on

SlowRota0on

Disk

NoDisk

MagnetosphericAccre0on:TheEvidence

•  Magne0cally‐sensi0velines(e.g.,TiIintheKband)areZeeman‐broadened

•  EasierintheIRduetoλ2dependence(vsλdependenceofrota0onalbroadening)

•  Averagefieldstrengthis1‐2kG,implyingdiskdisrup0onatseveralR*

I.Stellarmagne0cfields

Yang&Johns‐Krull2011(alsomanyearlierworks

byJohns‐Krulletal.)

B=3.45kG

MagnetosphericAccre0on:TheEvidence

•  CTTSshowmoreemissionatallwavelengthsthanexpectedfromayoung,coolstar

•  Thiscanbestudiedindetailwithmeasurementsoflineveiling

II.Op0cal/UVexcesscon0nuum

λ(µm)

logλF

λ(e

rg/s/cm

2 )

CTTS

WTTS

SED:spectralenergydistribu0on

MagnetosphericAccre0on:TheEvidence

II.Op0cal/UVexcesscon0nuum

λ(µm)

CTTSWTTS

rλ=Fexcess/Fphotosphere

Range Median

rB(0.48µm) 0.1‐6.3 1.0

rY(1.08µm) 0‐3.5 0.4

rK(2.2µm) 0.3‐10 1.9

ObservedSpectraof

TTSFromGullbringetal.ApJ1998

Heavilyaccre0ngCTTS

Non‐accre0ngWTTS

Sumofstellarandexcesscomponents

Note•  Con0nuumshape•  Balmerseries

ExcessSpectraof

TTSFromGullbringetal.ApJ1998

Heavilyaccre0ngCTTS

Mildlyaccre0ngCTTS

Thestellarcomponenthasbeensubtractedarermeasuringtheveiling

MagnetosphericAccre0on:TheEvidence

ConclusionsfromGullbringetal.1998

•  Accre0onratesrangefrom10‐9to10‐7M/yr•  Balmercon0nuum(λ <0.3647µm)isop0callythin

–  Theboundary‐layermodelpredictsthispartofthecon8nuumtobeop8callythick

II.Op0cal/UVexcesscon0nuum

Calvet&Gullbring(1998)modeledthespectraasarisinginhotaccre0onshockscovering0.1‐1%ofthestar•  Op0callythinshort‐

wavelengthemissionfromthepre‐shockandahenuatedpost‐shockregions

•  Op0callythicklong‐wavelengthemissionfromtheshock‐heatedphotosphere

CombinedStellarandExcessCon0nuum(Calvet&Gullbring1998)

λ(µm)1 20.40.2

Accre0onExcess

Star

Total

•  Fistheenergyfluxoftheaccre0onflow(cgs)

•  fisthefrac0onofthestarcoveredbyshocks

MagnetosphericAccre0on:

TheEvidenceIII.LineProfiles

•  Linemorphologiessuggestfree‐fall,notlowveloci0esexpectedfromboundary‐layeraccre0on–  Redshiredabsorp0on(vred>300km/s)

–  Broademission(FWHM>200km/s)

Walker&Burstein1980:YYOri

Muzerolleetal.2000:TWA3A

MagnetosphericAccre0on:

TheEvidenceIII.LineProfiles

•  CanonicalRTmodels(Muzerolleetal.2001)include–  ballis0cinfall–  axisymmetricdipolarflow

–  “highlyschema0c”temperaturestructure

•  HIandNaIprofilesareproduced

PhysicalProper0es

Hβ,i=60o,Rm=2.2–3R*

Muzerolleetal.2001

Comparisontoobserva0ons

Muzerolleetal.2001

•  Reasonablygoodmatchesatlow(DNTau)&intermediate(UYAur)accre0onrates

•  Poormatchathighaccre0onrates(DRTau)–  Probablya

contribu0onfromawind

ImprovedHαmodels

•  Hybriddiskwind+accre0onflow:Kurosawaetal.2006

Kurosawaetal.2006

Surfacebrightnessmap

Butnotebroadblueabsorp0on

HαmodelwithMacc=10‐7M/yr,Mwind/Macc= 0.05(dash‐dot) 0.10(solid) 0.20(dash)

.

. .

CluesfromHelium

•  Highexcita0onpoten0al(20eV)restrictsforma0ontoregionsofhightemperature(>20,000K)orhighionizingphotonflux

–  Accre0onshocksgenerateX‐raysthatcanionizethehelium

–  Restrictedforma0onregionreducesinfluencefromawindthatcomplicatesHα

CluesfromHelium

•  HeI5876:twocomponents(Beristainetal.2001)–  Narrow,centeredcomponent(NC)formsattheaccre0onshock

–  Broad,blueshiredcomponent(BC)formsinawind

Con0

nuum

‐sub

tractedflu

x

Velocity(km/s)‐400 4000

•  Correla0onbetweenveilingandNCstrengths–  TightifnoBC– WeakifBC

•  Presenceofawindaltersthemagnetosphericflow

CluesfromHelium

•  HeI10830:Subcon0nuumabsorp0on

•  Itslowerlevelismetastable–itactslikeagroundstate–soabsorp0onprofilesarecommon

λ10830

He10830Profiles

Velocity (km/s)

Nor

mal

ized

Flu

x

Blue Only (37%)

Wind

Blue + Red (34%) Red Only (13%)

Accretion Flow

WTTS Central (5%) None (11%)

(Sorted by morphology of subcontinuum absorption)

DR Tau AA Tau CY Tau

GK Tau CW Tau V819 Tau

Edwardsetal.2006

He10830RedAbsorp0onandVeiling

38 CTTS

< rY >

Num

ber o

f CTT

S 21 with Red Abs

17 w/o Red Abs Redabsorp0onisstrongestwhentheveilingislow

Fischeretal.2008

ComparisontoObserva0ons

One-Micron Veiling

Nor

mal

ized

He

1083

0 R

ed A

bsor

ptio

n S

treng

th

Iftheaccre0onflowislargeenoughforstrongabsorp0on,theresul0ngshocksgeneratelargeveiling

Data+‐‐+Resultsofscaheringmodels

Accre0onflow

Accre0onshock

Fischeretal.2008

FlowDilu0on

Manynarrowstreamlets•  Totalcoverageofshocks

onthestaris~1%aspredictedfromveilingstudies

•  Broadlydistributedstreamlets(withsometurbulence)presenttherangeofveloci0esneededfortheobservedabsorp0ons

Fischeretal.2008

z (R

star

)

R (Rstar)

NumericalMethods

•  Mosttreatmentsofmagnetosphericaccre0onthroughmid‐2000susedanaly0cflowsolu0ons–  Velocity:ballis0cinfall–  Temperature:simplehea0ng/coolingparameteriza0on

– Density:idealmagnetohydrodynamic(MHD)result

– Geometry:axisymmetricdipole,magne0caxisalignedwithrota0onaxis

•  Formorecomplexity,neednumericalMHDcodes

MHDResults

•  ComplexFields:Tilteddipole– Twofunnelstreams

– High‐la0tudespots

Romanova,Ustyugova,Koldoba,&Lovelace2003,2004

MHDResults

•  ComplexFields:Tilteddipole+quadrupole

Long,Romanova,&Lovelace2007

MHDResults

•  ComplexFields:Userealmagne0cfieldmeasurements

Dona0,Jardine,Gregoryetal.2008

BPTauDipole:1.2kGOctupole:1.6kG

MHDResults:ObservablesBPTau:Longetal.2011

Time‐dependent:arer10rota0ons

Magne0cfield

StellarhotspotsDensity

Ini0al Arer10rota0ons

Lineprofiles?

Accre0on/OuylowConnec0on

•  Accre0onmustdriveouylowsfromyoungstars–  Roughlyconstant10‐to‐1ra0oofmassaccre0onratetomasslossrate

–  Oneisneverpresentwithouttheother– MHDmodelsshowthisinac0on

Accre0on/OuylowConnec0onConicalWind:Romanovaetal.

Summary

•  Magnetosphericaccre0on:Thestellarmagne0cfieldtruncatesthedisk,andmaherfallstothestaralongstellarfieldlines–  TTauristars–  Compactobjects

•  Magnetosphericaccre0onregulatesthestellarrota0onrate

•  Evidenceformagnetosphericaccre0oninTTS–  Stellarmagne0cfieldsarestrongenoughtotruncatethedisk–  Bluecon0nuumemissionindicatesop0callythinshockemission–  Lineemission+absorp0onindicatefreefallinggastermina0nginahotshock

•  NumericalMHDmodelsenablethestudyofrealis0cflowgeometries

•  Accre0onmustprovidetheenergyforouylowsfromyoungstellarobjects