magnetospheric accreon - university of toledo
TRANSCRIPT
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