Table Of ContentAstronomy&Astrophysicsmanuscriptno.jetform February2,2008
(DOI:willbeinsertedbyhandlater)
Are jets in symbiotic stars driven by magnetic fields?
MatthiasStute1 andMaxCamenzind1
LandessternwarteHeidelberg,Ko¨nigsstuhl,D-69117Heidelberg,Germany
Received25November2004/Accepted26January2005
5
0
0 Abstract. Wecomparetwoscenariostolaunchjets–formationbyMHDprocessesorformationbythermalpressureinthe
2 boundary layer(BL)–withrespecttotheircompatibilitywithobservational dataofjetsinsymbioticstars,especiallyinthe
wellstudiedjetsourceMWC560.Finally,wediscusspointsoffurtherresearchtobedone.
n
a
J Keywords. ISM:jetsandoutflows–binaries:symbiotic
6
2
1. Introduction Anotherclass of accreting WDs are cataclysmic variables
1 (CV)whichareverycloseWDbinarieswithalowmassmain
v Althoughjetsareubiquitousphenomenainmanydifferentas- sequencestarascompanion.ThemasstransferisduetoRoche
6
trophysical objects as young stellar objects where they are lobeoverflow,notduetowindaccretion.Theshorttimescales
6
5 drivenbyprotostars,symbioticstars(whitedwarfs),X-raybi- inthesesystemsmakethemthebestunderstoodaccretingsys-
1 naries (neutron stars and stellar mass black holes) and active tems. Remarkably,they show no jet emission. Further related
0 galactic nuclei (supermassive black holes), their formation is objectsare supersoftX-ray sources, in which the temperature
5 relativelyunclear. andpressurein theboundarylayer(BL)oftheWD areinthe
0
Themasslossrateofthejetisfoundtobeconnectedtothe correct range to maintain steady nuclear burning on the WD
/
h massaccretionrateoftheunderlyingdiscfoundinmostobjects surface.
p
(e.g.Livio1997).Thereforethenecessarycomponentsseemto
-
o bewellknownandcommontoallobjects.Amorecarefulin- Injetformationmodelspresentedsofar,themagneticfield
r vestigationofonespecificclassofobjectsshouldpromisenew seems to play a key role. The first analytical work study-
t
s insightsalsoforthemechanismsintheotherclasses.Fromthe ingmagneto-centrifugalaccelerationalongmagneticfieldlines
a observationalpointofview,oneneedsobservationswithahigh threading an accretion disc was done by Blandford&Payne
:
v spatial resolution and kinematic informationsfrom regionsas (1982). They have shown the braking of matter in azimuthal
Xi nearaspossibletothejetsource.Theseaspectsmaketheclass direction inside the disc and their acceleration above the
ofsymbioticstarsidealtestbeds. disc surface by the poloidal magnetic field components.
r
a Toroidal componentsof the magnetic field then collimate the
Symbiotic stars are interacting binaries with orbital peri-
flow. Numerous semi-analytic models extended the work of
ods in the range of years. These systems show outburstssim-
Blandford&Payne(1982), eitherrestrictedtoself-similarso-
ilarto classicalnovae.Thestellar componentisa coolredgi-
lutionsandtheirgeometriclimitations(e.g.Pudritz&Norman
ant (RG), the hot component a white dwarf (WD) with tem-
1986; Vlahakis&Tsinganos 1998, 1999; Ferreira&Casse
peratures of 50000 – 200000 K. Both stars show mass loss
2004)orwithnon-self-similarsolutions(e.g.Camenzind1990;
throughsupersonicwinds.WindmaterialfromtheRGiscap-
Pelletier&Pudritz1992;Breitmoser&Camenzind2000).
turedbytheWDtoformanaccretiondisc.Theaccretionthen
causesthermonuclearexplosionsoftheWDsurfaceleadingto
Another approach is to use time-dependant numerical
anincreaseinluminosityfollowedbyjetemission.Jetsarede-
MHD simulations to investigate the formation and collima-
tectedin10outofalmost200symbioticstars(Brocksoppetal.
tion of jets. In most models, however, a polytropic equilib-
2004) and this process was directly observed in CH Cygni
riumaccretiondiscwasregardedasaboundarycondition(e.g.
(Taylor,Seaquist&Mattei1986).OtherfamoussystemsareR
Krasnopolsky,Li&Blandford 1999, 2004; Andersonetal.
AquariiandMWC560.Whilethefirsttwoobjectsareseenat
2004; Goodson,Bo¨hm&Winglee 1999). The magnetic feed-
highinclinations,thejetaxisinMWC560ispracticallyparal-
back on the disc structure is therefore not calculated self-
leltothelineofsight.
consistently.Onlyinrecentyearswerethefirstsimulationsin-
cludingthe accretion disc self-consistently in the calculations
Send offprint requests to: Matthias Stute, e-mail: ofjetformationpresented(e.g.Casse&Keppens2002,2004;
[email protected] Kato,Mineshige&Shibata2004).
2 MatthiasStuteandMaxCamenzind:Arejetsinsymbioticstarsdrivenbymagneticfields?
Due to the fact, that strong magnetic fields have been de- – orthemagneticluminosity(Camenzind1997)
tected so far only in one symbiotic system (Z Andromedae,
1
Sokoloski&Bildsten 1999), the jet formation by magneto- Lmag = Ω∗ R3∗BpBϕ =4.76×1034
2
centrifugal forces exclusively seems to be insufficient.
Radiativelaunchingcanbeexcludedduetotoosmallradiation × Ω∗ R∗ 3 Bp Bϕ ergs−1. (3)
fields.Anewpossibilitytoaccelerateplasmaclosetothecen- h−1 7×108cm MG MG
! (cid:18) (cid:19) ! !
tralobjectwasproposedinvolvingSPLASHs(SPatiotemporal
The flux in the UV band was measured as 10−9 erg cm−2 s−1
LocalizedAccretionSHocks)intheBL(Soker&Regev2003).
(Maranetal.1991)andtheUVluminosity–whichislikelyto
Locallyheatedbubblesexpand,mergeandaccelerateplasmato
be equal to the accretion power of the disk and the boundary
velocitieslargerthanthelocalescapevelocity.Soker&Lasota
layer–isthen
(2004)appliedthismodeltodisk-accretingwhitedwarfstoex-
plaintheabsenceofjetsinCV. Theyhavefoundacriticalac- d 2
cretion rate of ∼ 10−6 M⊙ yr−1 below which no jets should LUV =1.2×1035 ergs−1. (4)
kpc
bepresent.Thisscenariowasintroducedonlyinanalyticesti- !
mates. With a derived distance of 2.5 kpc, this suggests an minimal
In section 2, we give generalestimates based on observa- accretionrateof6.3×10−8 M⊙ yr−1 andthereforeanejection
tions of the well studied jet in MWC 560. After that we cal- efficiencyof14%.
culatethemagneticfieldnearthewhite dwarfrequiredforjet The minimal accretion rate is below the critical accretion
formationbyMHDprocessesinsection3.Insection4,welist rate derived by Soker&Lasota (2004) by a factor of 16. As
differentwayswithwhichamagneticfieldinMWC560could theseauthorsclaimanuncertaintyofa factorof∼ 10andthe
bedetected.Finallyadiscussionisgiven. remainingfactorcouldnaturallyarisefromourestimates,this
shouldnotconcern.
2. Generalestimates Using the observables M˙, Ljet and P˙0, one can fix in
principle the two free parameters χ and Γ of the model of
As observed by Schmidetal. (2001) and as used to simulate
Soker&Regev (2003). These parameters are the fractions of
the jet nozzle in Stute,Camenzind&Schmid (2005), the pa-
the mass outflow rate due to SPLASHs to the accretion rate
rametersofthejetinMWC560are
and of the initial kinetic energyto the final kinetic energyin-
– thevelocityv =1000kms−1 , sideaSPLASH,respectively.Fixingthemodelhighlydepends
jet
– thenumberdensityn =5×106cm−3,whichisequaltoa onestimatingtherightmassaccretionratewhich,however,is
jet
massdensityρ =8.4×10−18gcm−3,and ratherdifficult.
jet
– thejetradiusR =1AU.
jet
Usingtheequations 3. Requiredmagneticfieldsinthemagneticjet
formationscenario
M˙ = πR2 m n v
jet jet H jet jet
P˙ = M˙ v LetΨbeamagneticsurfaceanchoredattheinnerradiusofthe
0 jet jet
accretiondisk.Thenthissurfaceremainsalwaysinsidethejet
1 1
L = M˙ v2 = πR2 m n v3 , uptolargedistances.Usingtheconstantsofmotionalongthis
jet 2 jet 2 jet H jet jet
surfaceandthejetparameters,onecanmakedetailedestimates
thisspecifies followingfromthescenario.
– themassoutflowrateM˙jet =9.33×10−9 M⊙ yr−1, As magneto-centrifugally driven jets have fast-
– themomentumdischargeP˙ =5.93×1025gcm−1s−2and magnetosonic Mach numbers Mfm ∼ 3 (e.g.
0
– thekineticjetluminosityL =2.93×1033ergs−1. Krasnopolsky,Li&Blandford 2004), the total magnetic
jet
fieldinthejetshouldbe
This luminosity can be provided by several different mecha-
nisms: B2 +B2 = 1 4πρ v =0.34G∼ B , (5)
p,jet ϕ,jet 3 jet jet ϕ,jet
– bythe(insufficient)luminosityofthewhitedwarfs q q
astheazimuthalcomponentshouldbedominantinsidethejet.
L = 4πR2σT4 =
WD ∗ WiththeconservationofcurrentI = RB ,theazimuthalfield
ϕ
3.5×1030 R∗ 2 T 4 ergs−1 (1) nearthejetsourceisthen
7×108cm 104K
– the accretion powe(cid:18)r of the di(cid:19)sk(cid:18)(and a(cid:19)lmost the same Bϕ,0 ∼7.3kG. (6)
amountcomingfromaBL)ofarotatingmagnetizedWD AsB ≪ B ,thetotalmagneticfieldshouldbebyfarlarger.
ϕ,0 p,0
Lacc = GMR∗∗M˙ =1.19×1035 MM⊙∗ ABpn,jeutp≪per0.l3im4iGt,caanndbethfeocuonndsuersvinagtioangaoifnfleuqxuaΨtio=nB(5p)R,n2,amleaedly-
! ingtomagneticfieldsintherangeof
× R∗ −1 M˙ ergs−1 (2)
7×108cm 10−8M⊙yr−1 7.3kG≪ Bp,0 ≪155MG. (7)
(cid:18) (cid:19) !
MatthiasStuteandMaxCamenzind:Arejetsinsymbioticstarsdrivenbymagneticfields? 3
4. Observingthemagneticfield and Stute,Camenzind&Schmid (2005) have found that the
number density near the red giant should be of the order of
Following Brocksoppetal. (2004), jets are detected in 10 109 cm−3 andthenbedecreasingfollowinga1/r2-law,theto-
out of almost 200 symbiotic stars. Remarkably, this is
talcolumndensityindirectionswithlargeinclinationwouldbe
exactly the fraction of magnetized to un-magnetized iso-
oftheorderof1022 cm−2 whichistoolowtocausetheneces-
lated white dwarfs with field strengths larger than 30 kG
saryabsorption.ThecolumndensityofthejetinMWC560has
(Wickramasinghe&Ferrario2000).
similarvalues(Schmidetal.2001)leadingtothesameconclu-
Astrongmagneticfield,however,hasbeendetectedonlyin
sion. The non-detection therefore sets an upper limit for the
one symbiotic system (Z Andromedae, Sokoloski&Bildsten
magneticfieldwhichisatthelowerendoftherangeofeq.(7).
1999).Theyinterpretedanobservedperiodicphotometricvari-
ability as the Keplerian spin period at the magnetosphericra-
dius r , where the magnetic pressure of the WD’s field is 5. Discussion
mag
comparableto the ram pressureof the accretedmaterial. This
ByanalyticestimatesforthesymbioticstarMWC560,wehave
radiuscanbecalculatedas
tried to decide which of the two main mechanisms – forma-
B4R12 1/7 B 4/7 tion by MHD processes or formation by thermal pressure in
r = =3.6×109 (8)
mag 32GMM˙2 MG the boundarylayer (BL) – is more reliable to explain the ob-
R ! 12/7 M −1/7(cid:18) (cid:19) M˙ −2/7 servedparametersofjetsinsymbioticstars.Thisquestioncan
× cm, not(yet)beansweredbyobservations.
(cid:18)7×108cm(cid:19) M⊙! 10−8M⊙yr−1! TheX-rayobservationsofMWC560givinganupperlimit
forthemagneticfieldstrengthshowthatisstillconsistentwith
whichleadstoaKeplerianperiodof
the minimalmagneticfieldrequiredfora magneticjetforma-
r3 B 6/7 R 18/7 tion process.FurtherX-rayobservationsareneededto reduce
mag
P = 2π =116 thisdetectionlimitandtodecidewhetherthemagneticscenario
sGM MG 7×108cm
canbeexcludedornot.Untilveryhighspeedphotometricob-
(cid:18) (cid:19) (cid:18) (cid:19)
M −5/7 M˙ −3/7 servationsaremade,alsothefirstwaytodeducethemagnetic
× s. (9)
M⊙ 10−8M⊙yr−1 fieldstrengthdoesnotrevealanyreasonagainstamagneticjet
! !
formation process. At the moment, both mechanisms are still
In the case of Z And, the measured period was about28 min
possible.
(Sokoloski&Bildsten1999),equivalenttoamagneticfieldof
As the BL scenario was introduced only analytically and
about22MG.
thederivationofthemodelparametersbyobservationishighly
AsnooscillationshavebeendetectedinMWC560sofar,
difficult, however, numerical simulations of a compact object
themagneticfieldnearthewhitedwarfshouldhavelowerval-
withasolidsurfaceaccretingmatterwouldbeinteresting.Then
uesintherangeofeq.(7)–oftheorderof B = 100kG,with
thereliabilitycouldbequalitativelyandquantitativelychecked
whichtheperiodwouldbe16s andthereforecertainlybelow
in differentrangesof parameters,which should be interesting
anyobservablelimit.
notonlyforsymbioticstars,butalsoforotherkindsofbinaries.
Soker&Regev (2003), however, assumed that the mag-
Theeffectivityofjetlaunchingbythermalpressureinsidethe
neticfieldeitherinsidethejetornearthejetsourcewouldun-
BLshouldbeinvestigatedanddependenciesofjetparameters
dergofastreconnectionwhichheatsuptheplasmatotempera-
asoutflowrate,outflowvelocityandjetkineticluminosityfrom
turesmakingthegasvisibleinX-rays.Themaximumtempera-
initialmodelparametersase.g.themassaccretionrateshould
tureuptowhichthegasisheatedcanbeestimatedbyequipar-
be derived. The authors have started first simulations whose
tition(Tanumaetal.2003)as
resultswillbepresentedsoon.
n −1 B 2
T ∼106 K. (10) Acknowledgements. Parts of this work were supported by the
max 106cm−3 100mG
Deutsche Forschungsgemeinschaft (DFG). We acknowledge the im-
(cid:18) (cid:19) (cid:18) (cid:19)
Inside the jet, the maximum temperature, however, would be provingcommentsandsuggestionsbythereferee.
negligible.Nearthejetsourceitwouldbe∼ 7×107 Kwitha
densityof1.4×1016cm−3–whichcorrespondstoanaccretion
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