Magnetic chemically peculiar stars

0.7 MB·English

by  Markus Schöller

Magnetic chemically peculiar stars
MarkusScho¨llerandSwetlanaHubrig
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Abstract Chemicallypeculiar(CP)starsaremain-sequenceAandBstarswithab-

] normallystrongor weak linesforcertain elements.Theygenerallyhavemagnetic
R
fields and all observablestend to vary with the same period. Chemically peculiar
S
stars providea wealth of information;they are naturalatomic and magnetic labo-
.
h ratories. After a briefhistoricaloverview,we discuss the generalpropertiesof the
p magneticfieldsinCPstars, describetheobliquerotatormodel,explainthedepen-
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denceofthemagneticfieldstrengthontherotation,andconcentrateatthe endon
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r HgMnstars.
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1 Introduction
1
v
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ApandBpstarsaremainsequenceAandBstars,inthespectraofwhichlinesof
2
2 some elementsare abnormallystrong or weak (e.g.,Si, Sr, Cr, Eu, He, ...). They
4 generallyhavemagneticfieldsthatcanbedetectedthroughobservationsofcircular
0 polarizationinspectrallines.Observables,suchasthemagnitudesinvariousphoto-
1. metricbands,thespectrallineequivalentwidths,andthemagneticfield,varywith
0 thesameperiod,whichcanrangefromhalfadaytoseveraldecades.Abnormalline
5 strengthscorrespondtoelementoverabundances(byupto5−6dexwithrespectto
1
theSun)andareconfinedtothestellarouterlayers.Theclassofchemicallypeculiar
:
v (CP)starsisroughlyrepresentedbythreesubclasses:themagneticApandBpstars,
i themetallic-lineAmstars,andtheHgMnstars.Anoverviewofthedifferentgroups
X
ofCPstarscanbefoundinTable1.
r
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MarkusScho¨ller
European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany, e-mail:
[email protected]
SwetlanaHubrig
Leibniz-Institut fu¨r Astrophysik, An der Sternwarte 16, 14482 Potsdam, Germany e-mail:
[email protected]
1
2 MarkusScho¨llerandSwetlanaHubrig
Table1 Differentgroupsofchemicallypeculiarstars.
Peculiarity Spectral T magnetic spots
eff
Type Type range
He-strong B1-B4 17000−21000 yes yes
He-weak B4-B8 13000−17000 yes yes
Si B7-A0 9000−14000 yes yes
HgMn B8-A0 10000−14000 yes? yes!
SrCrEu A0-F0 7000−10000 yes yes
Am A0-F0 7000−10000 yes? no
2.5
HD101065
Normalized Flux + Constant1.521 CeII NdIIINdII NdPIIrIINdPIrICIIeII FeI HHDD291675522
0.5
5427 5428 5429 5430 5431 5432 5433 5434 5435 5436 5437 5438
Wavelength (A)
Fig.1 UVESobservationsofHD101065,HD217522,andHD965.Themagneticallyinsensitive
FeIlineatl 5434A˚ issharpandhasasimilarwidthinallthreespectra.–Credit:Hubrigetal.
(2002).
Chemically peculiar stars provide a wealth of information. E.g., Castelli &
Hubrig (2004) analyzed a spectrum of the HgMn star HD175640, observed with
UVES at a spectral resolutionof R∼90000−100000and a spectral coverageof
3040−10000A˚). TheyusedanATLAS12modelatmosphere(Kurucz1997)with
theSYNTHEcode(Kurucz1993)tomodelthisspectrum.Theywereabletoobtain
abundancesfor49ions,using200linesforabundancesoflightelements,230lines
for abundances of ion group elements, and 130 lines for abundances of elements
with Z ≥31. They identified 80 TiII emission lines, 40 CrII emission lines, and
used100linestostudytheMnIIhyperfinestructure,140linesfortheGaIIisotopic
structure, 15 lines for the BaII hyperfinestructure, and 30 lines for HgII isotopic
andhyperfinestructure.Still,thereremained170unidentifiedabsorptionlinesand
30unidentifiedemissionlines.
Thedifferencebetweenanon-peculiarstarandaCPstarcanbestriking.Look-
ingatthespectrumofVega,intheregionbetween5000and6000A˚ thereareonlya
fewlinesofNaI,MgI,SiII,andFeII.Ontheotherhand,CPstarscanhaveadozen
lines within a spectral range of 10A˚, which can be seen in Fig 1. The overabun-
dancesseeninCPstarsaretheresultofselectivediffusionofthedifferentelements
(Michaud1970).SeealsoChapter“Diffusionanditsmanifestationinstellaratmo-
spheres”.
Magneticchemicallypeculiarstars 3
2 A brief historicaloverview
The first detection of a magnetic field in a star other than the Sun was achieved
inCSVirbyBabcock(1947).Heessentiallydeterminedthelongitudinalmagnetic
fieldinthisstar.Today,meanlongitudinalmagneticfieldmeasurementsthroughout
thevariationperiodhavebeenobtainedfornomorethan100stars.Theresolutionof
magneticallysplitlinesrequiresastrongenoughmagneticfieldandsufficientlyslow
rotation.Resolvedmagneticallysplitlineswere firstdiscoveredin Babcock’sstar,
HD215441(Babcock1960),forwhichhemeasuredameanmagneticfieldmodulus
ofhBi∼34kG,andwhichisthestrongestmagneticfieldmodulusmeasuredinan
Apstartodate.In1987,twelvestarswithmagneticallyresolvedlineswereknown,
onlyfourofthosewerestudiedthroughouttheirvariationperiod.In2001,44stars
with magnetically resolved lines were known, 24 of those were studied through-
outtheir variationperiod(Mathyset al. 1997;Mathyset al.,inpreparation).First
systematicdeterminationsofthecrossovereffectandthemeanquadraticmagnetic
fieldwerepublishedbyMathys(1995a,b).Afullphasecoveragewasachievedfor
abouttwodozenstars.Thebulkofthepublishedmaterialonbroad-bandlinearpo-
larization(BBLP)wasgatheredbyLeroybetween1990and1995(Leroy1995,and
references therein). Variations in BBLP were well studied for about 15 stars. See
Chapter“Magneticfields”onadetaileddiscussionofstellarmagneticfields.
3 General properties ofmagneticfields inApstars
Thestrongestmagneticfieldstendtobefoundinmoremassivestars.Theyarealso
foundonlyinfast-rotatingstars(Hubrigetal.2000).Allstarswithrotationperiods
exceeding1000dayshavemagneticfieldsbelow6.5kG.Fromthe findingthatthe
longitudinalmagneticfieldaveragedoverthestellardiskisnotzero,onecandirectly
conclude that the magnetic field needs to be organized on a larger scale, i.e. it is
a dipoleora superpositionofadipoleanda quadrupole.Thecircularpolarization
fromtangled,solar-likemagneticfieldsmostlycancelsoutinadiskintegration.The
magneticfieldofApstarsthushasasignificantdipole-likecomponent.Foradipole,
the ratio between the longitudinal magnetic field and the magnetic field modulus
hB i/hBi is 0.3, for a quadrupoleit is 0.05. If toroidalor higher-ordermultipolar
z
componentsweresufficienttoaccountfortheobservedlongitudinalmagneticfield,
thesewouldinducestrongdistortionsofthespectrallineprofilesinStokesI,i.e.in
integrallight,whichwedonotsee.
Themagneticfieldcoversthewholestellarsurfacehomogeneously,i.e.thedis-
tributionofthefieldstrengthoverthestarisfairlynarrow.Evidenceforthiscomes
from the fact that the magnetic field is observed at all phases, the continuum is
reachedbetweenthesplitcomponentsofresolvedlines,andthattheresolvedmag-
neticallysplitcomponentsarerathernarrow(Mathysetal.1997).
Magneticfieldshavesevereeffectsonthestructureofstellarouterlayers.They
areresponsibleformagneticallycontrolledwindsandelementalabundancestratifi-
4 MarkusScho¨llerandSwetlanaHubrig
cation.Evidenceforabnormalatmosphericstructurecomesfromthefactthatpro-
files of hydrogen Balmer lines in cool Ap stars can not be fitted by conventional
models (Ryabchikova et al. 2002). This has also a potential impact on the longi-
tudinal magnetic field determination by Balmer line polarimetry. The core-wing
anomaly (Cowley et al. 2001) of the hydrogen Balmer lines leads to the impos-
sibility of fitting the Balmer lines with one effective temperature. E.g., to fit the
Hb lineinHD965,oneneedstoassumeT =5500Kforthecoreofthelineand
eff
T =7000Kforthewings.
eff
4 The oblique rotatorand thegeometric structure ofthe
magnetic field
Themagneticfieldisnotsymmetricwithrespecttothestellarrotationaxis.Other
surface features, e.g. the abundance distribution, are determined by the magnetic
field.Observedvariationsresultfromchangingaspectsofthevisiblehemisphereas
thestarrotates.Thus,thevariationperiodistherotationperiodofthestar.Nointrin-
sicvariationsofthemagneticfieldhavebeenobservedinApstarsovertimescales
ofdecades.
In early models of the magnetic field, a quasi-sinusoidal variation of the lon-
gitudinalmagnetic field was assumed. In the simplest model, a dipole centered at
the star’s center and with an axis inclined with respect to the stellar rotation axis,
wasemployed.Fromstarswithmagneticallyresolvedlines,itcanbeseenthatthe
mean magnetic field modulusgenerally has one maximum and one minimum per
rotationperiod,evenforstarswithareversinglongitudinalmagneticfield(Mathys
etal. 1997).Fromthese observations,a centereddipolecanberuledout.Alterna-
tive models include a dipole that is offset along its axis (parameters:i, b , B , a),
d
or a collineardipole plusa quadrupole(parameters:i, b , B , B ), with i the incli-
d q
nationangleof thestar withrespecttothe lineofsight,b the inclinationangleof
themagneticfieldwithrespecttoi,B thestrengthofthedipole,B thestrengthof
d q
thequadrupole,andatheoffsetofthedipolewithrespecttothestar’scenter.The
models have to make a good match with four observables: the maximum and the
minimumof both the longitudinalmagnetic field and the magnetic field modulus.
Bothmodelsareequivalenttofirstorder.
Additionalconstraintsonthemagneticfieldgeometrycancomefromthecross-
over and the mean quadratic magnetic field. A collinear dipole plus a quadrupole
and an octupole give good first approximations in many cases (Landstreet &
Mathys 2000). The dipole primarily accounts for the longitudinal magnetic field,
thequadrupolegivesthefieldstrengthcontrastbetweenthepoles,andtheoctupole
isresponsiblefortheequator-to-polefieldstrengthcontrast.Asymmetricvariation
curves can be determined from some magnetic field moments. They exist, if the
magneticfieldisnotsymmetricaboutanaxispassingthroughthecenterofthestar
(Mathys1993)andcanbedescribedwithageneralizedmultipolarmodel(Bagnulo
etal.2000,andreferencestherein).Theinputobservablesforthesemodelsareall
Magneticchemicallypeculiarstars 5
availableobservablesofthemagneticfield:hBzi,hxBzi,phB2i+hB2zi,hBi,andthe
BBLP. A c 2 minimization between the predicted and the observed values of the
observablesatphasesdistributedthroughouttherotationperiodwilldeterminethe
finalmodelforthegeometricstructureofthemagneticfield.
Ultimately,adirectinversionofthelineprofilesrecordedinallfourStokespa-
rameters will allow one to derive magnetic field maps without a priori assump-
tions. Since the inversion is an ill-posed problem, a regularization condition is
needed.Thisis achievedwith the magneticDopplerimagingtechnique(Piskunov
&Kochukhov2002).Itisverydemandingintermsofthesignal-to-noiseratiointhe
data,spectralresolution,andphasecoverage.Sofar,theseinversionsarerestricted
toafewindividualstars(e.g.,Lu¨ftingeretal.2010;Kochukhovetal.2004).
5 Fieldstrength distribution and rotation
Themeanlongitudinalmagneticfielddistributionextendsallthe waydowntothe
detectionlimitof100Gorless(Landstreet1982).Thermsmeanlongitudinalmag-
neticfieldaveragedoverastellarrotationperiodisoftheorderof300Gfor“clas-
sical” Ap stars, and larger (∼1kG) for hotter He weak and He strong Bp stars
(Landstreet1982).
The mean magnetic field modulus much better characterizes the intrinsic stel-
lar magneticfield than the mean longitudinalmagnetic field, which is muchmore
dependentonthegeometryoftheobservation.MostApstarswithmagneticallyre-
solvedlineshaveameanmagneticfieldmodulus(averagedoverthestellarrotation
period)comprisedbetween3and9kG.Butthereisalowercutoffofthedistribution
at2.8kG.Oneexpectstobeabletoresolvelinesdownto1.7kGorloweratsome
rotationphasesofsomestars,butonlyforonetargetitisobserveddownto2.2kG.
The lower limit of the magneticfield distributionis roughlytemperatureindepen-
dent;hotterstarsmayhavestrongermagneticfieldsthancoolerstars(Mathysetal.
1997).
Ap star variation periods span five orders of magnitude. Until recently, there
seemedtobenosystematicdifferencesbetweenshortandlongperiodstars.Acon-
firmation that very long periods are indeed rotation periods has been brought by
BBLP (Leroy et al. 1994). The systematic study of Ap stars with resolved mag-
neticallysplitlineshasdoubledthenumberofknownstarswith P>30days.The
distribution of periodslonger than 1year is compatible with an equipartitionon a
logarithmicscale. No star with P>150d has a mean magnetic field modulusex-
ceeding7.5kG.Morethan50%ofthestarswithresolvedlinesandshorterperiods
haveamagneticfieldmodulusabovethisvalue(Mathysetal.1997).Inthecollinear
dipoleplusquadrupoleandoctupolemodel,theanglebetweenthemagneticandro-
tationaxisb isgenerallysmallerthan20◦ forstarswithP>30d,unlikeforshort
periodmagneticApstars,forwhichthisangleisusuallylarge(Landstreet&Mathys
2000).
6 MarkusScho¨llerandSwetlanaHubrig
0.4 HMJDD 523139763.036 0.4 HMJDD 523179003.134 4 HD 27376 0.4 HD 28217 2 HD 32964 0.4 HD 33647 2 HD 33904 2 HD 35548
0.2 0.2 2 0.2 1 0.2 1 1
(arcsec)0.0 (arcsec)0.0 (arcsec)0 (arcsec)0.0 (arcsec)0 (arcsec)0.0 (arcsec)0 (arcsec)0
--00..42 -0 .4 -0 .2 (ar0c .s0ec) 0 .2E0 .4N --00..42 -0 .4 -0 .2 (ar0c .s0ec) 0 .2E0 .4N --42 - 4 - 2 (arc0 sec) 2 E N4 --00..42 -0 .4 -0 .2 (ar0c .s0ec) 0 .2E0 .4N --21 - 2 - 1 (arc0 sec) 1 E N2 --00..42 -0 .4 -0 .2 (ar0c .s0ec) 0 .2E0 .4N --21 - 2 - 1 (arc0 sec) 1 E N2 --21 - 2 - 1 (arc0 sec) 1 E N2
4 HD 36881 0.4 HD 41040 2 HD 42657 2 HD 53244 4 HD 53929 2 HD 59067 2 HD 72208 2 HD 73340
2 0.2 1 1 2 1 1 1
(arcsec)0 (arcsec)0.0 (arcsec)0 (arcsec)0 (arcsec)0 (arcsec)0 (arcsec)0 (arcsec)0
--42 - 4 - 2 (arc0 sec) 2 E N4 --00..42 -0 .4 -0 .2 (ar0c .s0ec) 0 .2E0 .4N --21 - 2 - 1 (arc0 sec) 1 E N2 --21 - 2 - 1 (arc0 sec) 1 E N2 --42 - 4 - 2 (arc0 sec) 2 E N4 --21 - 2 - 1 (arc0 sec) 1 E N2 --21 - 2 - 1 (arc0 sec) 1 E N2 --21 - 2 - 1 (arc0 sec) 1 E N2
2 HD 75333 2 HD 78316 2 HD 90264 2 HD 101189 2 HD 110073 4 HD 120709 4 HD 129174 4 HD 165493
1 1 1 1 1 2 2 2
(arcsec)0 (arcsec)0 (arcsec)0 (arcsec)0 (arcsec)0 (arcsec)0 (arcsec)0 (arcsec)0
-1 N -1 N -1 N -1 N -1 N -2 N -2 N -2 N
-2 - 2 - 1 (arc0 sec) 1 E 2 -2 - 2 - 1 (arc0 sec) 1 E 2 -2 - 2 - 1 (arc0 sec) 1 E 2 -2 - 2 - 1 (arc0 sec) 1 E 2 -2 - 2 - 1 (arc0 sec) 1 E 2 -4 - 4 - 2 (arc0 sec) 2 E 4 -4 - 4 - 2 (arc0 sec) 2 E 4 -4 - 4 - 2 (arc0 sec) 2 E 4
0.4 HD 216494 2 HD 221507 4 HD 34880 2 HD 158704 4 HD 178065 4 HD 66259 0.4 HD 66259
0.2 1 2 1 2 2 0.2
(arcsec)0.0 (arcsec)0 (arcsec)0 (arcsec)0 (arcsec)0 (arcsec)0 (arcsec)0.0
--00..42 -0 .4 -0 .2 (ar0c .s0ec) 0 .2E0 .4N --21 - 2 - 1 (arc0 sec) 1 E N2 --42 - 4 - 2 (arc0 sec) 2 E N4 --21 - 2 - 1 (arc0 sec) 1 E N2 --42 - 4E N- 2 (arc0 sec) 2 4 --42 - 4 - 2 (arc0 sec) 2 E N4 --00..42 -0 .4 -0 .2 (ar0c .s0ec) 0 .2E0 .4N
Fig.2 CompanioncandidatestoHgMnstars,detectedwithNACObyScho¨lleretal.(2010). –
Credit:Scho¨lleretal.,A&A,522,A85,2010,reproducedwithpermission(cid:13)cESO.
6 HgMnstars
HgMn stars are chemically peculiar stars with spectral type B8 to A0 and T =
eff
10000−14000K. They show extreme overabundanceof Hg (up to 6dex) and/or
Mn (up to 3dex).They display the most obviousdeparturesfrom abundancesex-
pectedwithinthecontextofnucleosynthesis(Cowley&Aikman1975).Morethan
150HgMnstarsareknown,manyofwhicharefoundinyoungassociations(Sco-
Cen,OrionOB1).Theyareamongthemostslowlyrotatingstarsontheuppermain
sequenceandhaveexceptionallystableatmosphereswithanaveragerotationalve-
locityofhvsinii=29km/s,whichleadstoextremelysharp-linedspectra.Theyare
the best suited targets to study isotopic and hyperfine structure. More than 2/3 of
the HgMn stars belongto SB systems with a prevalenceof P ≈3−20d. Many
orb
HgMn stars are in multiple systems. The spectrum variabilityseen in HgMn stars
isduetothepresenceofchemicalspots.Theydonothavestronglarge-scaleorga-
nized magnetic fields, but tangled magnetic fields are possible. They do not have
enhancedstrengthsofrareearthelements,butoftheheavyelementsW,Re,Os,Ir,
Pt,Au,Hg,Tl,Pb,andBi,whichmakesthemanaturallaboratoryforthestudyof
heavy elements. They also show anomalous isotopic abundancesfor the elements
He,Hg,Pt,Tl,Pb,andCa.
Scho¨lleretal.(2010)studiedthemultiplicityoflate-typeBstarswithHgMnpe-
culiarity.Fromobservationsof57HgMnstarsobtainedattheVLTwiththeNACO
instrumentin Ks withthe S13 camera,theyfound34companioncandidatesin 25
binaries,threetriples, andonequadruple(see Fig. 2).Nine companioncandidates
were foundfor the first time, five objectsare verylikely chanceprojections.Only
fivestarsinthetotalsampleshownoindicationofmultiplicity,takingintoaccount
that 44 systems are confirmed or suspected spectroscopic binaries. On the other
hand,inastudyofrapidlyoscillatingAp(roAp)stars,Scho¨lleretal.(2012)found
Magneticchemicallypeculiarstars 7
Table2 Multiplicityofdifferentstellartypes.
Type Percentage Reference SB
NormalA ∼35% Kouwenhovenetal.2005
NormalB ∼30% Kouwenhovenetal.2005
MagneticAp 43% Carrieretal.2002 VeryfewSB2
MagneticBp ∼20% Renson&Manfroid2009 VeryfewSB2
HgMn >90% Scho¨lleretal.2010 2/3
Am >90% Renson&Manfroid2009 >90%
roAp 24% Scho¨lleretal.2012 2outof∼45
90 -5.00 90 -5.00 90 3.69 -5.00
457.25 -5.75 454.19 4.19 -5.75 45 -5.75
Latitude 0 7.25 -6.50 Latitude 0 4.56 4.564.56 -6.50 Latitude 0 4.44 3.69 -6.50
--94050 7.75 8.2590 8.258.75 180 7.75 8.25270 7.75360--87..0205 --940504.19 4.194.194.56 94.560 4.945.31 1804.94 2704.19 360--87..0205 --940503.69 4.06 4.44 94.810 4.06 4.064.44184.814.440 4.064.062704.064.44 360--87..0205
Longitude Longitude Longitude
phase=0.0 phase=0.25 phase=0.0 phase=0.25 phase=0.0 phase=0.25
phase=0.50 phase=0.75 phase=0.50 phase=0.75 phase=0.50 phase=0.75
Fig.3 MapsoftheabundancedistributionforFe(left),Sr(middle),andY(right)onthesurface
oftheprimaryinthesystemARAur.–Credit:Hubrigetal.,A&A,547,A90,2012,reproduced
withpermission(cid:13)cESO.
a total of six high probabilitycompanioncandidatesin a surveyof 28 roAp stars.
roAp stars pulsate in high-overtone,low-degree, nonradial p-modes, with periods
in the range from 5.6 to 21min and typical amplitudes of a few millimagnitudes
(e.g.Kurtzetal.1982).Theyareidealtargetsforasteroseismology.Theintriguing
questionisifandhowmultiplicitycanshapetheappearanceofchemicallypeculiar
stars.AnoverviewabouttheprevalenceofbinariesindifferentclassesofCPstars
andnormalstarscanbefoundinTable2.
One of the most exciting objects containing a HgMn star is the triple system
ARAur.Theinnertwostarsconstitutetheonlyknowneclipsingbinaryencompass-
ing a HgMn star. This binaryhas an orbital periodof 4.13d and an age of 4Myr.
The two stars are of spectral typesB9V andB9.5V, andwhile the primaryHgMn
star is exactly on the ZAMS, the secondary is still contracting(e.g. Nordstro¨m&
Johansen1994).Hubrigetal.(2012)usedobservationswithSOFIN attheNordic
OpticalTelescopetostudythedistributionofdifferentelementsoverthesurfaceof
the primary HgMn star, using the Doppler mapping technique (see Fig. 3). From
thesamedataset,theyalsodeterminedthemagneticfieldinbothprimaryandsec-
ondary(Fig.4).ARAurshowsasimilarbehaviortootherHgMnsystemsdiscussed
8 MarkusScho¨llerandSwetlanaHubrig
400
G] 200
B> [z 0
<-200
-400 Y
400
G] 200
B> [z 0
<-200
-400 Fe
400
G] 200
B> [z 0
<-200
-400 Ti
-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2
Fig.4 Measurementsofthemeanlongitudinalmagneticfieldpresentedasafunctionofthero-
tationphaseforARAur.Theywerecarriedoutseparately fortheelementsTi,Fe,andY(from
bottomtotop).Thesolidlinedenotestheprimarycomponent,whilethedashedlinedenotesthe
secondarycomponent.Filledcirclesindicate3s measurements.–Credit:Hubrigetal.,A&A,547,
A90,2012,reproducedwithpermission(cid:13)cESO.
byHubrigetal.(2012).Theresultssuggesttheexistenceofacorrelationbetween
themagneticfield,theabundanceanomalies,andthebinaryproperties.Forthesyn-
chronouslyrotatingcomponentsoftheSB2systemARAur,itlooksasifthestellar
surfaces facing the companion star usually display low-abundance element spots
and negativemagnetic field polarity.The surface of the oppositehemisphere,as a
rule,iscoveredbyhigh-abundanceelementspotsandthemagneticfieldispositive
atthe rotationphasesof thebest-spotvisibility (Hubrigetal. 2010).Still, thedis-
cussionaboutthe presenceofweakmagneticfieldsin HgMnstarsisstill ongoing
(seeKochukhovetal.2013).
7 Summary
Chemicallypeculiarstarsareprobablythemostchallengingmainsequencestarsto
modeldue to their magnetic fields, and the element segregationand stratification.
Theyareidealatomicphysicslaboratories.Theyarethebestobjectstolearnabout
magneticfieldmodels,tobeappliedtootherclassesofstars.Binarityandmultiplic-
ityforthedifferentclassesisdifferent,whichpotentiallyleadstonewinsightsinto
starformationmechanisms.
Magneticchemicallypeculiarstars 9
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