Explaining the transient fast blue absorption lines in the massive binary system Eta Carinae

0.63 MB·English

by  Amit Kashi

Mon.Not.R.Astron.Soc.000,1–8(2010) Printed1February2011 (MNLATEXstylefilev2.2)
Explaining the transient fast blue absorption lines in the massive
binary system η Carinae
1 Amit Kashi1⋆, Noam Soker1†, and Muhammad Akashi1‡
1
0 1DepartmentofPhysics,Technion–IsraelInstituteofTechnology,Haifa32000,Israel
2

n
Accepted0000;Received0000;inoriginalform2010October3
a
J

9
ABSTRACT
2
We use recent observations of the He I λ10830Å absorption line and 3D hydrodynamical

] numericalsimulationsofthewindscollision,tostrengthenthecaseforanorientationofthe
R semimajoraxisofthemassivebinarysystemηCarinaewherethesecondarystaristowardus
S atperiastronpassage.Thoseobservationsshowthatthefastblueabsorptioncomponentexists
. for only severalweeks prior to the periastronpassage. We show that the transientnature of
h
thefastblueabsorptioncomponentsupportsageometrywherethefastsecondarywind,both
p
preandpost-shockmaterial,passesinfrontoftheprimarystarnearperiastronpassage.
-
o
r Key words: Stars: individual: η Car – Stars: winds, outflows – Stars: mass-loss – (Stars:)
t binaries:general–Infrared:stars
s
a
[

2
v 1 INTRODUCTION istowardusatperiastronpassage.Severalpropertiesofthebinary
0
systemhavebeenusedtodeducetheorbitalorientation,withcon-
0 η Car isavery massive stellar binary system, withan orbital pe-
tradictingresults(fordetailsseeKashi&Soker,2008b,2009c).
6 riod of 5.54 yr (Damineli 1996), as observed in all wavelengths
Oneofthepropertiesthatledtoacontradictingconclusionon
2 (e.g.,radio,Duncan&White2003;IR,Whitelocketal.2004;vis-
theorientation isthebehavior of the blue absorption wing of the
4. ible, van Genderen et al. 2006, Fernandez-Lajus et al. 2009; UV, He I λ10830Å line. In Kashi & Soker (2009b) we constructed a
0 Smith et al. 2004; emission and absorption lines, Nielsen et al.
toy model where the material responsible for the blue absorption
0 2009,Daminelietal.2008a,b;X-ray,Corcoran2005,2010,Ham-
wing was assumed to reside in the colliding winds region – the
1 aguchietal.2007).Thehigheccentricityofe≃0.9resultsinrapid
conicalshell–closetothebinarysystem.Thismodelisabletoac-
: changesinemissionandabsorptionlines,aswellasinthecontin-
v countforthetransientappearanceoftheblueabsorbingwingand
uum,neareachperiastronpassage.Theseveralweeksoftherapid
i tothefindingthatthemaximumabsorbingvelocityisreachedsev-
X changesoccurringeveryorbitalperiodistermedthespectroscopic
eraldaysbeforeperiastronpassage,onlyifthesecondaryistoward
r event.Theselinesmightoriginateindifferentplacesinthebinary usnearperiastronpassage,i.e.,ω≃90◦.InarecentpaperGrohet
a system:theprimarystar,thesecondarystar,theirrespectivewinds,
al.(2010;hereafterG2010)reachedanoppositeconclusion.Com-
andthecollidingwindsstructurewhichistermedtheconicalshell.
paringtheirobservationswithamodelbasedon3Dnumericalsim-
As η Car is the best studied binary luminous blue variable
ulationsofthecollidingwindsstructure,G2010suggestedthatthe
(LBV), it holds the key to our understanding of other LBVs. It
orientationisthatofω=243◦.Namely,theprimaryisclosertous
isparticularyimportanttounderstandthebehaviornearperiastron
justbeforeperiastronpassage.Inthispaperwecriticallyreexamine
passage,wherethestrongestbinaryinteractiontakesplace,andfor
bothmodels.
thatiscrucialtoknowtheorientationofthebinarysystem.Namely,
thedirectionoftheprimarymoremassiveLBVstarrelativetoits
lessmassivebuthottercompanionatperiastronpassage.Whileitis
agreedthattheinclinationofthebinarysystemisi ≃ 41◦ (David- 2 PROPERTIESOFTHEABSORPTIONPROFILE
sonetal.2001;Smith2006),thereisnoagreementonthedirection
ofthesemimajoraxis,termedperiastronlongitude.Theorientation TheHeIλ10830Ålinehasbeenobservedformorethanadecade,
ismeasuredbytheangleω:ω=0◦forthecasewhenthesecondary but only occasionally (Damineli et al. 1998, Groh et al. 2007;
movestowardusbeforeperiastronpassageandthesemimajoraxis Daminelietal.2008b; G2010). ThelineshowsaP-Cygniprofile
isperpendiculartothelineofsight,ω = 90◦ whenthesecondary varyingintime,especiallyclosetothespectroscopicevent.Ithas
istowardusatperiastronpassage,andω=270◦whentheprimary a complicated emission with three spikes, and absorbing compo-
nents with velocities of up to −v ≃ 1900 km s−1. In this section
wedescribetherelevantpropertiesoftheblueabsorptionwingthat
rapidlyappearsanddisappearsnearperiastronpassages.
⋆ [email protected]
† [email protected] G2010observedtheHeIλ10830Åacrossthe2009periastron
‡ [email protected] passage.InFig.1weshowtheabsorptionprofileatphase11.998,
2 A. Kashiet al.
ittothecollidingwinds,inthemodelofG2010theabsorbinggas
1.4
isat a distance of tensof AU from the binary systems, whilein
1.2 F: our model theabsorbing material isrelatively closeto thebinary
fast transient system.
component:
x 1 secondary Onethingtonoticeisthatthefirstobservationafterthe2009
ed Flu0.8 wthien ds eacroounnddary sthpeecXtr-orascyompiicniemveunmti(sCaotrpchoarsaen1220.01104),,abnedfobreeftohreestyhsetehmardexiiotsnifzrionmg
z M:
mali0.6 extra− radiation from the secondary resumes after exiting the dense pri-
or accelerated mary wind. As we note in section 3, this holds difficulties to the
N primary
0.4 wind modelofG2010.
P:
0.2 primary
wind
0 3 PROBLEMSWITHTHEMODELOFG2010
−2200−2000−1800−1600−1400−1200−1000 −800 −600 −400
Velocity (km s−1) The purpose of this section is to set the motivation for a model
wherethefastabsorbingmaterialresideclosetothesecondarystar,
Figure1. Theflatpartofeachofthethreeabsorptioncomponentsidentified
and the secondary star is in front of the primary near periastron
inthepaper,markedontheabsorptionprofileatphase11.998(fourdays
beforetheperiastronpassageofthe2009spectroscopicevent)takenfrom passage. We critically examine a model with an opposite binary
G2010. orientation,andrevealitsproblems.
G2010 present (intheir fig.12m-o) thetotal columndensity
asafunctionofgasvelocityat5phasesaccordingtotheirassump-
four days before the 2009 spectroscopic event, assumed to occur tions.Theirmainassumptionsarethat:(1)Thephotosphericradius
atphase 12.000 (Damineli etal.2008b, Groh&Damineli2004), of the primaryat 1.083 µmis2.2 AU.(2) A point source can be
takenfromfig.1ofG2010.Theprofileatphase11.991ispracti- used for the central source of the1.083 µm continuum emission.
callysimilarinshape,buttheabsorptionissomewhatweaker. (3)Thecentralsourceistheonlysourceofthecontinuum(below
Atphases11.998and11.991 wecanidentifythreeflatparts weclaimthattheconicalshellisanadditionalsource).(4)G2010
inthespectrum,asmarkedonFig.1. didnotconsiderevenqualitativelythechangeinlevelpopulation.
Intheirmodelthechangeinlevelpopulationmustbesignificant;
(i) P-component. The deepest component can be safely iden-
belowwearguewhythisisunlikely.
tified with the regular primary wind. This part exists during all
Inmore details,their fig.12 givesthe densityof material as
phases,itisdeep,anditsvelocitymatchesthatoftheprimarywind.
a function of line of sight distance to the primary for 5 orbital
Theprimarywindreachesterminalvelocitiesof500–650kms−1,
phases (11.875, 11.991, 11.998, 12.014 and 12.041) and 6 orien-
dependingonlatitude(Smithetal.2003).Thereisnodisputeabout tations(ω = 0◦,50◦,90◦,180◦, 243◦, and270◦). Inaddition, the
theoriginofthiscomponent.
column density along the same assumed line of sight to the pri-
(ii) M-component. Althoughlessprominent, theM-component
mary star is given as a function of the line of sight velocity, for
hasaflatprofileinthevelocityrange−v=620–720kms−1.Weare
eachorientationandphase.Fig.12ofG2010allowsustodeduce
notsureaboutitsnature,butwedoseesignaturesthatitexistsat
thebasicpropertiesoftheirmodel.
phases11.875,12.014and12.041,althoughlessprominently.We
G2010 didnot relatethetotal columndensity totheabsorp-
attributeittopartsintheprimarywindthatareacceleratedtosome-
tionoftheHeIλ10830Åline.Weattempttodosoandcheckwhat
what higher velocities. These may be, for example, clouds with
theresultsofG2010imply.Aswenowshow,theirmodel,witha
higheropacity,e.g.,duetodustformation.Wenotethatbeforethe preferred orientation of ω = 243◦, suffers from severe problems
2009 periastron passage the primary wind might have undergone
becausethecolumndensitydoesnotfollowevenqualitativelythe
somefluctuationsinitsvelocity,whichmayhavebeenevidentin
behavioroftheobservedabsorption.Intheirattempttoresolvethis
theX-raylightcurve(Kashi&Soker2009d).
discrepancy, G2010 claim that the presence of high velocity gas
(iii) F-component.Weidentifythemainpartsofthiscomponent
inour lineof sight isanecessary, but not sufficient condition for
with the flat part in the velocity range −v ≃ 1150–1400 km s−1,
thepresenceofhighvelocityabsorption.Astheamountofabsorp-
and the tail formed by faster and lower column density gas with
tion depends on the population of the lower energy level of the
absorption extending to −v = 1900 km s−1. This tail is not due HeIλ10830Åline(23S),theyattributethedifferencesinabsorp-
to line broadening in the main part of the absorbing component. tiontodifferentpopulationofthe23S atomiclevel.
The F-component does not exist at all at phases 11.875, 12.014
and 12.041. Namely, it is a transient component appearing for a
shorttimebeforeperiastronpassage.Initsflatpartthiscomponent 3.1 Pre-periastronpassage
absorbs∼ 25−30percentofthecontinuumatphase11.998,and
For the three phases before periastron passage (11.875, 11.991,
∼20percentatphase11.991.
11.998), G2010 model (in their fig. 12o) predicts the absorbing
WeattributetheF-componenttopreandpost-shocksecondary columndensityintherange−v = 2000–3000 kms−1 tobeabout
windmaterialclosetothebinarysystem(Kashi&Soker2009b). equaltothatinthevelocityrange−v=1600–1800kms−1.Obser-
Thepost-shockgasresidesintheconicalshell.Wefurtherassume vationsshowsubstantialabsorptioninthe−v=1600–1800kms−1
thatthesecondarywindgasabsorbsmostofthecontinuumemitted rangeatphases11.991(∼ 3−10percentabsorption)and11.998
bythegasintheconicalshell(emissionthatincreasesjustbefore (∼ 5 − 13 per cent absorption), while there is no absorption at
periastronpassage), inadditiontoabsorbing somefractionofthe −v > 1900 km s−1 (fig. 1 of G2010). G2010 attribute the differ-
continuumemittedbytheprimarywind.Itisthiscomponentthatis encetostratificationinthepopulationofthe23S level,fromwhich
indisputebetweenG2010andus.AlthoughG2010andusattribute theHeIλ10830Å lineisabsorbed. However, theydonotpresent
Transientabsorptionlinesin ηCar 3
aqualitativestudyofthewaythepopulationofthe23S levelcan observations,asthereisnoabsorptionatallfor−v>1100kms−1
evolvesorapidly. Wefinditextremelyunlikely(unlessthereisa atphase12.014(fig.1ofG2010).Thisisasevereproblemforthe
carefully arranged fine tuning) that differences in populating the modelpresentedbyG2010.
23S levelcanaccountforthisdifference.Thereasonisasfollows. NotonlythemodelofG2010,butanyothermodelthatwould
AccordingtoG2010modelduringthephaseinterval11.991– assume absorption bymateriel residing at alargedistance (afew
11.998 the absorbing gas at −v = 2000 km s−1 resides at a dis- ×10aormore)isproblematic.Thereasonisthattheabsorptionof
tance of r ≃ 3.5a fromthe binary system, where a ∼ 16 AU is theHeIλ10830Ålineinhighvelocitiesisatransientevent,which
a
thesemimajoraxis(theirfig.12n).Theyalsofindthatthegasab- occurs very close to periastron, and must be related to variations
sorbing in the velocity range −v = 1600–1800 km s−1 resides at inthesystemshortlybeforeperiastron. Thematerialfarfromthe
r ≃ 2.5−3a. During thisphase interval, the binary orbital sep- binary system does not suppose to show fast variations close to
a
arationisr . 0.2a. Therefore, thedifferences intheradiationof periastron,andforthatitisabadcandidateforanabsorberofthe
the two stars at r ≃ 2.5−3a and r ≃ 3.5a are small, and can- HeIλ10830Åline.
notaccountfortherequiredlargedifferencesinthepopulationof
the23S level.Moreover,theincreaseintheamountofabsorption
asthesystemapproaches periastroninthemodel ofG2010 isat-
tributedtoanincreaseinthepopulationofthe23S level.Thisis 4 THEMODEL
probably(G2010donotspecifythereason)becausethesecondary
4.1 Thecolumndensityoftheabsorbinggas
getsdeeperintotheprimarywind,andlessofitsradiationreaches
theabsorbinggas.Itisexpectedthattheouterregions,wheregas We perform hydrodynamic simulations of the colliding winds to
movingat−v=2000kms−1resides,willgetlessofthesecondary show that the column density of the conical shell and the pre-
radiation,andhencewillabsorbmore.Thisisoppositetowhattheir shocked secondary wind at high velocities is high enough to ac-
modelrequires. countfortheabsorptionoftheHeIλ10830Åline.Thesimulations
A potential way to handle the problem is that the material areperformedwithVirginiaHydrodynamics-I(VH-1),ahighreso-
closer-in is of higher density, and it recombines first or its colli- lutionmultidimensional astrophysicalhydrodynamicscodedevel-
sionalexcitationismoreefficient.Thiswouldnotworkeither.The opedbyJ.Blondinandcollaborators(Blondinetal.1990;Stevens
rateofthesetwoprocessesislinearwithdensity.Thedensityratio etal.,1992;Blondin1994).Thecodeincludesradiativecoolingat
betweentheregionabsorbinginthe−v=1600–1800kms−1range alltemperaturesofT > 2×104 K.Theradiativecoolingissetto
tothatabsorbinginthe−v≃2000kms−1rangeis∼2(G2010;fig. zerofor temperaturesofT < 2×104 K,andthegascannot cool
12m,n).Therefore,theabsorptionat−v≃2000kms−1isexpected tolowertemperatures.ThecoolingfunctionΛ(T)(forsolarabun-
tobeonehalfofthatinthe−v=1600–1800kms−1velocityrange. dances)istakenfromSutherland&Dopita(1993;theirtable6).
Thisamountsto∼1.5−5percentofthefluxatphase11.991and The numerical simulations were performed in the Cartesian
∼ 2.5−6.5 per cent of the flux at phase 11.998. This is clearly geometry(x,y,z)modeof thecode(a3Dcalculation), wherethe
ruledoutbyobservations,astheabsorptionat−v = 2000kms−1 orbitalplaneisthe(x,y)planewiththe xaxistakentobeparallel
observedinthesetwophasesispracticallyzero. tothelineconnectingthetwostars.Ournumericalsimulationgrid
has112equal-sizegridpointsalongeachaxis.Theprimaryandthe
secondaryarelocatedinthemiddleoftheyandzaxes.Thebinary
3.2 Post-periastronpassage
separationbetweenthetwostarsissettobeconstantateachnumer-
Inphase12.014(28daysafterphase12)thecolumndensitycalcu- icalrun(forthejustificationseeAkashi&Soker2010).Themass
latedbyG2010inthevelocityrange−v= 1200–1400kms−1isa lossratesandvelocitiesofthewindsareM˙ =3×10−4M yr−1and
1 ⊙
factorof2-3lowerthanthatatphase11.998,whilein−v=1600– M˙ = 10−5 M yr−1,andv = 500kms−1 andv = 3000kms−1,
2 ⊙ 1 2
1800kms−1 thisratiois< 2(theirfig.12o).Atphase12.014the respectively(e.g.,Pittard&Corcoran2002;Kashi&Soker2008b
secondaryradiationhasnotstartedyettoionizethematerialatlarge andreferencestherein).Weuseeccentricityofe = 0.9andsemi-
distances,asitisstillinsidethedenseregionoftheprimarywind. majoraxisofa = 16.64AU.Weperformedfourrunswithbinary
Weknow thisfromthebehavior of thehighly ionized lines, e.g., separationofr=2,3,4,and10 AU,andineachcreatedadensity
intheWeigeltblobs,whichshow thatthehardradiationresumed profileandwindvelocityfieldaroundthetwostars.Atthelocation
(inthe2003.5spectroscopicevent)wellaftertheX-rayminimum ofeachstarweinjectitsappropriatewind.Welettheflowreachan
ended (Damineli et al. 2008b; Mehner et al. 2010). In the 2009 approximatelysteadystate.Anexampleoftheresultsofthesimu-
spectroscopiceventtheX-rayminimumendedatphase∼ 12.014 lationisshowninFig.2.
(Corcoran 2010), when one of the He I λ10830Å line measure- Foreach runwecalculatethedirectiontotheobserver fora
ments was conducted. Namely, at this phase the secondary hard systemwith(ω,i)=(90◦,41◦),andforthephasebeforeperiastron
ionizingradiationdidnotionizetheHeintherelevantdistanceof correspondingtothebinaryseparationoftherun.Wetakeaplane
tensof AU.Thedensityintherelevantregionatphase12.014is perpendicular tothelineofsight andlocatedbehind theprimary,
notmuchdifferentfromthatatphase11.998.Ifitwasforrecom- andfromthisplanewecalculatethecolumndensityalong agrid
bination, there should have been more time for the gas at phase oflinesofsighttotheobserver.Weselectthelocationofthisplane
12.014torecombine. inanapproximatelyoptimalwaytoincludeasmuchofthegridas
Over allinthevelocityranges−v = 1200–1400 kms−1 and possible. Thisprocedure favorsaccuracy inthecalculationof the
−v = 1600–1800kms−1 thecolumndensitycalculatedbyG2010 columndensityofthehighvelocitygastowardstheobserver.Each
atphase12.014is∼0.3−1thatatphase11.998,andtheionization lineofsightisdividedintocells,asthenumberofzplanestheline
stateshouldbethesame.Therefore,itisexpectedthattheabsorp- ofsightcrossesinourgrid.Wecalculatethecolumndensityalong
tionatphase12.014inthevelocityranges−v=1200–1400kms−1 each line of sight as function of the velocity projected along the
and−v=1600–1800kms−1wouldbe∼0.3−1timesthatatphase lineofsight.LikeG2010weusevelocitybinsof50kms−1.
11.998,orevenaboveitsvalueatphase11.998.Thisiscontraryto We adopt the conclusions of G2010 that the main source of
4 A. Kashiet al.
1023
r=2AU
r=3AU
−1s)1022 r=4AU
m r=10AU
k
50 1021
−2m /
y (c1020
sit
Den1019
n
m
olu1018
C
1017
−3000 −2500 −2000 −1500 −1000 −500 0
v (km s−1)
Figure3. Thecalculatedcolumndensityasafunctionofprojectedvelocity
alongthelineofsight.
fig.12ofG2010thisisnotthecase).(4)Thecolumndensityde-
creasestowardshigherbluevelocities,despiteofminorfluctuations
duetothelimitednumericalresolution.(5)Atshortbinarysepara-
Figure2. Ourhydrodynamicalsimulationsshowingdensityandvelocity tions(r.4AU;t&0.987)thereisaplateauinthecolumndensity
mapsforanorbitalseparationofr =2AU.Thebarontherightgivesthe between∼1400kms−1and∼2100kms−1,followedbyadecrease
densitycolor-codeinunitsoflog[ρ(gcm−3)].Thethinarrowsrepresentthe
incolumndensitytowardshigherbluevelocities.
velocityinthe(x,y)plane(theorbitalplane)andarescaledwiththeaxes
Our model gives that at very high velocities of −v >
suchthatoneunitrepresents5000kms−1.Thesecondaryisthestaronthe
1900kms−1thecolumndensityisalsolarge,namelywehavemore
rightat(x,y) = (5,4)AU.Thearrowsareinthelineofsightdirectionto
anobserverat(ω,i)=(90◦,41◦)andatphase0.9964.Thecolumndensity absorbing gas thanrequired toaccount for theobservations. This
fromeachlineofsightisweightedwithaGaussian(equation1). discrepancy may be the result of an anisotropic secondary wind
velocityclosetoperiastronpassage,thatcanoccuriftheaccelera-
tionofthesecondarywindisnotisotropic,aneffectwhichwasnot
the1.083µmcontinuumemissioncomesmostlyfromaregionof takenintoaccountinoursimulation.Suchaneffectcanreducethe
size∼ 2.2AUcenteredontheprimarystar.Wetermitthecentral amountofgaswhichoutflowsat−v>1900kms−1.Theaccelera-
source. Though G2010 usedapoint sourcefor thecentral source tioncanbeefficientalongpolardirections,butbelowi≃60◦radi-
ofthe1.083µmcontinuum emission,theymentionedthatitisof
ationfromtheprimaryandpreviousaccretionofblobsmightcause
sizeofseveral AU,basedonobservationsbyWeigeltetal.(2007)
thesecondarywindtobeslower.Thisexplanationcannotworkfor
intheK-band,whichwerecalibratedto1.083µm(whichiswithin
correctingthesurplusabsorbinggasinthemodelofG2010,astheir
theI-band),withthehelpofthetheoreticalresultsofHillieretal.
absorbingmaterialismuchfurtherawayfromthecollidingwinds
(2001).Weigeltetal.(2007)foundthedecreasefunctionofthecen-
region.
tralsourceintensitywithprojecteddistancefromthecenterr tobe
p
closetoaGaussian(theirfig.6).Wethereforetaketheemissionto
befromaradiusofR =4.4AU,andweightedtheintensityasa
em 4.2 Coveringtheemissionsourcewiththeconicalshell
Gaussianwithσ=2.2AU
From Fernandez-Lajus et al. (2010) we find that the I-band con-
I=I exp(−r2/2σ2); forr 6R , (1)
0 p p em tinuum has a quiescence value of I = 3.55 mag. We sample the
whereI istheintensityatthecenter(thecut-offatR isuseddue increaseintheI-bandcontinuumfluxabovethequiescencevalue,
0 em
tonumericalconstrainsandresultsinanegligibleerror). f (t),fromtheobservationsofFernandez-Lajusetal.(2010).These
c
Thecolumndensityfromeachlineofsightisweightedwith observationsshowthatclosetothe2009periastronpassagetheI-
theGaussianinequation(1)accordingtoitsdistancer fromthe bandcontinuumflux(andthereforethe1.083µmlineflux)hasin-
p
line of sight of the primary (Fig. 2). This procedure gives us an creasedby f (t=0)≃20percentabovethequiescencevalue(Fig.
c
effectivecolumndensityineachvelocityintervalandateachphase. 5).Weattributemostofthisextraemissiontotheconicalshellclose
TheresultsarepresentedinFig.3. tothebinarysystem.Wefurtherassumethatthereisastrongab-
From Fig. 3 we learn the following: (1) For ω = 90◦ the sorptionbytheconicalshellandthepre-shocked secondarywind
column density in the conical shell and the undisturbed wind is oftheHeIλ10830Åline.
high enough to account for the absorption of the He I λ10830Å Weanalyticallycalculatethefractionofthecentralsourceof
line for our favored orientation of ω = 90◦. As the model of the1.083µmcontinuumemissionregionthatiscoveredbythecon-
G2010,ourmodelisunabletocalculatethepopulationofthe23S icalshell(namely,coveredbehindtheconicalshellfromthelineof
level of He I, and hence we cannot calculate the exact absorp- sightoftheobserver).Theparametersofthestarsandtheconical
tion in the He I λ10830Å line. (2) The high column density for shellaretakenasinourpreviouspapers(Kashi&Soker2009b,c,
−v&1000kms−1isatransientcomponentwhichappearsonlyfor andreferencestherein).Thecontactdiscontinuityisapproximated
a short period before periastron passage. (3) The column density as an hyperboloid with semi-opening angle φ ≃ 60◦. This is an
a
clearlyincreasesasthesystemapproachesperiastron(notethatin approximation,asnumericalsimulationsshowthatthestructureis
Transientabsorptionlinesin ηCar 5
1
0.9
F (t)/F (t) :
obs continuum
0.8 The predicted 1.08 µm flux
for: f=0.5
0.7 s
f=1
s
0.6
0.5
0.4
f(t) : The increase in the f(t) : The covered part of the 1.08 µm
0.3 c p
I−band continuum flux central continuum emission source
0.2
0.1
0
0.96 0.965 0.97 0.975 0.98 0.985 0.99 0.995 1
t
Figure5. Behaviorofseveralpropertiesjustbeforeperiastronpassage.The
I-bandcontinuumfluxesbeforethe2009.0and2003.5periastronpassages
Figure4. Thegeometryofthebinarysystemandthe1.083µmcontinuum
areverysimilar.Circlesandsquares:Theobserveddecreaseintheflatpart
emissionregion.
of the F-component prior to the 2009.0 and 2003.5 periastron passages,
respectively.Theerrorbarsarecalculatedfromthemaximumandminimum
absorptionvaluesinthevelocityrangeoftheF-component.
muchmorecomplicated(Akashi&Soker2010;Parkinetal.2011).
Wetakeintoaccounttherotationoftheconicalshellclosetoperi-
astron,causedbythenonnegligibleorbitalvelocityrelativetothe
primary wind velocity, and the acceleration of the primary wind (theprimarystar).Whenω = 90◦ ourmodelaccountsforthefast
(Kashi&Soker2009b,c).Theeccentricityistakentobee =0.9, increaseinabsorptionoftheflatpart(F-component;Fig.1).Con-
suchthattheorbitalseparationatperiastronisa =1.66AU.This sideringthemanyuncertaintiesandthesimplicityofourmodel,our
p
impliesthatthesecondarystarandpartoftheconicalshellarein- resultsareinreasonableagreementwiththeobserveddecreasein
sidethecentral1.083µmcontinuumemissionregionnearperias- theflatpartoftheF-componentinthevelocityrange−v ≃ 1150–
tronpassage.Ourassumptionsbreakdownafterperiastronpassage, 1400kms−1(thetailto−v=1900kms−1ispartofthiscomponent,
whentheconicalshellcollapses. andbehavesinthesameway).
Weagainuseourpreferredorientation(ω,i)=(90◦,41◦)The Thesizesofthecentralandextraemissionsourcesareimpor-
geometry of the calculation is shown in Fig. 4. For every orbital tantforthismodelling.G2010useapoint-likecontinuumsourcein
phasewedividetheobservedhalfspherearoundtheprimarythat theirquantitativecalculationofthecolumndensityoftheabsorbing
isclosertotheobserverintosphericalvolumeelements(bins).For material(totalcolumndensity,notonlythatoftherelevantatoms).
eachemissionsourceelementwecheckwhethertheconicalshellis ForthemodelofG2010,whereω = 243◦,itdoesnotmakemuch
betweentheobserverandtheemissionsourceelement,orwhether differencetouseapointsource,astheirabsorbingmaterialissev-
itiswithintheconicalshellitself.Iftheconicalshellisbetweenthe eraltensof AUfromthesource.However,calculatingthecolumn
observerandtheemissionsourceelement,weuseourassumption densitytoapointsourcefailstoreproduceresultsofamodelwhere
of high optical depth in the conical shell, and assume the emis- theabsorptionoccursafew AUfromthestars,suchasourmodel
sionfromthecoveredsourceistotallyabsorbed.Inotherwords,if (Kashi&Soker2009b),whichweimprovehere.G2010dodiscuss
theemissionsource element iseitherofthetwo(behindthecon- (intheirsection5.3) a2.2AUextended sourcefor anorientation
ical shell or inside it), we consider this element to contribute to ofω = 90◦,andfindthedurationof thehighvelocityabsorption
the1.083µmlineabsorption.Thecontributionofeachelementis tobetooshort.Ourpointhereisthatanextrasourcemustbein-
weighedastheGaussian,withtheprojecteddistancetothelineof cluded totake intoaccount theincreaseintheI-band andfor the
sight,asdiscussedabove. extraabsorption.Inourmodeltheextrasourceistheconicalshell.
Thecalculatedfluxofthe1.083µmfluxlineaccordingtoour Ourresultsshow that onemustuseanextended emissionsource,
modelis bothforthecentralsource(theprimarystar)andtheextrasource
F (t) 1− f (t)+ f (t)(1− f ) fromtheconicalshell.Usingthetwoextendedsourcesovercomes
obs = p c s ; F−component (2) theobjectionofG2010totheω=90◦orientation.
F (t) 1+ f (t)
continuum c
whereF (t)isthe1.083µmcontinuumflux(givenbyGroh
continuum
etal.2010,asdefinedabove), f isthefractionofthecentralcontin-
p
uumsourcecoveredbytheconicalshell, f istheincreaseintheI-
c 5 SUMMARYANDDISCUSSION
bandcontinuumfluxabovethequiescencevalue(Fernandez-Lajus
etal.2010),and f isthefractionoftheconicalshellemissionthat TheHeIλ10830ÅlineofηCarhasbeenobservedacrossthe2009
s
isabsorbedbytheF-component.Iftheconicalshellselfabsorball periastron passage by G2010. We identify three main absorbing
itsemission,then f = 1,whileifitabsorbsonlyfromitsvolume components intheHeI λ10830Å lineprofile(Fig. 1). Twocom-
s
that is behind the conical shell,then f = 0.5. We present results ponentscanbeidentifiedwiththedenseprimarystellarwind;the
s
for both these values that bound our expectation. Fig. 5 presents P-component(500–650kms−1)andtheM-component(−v=620–
F (t)/F (t) close to periastron, as well as f and f . The 720kms−1).Thethird,moreinterestingF-componentshowsaflat
obs continuum c p
conical shell absorbs up to f (t = 0) ≃ 25 per cent of theinten- partinthevelocityrange −v ≃ 1150–1400 kms−1,withatailto
p
sityof the1.083µmcontinuum emissionfromthecentralsource −v = 1900 km s−1. The tail results from faster moving gas, and
6 A. Kashiet al.
not from line broadening. There is no absorption in this line at tinuum emission source (equation 2). By that we show that for
−v>1900kms−1. ω = 90◦ our model explains the observed increase in absorption
G2010presenttheirinterpretationsoftheirobservations,con- strength of the F-component close to periastron passage (Fig. 5),
cludingthattheorientationofthebinarysystemissuchthatω = andexplainsitstransientnature.
243◦ (theprimaryclosertotheobserverjustbeforeperiastron).In (xi) Wethereforeconcludethattheorientationisindeedthatthe
section3wepoint out someproblems inthemodel of G2010, in secondaryistowardusnearperiastron(ω=90◦).
particular that they cannot account for the transient nature of the (xii) Inaddition,usinghydrodynamicsimulations(Fig.2and3)
line.ThemodelofG2010failstoreproduce,evenqualitatively,the we find that the He I in the conical shell and in the pre-shocked
observed absorption profiles of the He I λ10830Å line, and can- secondary wind has a substantial column density in the velocity
not account for the absence of the F-component after periastron range−v=1150–1400kms−1.Weassumethatthefraction(outof
passage.Inanalyzingour(Kashi&Soker2008b,2009b)preferred totalHeatomsandions)oftheHeIinthe23S levelishighenough
orientationofω = 90◦,G2010useapoint-likecentralcontinuum toabsorbmostradiationinthe−v<1400kms−1(withatailupto
sourceforcalculatingthecolumndensityoftheabsorbingmaterial. −v≃1900kms−1).
Basedonthat,theyclaimthattheω = 90◦ orientationisnotcom-
Thefindingthatthesecondaryisclosertousnearperiastron
patiblewiththeirobservations.Insection4wefindthattheusage
requiresthatsomepropertiesnearperiastronpassagebeexplained
ofacentralpointsourcedoesnotdojusticetotheω = 90◦ case.
bytheaccretionoftheprimarywindontothesecondarystar(Kashi
Thisisthemainreason whyG2010 failtoreproduce resultsof a
&Soker2008a,2009d).Foraccretiontooccurontothesecondary
modelwheretheabsorptionoccursafew AUfromthestars,like
star, that has a strong wind of his own, the binary must be close
inourmodel.
(Akashi&Soker 2010) andtointeractstronglywiththeprimary.
Weimproveourperviousmodel(Kashi&Soker2009b)which
Theaccretionthatoccursnearperiastronpassageiscrucialtothe
assumedthattheconicalshellisthemainabsorberofthehighve-
understanding not onlythepresent behavior ofηCar,butalsoits
locitycomponentoftheHeIλ10830Åline(section4).Ourmodel
behaviorduringthemajoreruptionsitundergoneinthe19thcen-
andresultscanbesummarizedasfollows:
tury,theGreatEruption(GE)andLesserEruption.
Thedebateontheabsorbingsourceofthebluewinghasim-
(i) Weassume that thecentral star isaGaussian weighted ex-
plicationsfarbeyondthespecificquestionontheorientationofthe
tendedcontinuumsourceasgivenbyequation(1).Thisassumption
majoraxisof thebinarysystem. Theessence of thedebate isthe
standsonasolidground(G2010;Weigeltetal.2007).
natureofthebinaryinteractionprocess,whichhasimplicationson
(ii) Weassumethattheincreaseinthe1.083µmcontinuumnear
thenatureofLBVmajoreruptions,andmasslossbyverymassive
periastronpassage comesfromtheconical shell (thecollisionre-
stars(e.g.,Soker2001,2005,2007;Kashi&Soker2010;Smithet
gion of the two winds). The main source is the shocked primary
al.2010;Smith2010a,b).
wind.Thisisbasedonanalysisweperformedinpreviouspapers.
The GE is the best studied example for a major LBV erup-
(iii) WeassumethatthemainabsorbinggasoftheHeIλ10830Å
tion,andservesasatestcasefortheoriesandmodels.Ahighrate
lineisbothpreandpost-shockmaterialinthefastsecondarywind.
accretionduringtheGEcouldhavesuppliedtheextraluminosity
Thisassumptionstandsonasolidground.Basedonrecentnumer-
for20years,andtheaccretingsecondarystarcouldhavelaunched
ical simulations (Akashi & Soker 2010), weshowed in section 4
twojetsthatshapedthebipolarstructure–theHomunculus(Soker
thatthecolumndensityofthefastmovinggasisashighasfound
2001). Most likelyother LBVmajor eruptions arealsorelatedto
by G2010. Some segments of the wind are at a temperature of
∼ 104 K, and there are enough recombining He atoms. Thiswill binary interaction (Kashi & Soker 2010), as was argued for the
17thcenturyeruptionofPCygni(Kashi2010),ratherthanasin-
becalculatedinafuturepaperbasedonhighresolutionnumerical
glestar phenomena assuggested by, e.g.,Smith(2007). Smith&
simulations.
Owocki (2006) suggested that LBVs lose most of their envelope
(iv) WeconcentrateontheabsorptionoftheF-component.We
massduringmajoreruptions.Therefore,thepresenceofastrongly
assumethatintheflatvelocityrangeoftheF-component(seeFig.
interacting companion can play a major role in the evolution of
1),theabsorbinggasintheconicalshellabsorbsmostoftheconical
verymassivestars.AssuggestedbyKashietal.(2010),majormass
shellemission.Wethereforeconsider twolimitingvaluesforthis
transfereventsinLBVsarerelatedtoopticaltransientobjectsand
parameter f =0.5and f =1.
s s
haveacommonpoweringmechanism–accretionontoacompan-
(v) Weassumethattheopticaldepthoftheconicalshellinthe
ion star. Understanding the binary interaction in η Car will shed
flat velocity range of the F-component is very high, such that it
lightonotherobjectswherebinaryinteractionisthoughttoshape
absorbsalltheradiationofthecentralsourceithidesfromourline
circumstellar nebulae, like planetary nebulae, symbiotic systems,
ofsight(intheflatpart).
andrelatedobjectssuchastheRedRectangle.
(vi) Weassumethatthecollidingwindsshellhasanhyperbolic
shape.Weconsideritstiltduetotheorbitalmotion.Incalculating
theshapeandtilttheprimarywindaccelerationzoneisconsidered.
(vii) Weassume,basedonourpreviouspapers(Kashi&Soker
ACKNOWLEDGMENTS
2008b,2009c),thatthesecondaryistowardusnearperiastron(ω=
90◦).Wetakeaninclinationangleofi = 41◦,andtheothercom- We thank Jose H. Groh for very valuable and enlightening com-
monlyusedbinaryparameters(semimajoraxis,eccentricity,etc.). ments,andananonymousrefereeforcommentsthathelpedtoim-
(viii) Wenotethatafterperiastronpassagesomeoftheassump- provethepaper.WeacknowledgeEduardoFernandez-Lajusforus-
tions break down, because accretion is likely to occur (Kashi & ing his I-band observations, and Augusto Damineli for using his
Soker2009a;Akashi&Soker2010). observations of the He I λ10830Å line across the 2003.5 perias-
(ix) Wecalculatethepart of thecentral sourcecovered bythe tronpassage. Thisresearchwassupported by theAsher Fundfor
conicalshell,forω=90◦. SpaceResearchattheTechnionandagrantfromtheIsraelScience
(x) Wecalculatethecoveredpartofthe1.083µmcentralcon- Foundation.
Transientabsorptionlinesin ηCar 7
REFERENCES Sutherland,R.S.,&Dopita,M.A.1993,ApJS,88,253
vanGenderenA.M.,Sterken,C.,Allen,W.H.&Walker,W.S.G.2006,
Akashi,M.&Soker,N.2010(arXiv:1006.3333)
JAD,12,3
Behar,E.,Nordon,R.&Soker,N.2007,ApJ,666,L97
Weigelt,G.etal.2007,A&A,464,87
BlondinJ.M.,1994,TheVH-1UsersGuide,Univ.Virginia
Whitelock,P.A.,Feast,M.W.,Marang,F.,&Breedt,E.2004,MNRAS,
BlondinJ.M.,KallmanT.R.,FryxellB.A.,&TaamR.E.,1990,ApJ,356,
352,447
591
Clayton,G.C.,Geballe,T.R.&Bianchi,L.2003,ApJ,595,412 APPENDICES(ASTRO-PHVERSIONONLY)
Corcoran,M.F.2005,AJ,129,2018
Corcoran,M.F.,Hamaguchi,K.,Pittard,J.M.,Russell,C.M.P.,Owocki,
S.P.,Parkin,E.R.&Okazaki,A.2010,ApJ,725,1528
SeealsoTheRXTEX-rayLightCurveofEtaCarinaesite APPENDIXA: THEBRIEFABSORBTIONINTHE
(http://asd.gsfc.nasa.gov/Michael.Corcoran/etacar CONICALSHELL
/etacarrxtelightcurve/index.html)
Asupport toashortdurationabsorptionbytheconicalshellnear
Damineli,A.etal.2008aMNRAS,384,1649
periastronpassagemightcomefromthebehavioroftheHydrogen
Damineli,A.etal.2008bMNRAS,386,2330
Damineli,A.,Stahl,O.,Kaufer,A.,Wolf,B.,Quast,G.&LopesD.F.1998, Hδ line. The Hydrogen atom is a simple atom, lacking the com-
A&AS133,299 plicationsresultingfromabsorptionfromameta-stablestateasfor
Davidson,K.etal.2005,AJ,129900 the HeI λ10830Å line. The complication is inthat it isnot sim-
SeealsoTheanimationoftheHαline pletoestimatethepopulationoftherelevantatomicstate.Nielsen
http://etacar.umn.edu/news/ etal.(2009)present highspectralresolution(R ∼ 80,000) ofthe
DavidsonK.,SmithN.,GullT.R.,IshibashiK.,&HillierD.J.2001,AJ, hydrogen Hδ line,aswellasof theHeIλ7067Å line, acrossthe
121,1569
2003.5spectroscopicevent.TheHeIλ7067ÅlineshowsDoppler
Damineli,A.1996,ApJ,460,49
shiftvariationswiththeorbitalphasethatfitemissionandabsorp-
Duncan,R.A.,&White,S.M.2003,MNRAS,338,425
tionintheaccelerationzoneofthesecondarywind(Kashi&Soker
Fernandez-Lajus,E.etal.2010,NewAstr.,15,108
2007, 2008b). The variations in the Doppler shift of the Hδ line
Groh,J.H.,Damineli,A.&Jablonski,F.2007,A&A465,993
aremuch smaller,perhaps ∼ 0.3timesthoseoftheHeIλ7067Å
Groh,J.H.&Damineli,A.2004,IBVS,5492,1
Groh,J.H.etal.2010,A&A,517A,9G line(hardtotell).Thismaysuggest thatpartoftheabsorptionin
Hamaguchi,K.etal.2007,ApJ,663,522 theHδlineisfromtheconical shell,inaddition toanabsorption
Henley,D.B.,Corcoran,M.F.,Pittard,J.M.,Stevens,I.R.,Hamaguchi, component associated withtheprimarywind. Closetoperiastron
K.&Gull,T.R.2008,ApJ,680,705 (phase1.001infigure5ofNielsenetal.2009)theHδlineshows
Hillier,D.J.,Davidson,K.,Ishibashi,K.&Gull,T.2001,ApJ,553,837 ananomalouswideblueabsorptionwing,reachingvelocitiesupto
Kashi,A.2010,MNRAS,405,1924 −v ≃ 900 kms−1.Attheclosest observation times,phases 0.985
Kashi,A.,Frankowski,A.&Soker,N.2010,ApJ,709,L11 and 1.115, this broad absorption does not exist. Though the blue
Kashi,A.,&Soker,N.2007,NewA,12,590
wing is of slower velocities than observed in the He I λ10830Å
Kashi,A.,&Soker,N.2008a,NewA,13,569
line,aresemblancebetweenthetwoisverylikely.Bothlinesshow
Kashi,A.,&Soker,N.2008b,MNRAS,390,1751
ahighvelocityabsorptioncomponentjustbeforeperiastron,which
Kashi,A.,&Soker,N.2009a,NewAstron.,14,11
later disappears. Like the He I λ10830Å line, it is possible that
Kashi,A.,&Soker,N.2009b,MNRAS,394,923
the Hδ absorption line originates in the conical shell, but in re-
Kashi,A.,&Soker,N.2009c,MNRAS,397,1426
Kashi,A.,&Soker,N.2009d,ApJL,701,L59 gions with higher density and lower velocity. We note that qual-
Kashi,A.,&Soker,N.2010,ApJ,2010,ApJ,723,602 itativelysimilar blue absorption wing close to periastron passage
Mehner,A.,Davidson,K.,Ferland,G.J.,&Humphreys,R.M.2010,ApJ, alsoappears intheHαand Paschenη lines,although lesspromi-
710,729 nent(Davidsonetal.2005;fortheHαthistrendisbetterseenin
Nielsen,K.E.,Kober,G.Vieira,Weis,K.,Gull,T.R.,Stahl,O.&Bomans, theanimationpreparedfromtheseobservations). Notethatinthe
D.J.,2009ApJS,181473 lastperiastronpassageof2009theHα(observedbyRichardsonet
Parkin,E.R.,Pittard,J.M.,Corcoran,M.F.&HamaguchiK.2011ApJ,
al.2010)bluewingabsorptioncomponentisnotprominent,butit
726,105
mayexistandbedifficulttosee.
Pittard,J.M.,Corcoran,M.F.2002,A&A,383,636
Richardson,N.D.,Gies,D.R.,Henry,T.J.,Fernndez-Lajus,E.,Okazaki,
A.T.2010,AJ,139,1534
Smith,N.2006,ApJ,644,1151 APPENDIXB: ABSORPTIONINAJET?
Smith,N.2010a(arXiv:1010.3721)
Smith,N.2010b(arXiv:1010.3770) We here check whether an alternative absorbing material for the
Smith,N.2007,Asymmetrical Planetary Nebulae IV,eds.RomanoL.M. HeIλ10830Åcanbeacollimatedfastwindorajetemittedfrom
Corradi,ArturoManchadoandNoamSoker,I.A.C.electronicpublica- thesecondary.Beharetal.(2007)analyzedCHANDRAMEG/HEG
tion,2009,p.561 observationsinthe3.7−7.5Åband,wherehighexcitationS,Siand
Smith,N.,Davidson,K.,Gull,T.R.,Ishibashi,K.&Hillier, D.J.2003, Arlinesareobserved.Farfromperiastrontheselinesshowasym-
ApJ,586,432
metricprofile withno Doppler shift. Close toperiastron the cen-
Smith,N.,Morse,J.A.,Collins,N.R.&Gull,T.R.2004,ApJ,610,105 troidmovesto∼ −200kms−1,andawidebluewingisdeveloped,
Smith,N.,Owocki,S.P.2006,ApJ,645L,45
reachingvelocitiesofupto∼−2000kms−1.Beharetal.(2007)in-
Smith,N.,Weidong,L.Silverman,J.M.;Ganeshalingam,M.&Filippenko,
terpretedthecentroidmotiontotheorbitalmotionofthesecondary,
A.V.2010(arXiv:1010.3718)
takingω=90◦.Wenotethatthoughtheobservedvelocityatphase
Soker,N.2001,ApJ,557,256
Soker,N.2005,ApJ,635,540 −0.006wasoutoftheirDopplerlines(theirfig.4),takingupdated
Soker,N.2007,ApJ,661,490 inclinationangle of i = 41◦ insteadof i = 53◦ asused by Behar
Stevens,I.R.,Blondin,J.M.,&Pollock,A.M.T.1992,ApJ,386,265 etal.(2007)makestheirfitmuchbetterandstrengthentheirclaim
8 A. Kashiet al.
for the emission source for the centroids. The high velocity blue
wingseenintheselineswasattributedbyBeharetal.(2007)toa
collimatedfastwindorajetlaunchedbythesecondary.
Thewidebluewingathighvelocitiesisclearlyseenatphases
−0.028(11.972)and−0.006(11.994)infig.3ofBeharetal.(2007),
where the mean velocity profiles constructed from nine different
spectrallinesfromfiveobservationsareplotted.Thefluxisstronger
atphase−0.028forvelocities−v.1400kms−1.Fig.3ofBeharet
al.(2007)presentsnormalizedobservation.Fromtheoriginaldata
presentedinHenleyetal.(2008)welearnthatthetruefluxatphase
−0.028is∼1.7timesasstrongasatphase−0.006sothatthewide
blue wing at phase −0.028 is even more pronounced. Correcting
forthetrue,notnormalized,flux,wefindthatthewidebluewing
atphase−0.028isstrongerthaninphase−0.006fornotonlyfor
−v.1400kms−1 butupto−v.2000kms−1.Inthiscontext,there
issomedifficultyinadoptingthejetexplanationforHeIλ10830Å
lineabsorption at high blue velocities. At phase −0.028, 57 days
beforeperiastron,theaccretionprocesshavenotyetstarted(Kashi
&Soker2009a),andhenceanaccretiondiskaroundthesecondary
which is essential for producing jets does not exist. This is also
supported by the X-ray flux which is strong and raising at phase
−0.028, while weaker and decreasing at phase −0.006 (Corcoran
2010),suggestingthatsubstantialaccretionhasnotyetstartedac-
cordingtotheaccretionmodel(Soker2005;Kashi&Soker2009a).
Wethereforeregardtheexplanationofafastwindfromajetasthe
absorbingmaterialfortheHeIλ10830Åunlikely.
ItispossiblethatthegasemittingthehighlyionizedwideS,
Ar, and and Si lines and the He I λ10830Å absorption do have
acommon origin. Theemissionand absorptionoccur indifferent
places along the outflow. It is therefore possible that the X-ray
emitting gas is not a jet, but rather the secondary wind (pre and
post-shock)itself.