VLTI-AMBER velocity-resolved aperture-synthesis imaging of η Carinae with a spectral resolution

20 Pages·2016·7.61 MB·English

Astronomy&Astrophysicsmanuscriptno.Weigelt-ver6 (cid:13)cESO2016
September9,2016
VLTI-AMBER velocity-resolved aperture-synthesis imaging of
η Carinae with a spectral resolution of 12000.
Studies of the primary star wind and innermost wind-wind collision zone(cid:63) (cid:63)(cid:63)
G.Weigelt1,K.-H.Hofmann1,D.Schertl1,N.Clementel2,M.F.Corcoran3,4,A.Damineli5,W.-J.deWit6,
R.Grellmann7,J.Groh8,S.Guieu6,T.Gull9,M.Heininger1,D.J.Hillier10,C.A.Hummel11,S.Kraus12,T.Madura9,
A.Mehner6,A.Mérand6,F.Millour13,A.F.J.Moffat14,K.Ohnaka15,F.Patru16,R.G.Petrov13,S.Rengaswamy17,
N.D.Richardson18,T.Rivinius6,M.Schöller11,M.Teodoro9,andM.Wittkowski11
(Affiliationscanbefoundafterthereferences)
ReceivedSeptember15,1996;acceptedMarch16,1997
ABSTRACT
Context.Themasslossfrommassivestarsisnotunderstoodwell.ηCarinaeisauniqueobjectforstudyingthemassivestellarwindduringthe
LuminousBlueVariablephase.Itisalsoaneccentricbinarywithaperiodof5.54yr.Thenatureofbothstarsisuncertain,althoughweknowfrom
X-raystudiesthatthereisawind-windcollisionwhosepropertieschangewithorbitalphase.
Aims.WewanttoinvestigatethestructureandkinematicsofηCar’sprimarystarwindandwind-windcollisionzonewithahighspatialresolution
of∼6mas(∼14au)andhighspectralresolutionofR=12000.
Methods.ObservationsofηCarwerecarriedoutwiththeESOVeryLargeTelescopeInterferometer(VLTI)andtheAMBERinstrumentbetween
approximatelyfiveandsevenmonthsbeforetheAugust2014periastronpassage.Velocity-resolvedaperture-synthesisimageswerereconstructed
fromthespectrallydispersedinterferograms.Interferometricstudiescanprovideinformationonthebinaryorbit,theprimarywind,andthewind
collision.
Results.Wepresentvelocity-resolvedaperture-synthesisimagesreconstructedinmorethan100differentspectralchannelsdistributedacrossthe
Brγ 2.166µmemissionline.Theintensitydistributionoftheimagesstronglydependsonwavelength.Atwavelengthscorrespondingtoradial
velocities of approximately −140 to −376 kms−1 measured relative to line center, the intensity distribution has a fan-shaped structure. At the
velocityof−277kms−1,thepositionangleofthesymmetryaxisofthefanis∼ 126◦.Thefan-shapedstructureextendsapproximately8.0mas
(∼18.8au)tothesoutheastand5.8mas(∼13.6au)tothenorthwest,measuredalongthesymmetryaxisatthe16%intensitycontour.Theshape
oftheintensitydistributionssuggeststhattheobtainedimagesarethefirstdirectimagesoftheinnermostwind-windcollisionzone.Therefore,the
observationsprovidevelocity-dependentimagestructuresthatcanbeusedtotestthree-dimensionalhydrodynamical,radiativetransfermodelsof
themassiveinteractingwindsofηCar.
Keywords. Stars:winds,outflows–stars:individual:ηCarinae–stars:massive–stars:mass-loss–binaries:general–techniques:interferometric
1. Introduction oftheX-raylightcurves(e.g.,Maduraetal.2013)andthenear
constancy of Heii0.4686µm (e.g., Teodoro et al. 2016), which
Studies of the mass-loss process are of crucial importance for indicatemuchsmallerchangesinthemass-lossrate.
improvingourunderstandingofstellarevolution.Infraredlong-
The distance of η Car is ∼ 2.35 kpc (Allen & Hillier 1993;
baseline interferometry provides us with a unique opportunity
Davidson&Humphreys1997;Smith2006).Itissurroundedby
to study the mass loss from the colliding wind binary η Car.
the spectacular bipolar Homunculus nebula. The inclination of
The primary star of the system is an extremely luminous and
thepolaraxisoftheHomunculusnebulawiththeline-of-sightis
massive (M ∼ 100 M(cid:12)) unstable Luminous Blue Variable star ∼41◦(Davidsonetal.2001;Smith2006),andthepositionangle
(LBV) with a high mass-loss rate of ∼ 10−3M(cid:12)yr−1 (Davidson (PA) of the projected Homunculus axis is ∼ 132◦ (Davidson &
& Humphreys 1997; Davidson et al. 2001; Hillier et al. 2001,
Humphreys1997;Davidsonetal.2001;Smith2006).
2006; Groh et al. 2012b). A decrease in the observed strength
of Hα and Feii emission lines, and a change in the strength of Using ESO’s Very Large Telescope Interferometer (VLTI),
Niilines,provideindicationsthattheprimarywinddensityhas the diameter of η Car’s wind region was measured to be about
5 mas (50% encircled-intensity diameter measured in the field-
decreased,eitherduetoaglobalreductioninmasslossortolat-
of-view,FOV,of70mas)intheK-bandcontinuum(vanBoekel
itudinalchanges(e.g.,Davidsonetal.2015;Mehneretal.2012,
et al. 2003; Kervella 2007; Weigelt et al. 2007). The measured
2010b).However,thereiscontradictoryevidencefromanalysis
visibilitiesareingoodagreementwiththepredictionsfromthe
detailed spectroscopic model by Hillier & Miller (1998) and
(cid:63) BasedonobservationscollectedattheEuropeanOrganisationfor
Hillieretal.(2001,2006).AgoodagreementbetweentheHillier
Astronomical Research in the Southern Hemisphere under ESO pro-
model and interferometric observations of the LBV P Cyg was
gram092.D-0289(A).
reportedbyRichardsonetal.(2013).
(cid:63)(cid:63) The images in Fig. 4 are available in electronic form at the
CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via Firstspectro-interferometricVLTI-AMBERobservationsof
http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/ ηCar(Weigeltetal.2007)withmediumandhighspectralreso-
Articlenumber,page1of20
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lutionintheHei2.059µmandtheBrγ2.166µmemissionlines Table1.Summaryofobservations
allowed us to study the spatial structure of η Car’s wind as a
functionofwavelengthwithahighspatialresolutionof∼5mas Date Nobsa Nvisb Cal
andspectralresolutionsof1500and12000.Themeasuredline 2013Dec31 1 2 HD92305
2014Jan04 1 3 HD28749
visibilitiesagreewithpredictionsoftheradiativetransfermodel
2014Jan19 2 1 HD18660
of Hillier et al. (2001). We derived 50% encircled-intensity di-
2014Jan20 9 5 HD23319
ameters of 4.2 mas (9.9 au), 6.5 mas (15.3 au), and 9.6 mas
2014Jan21 10 0 -
(22.6au)inthe2.17µmcontinuum,theHei2.059µm,andthe
2014Feb03 1 0 -
Brγ 2.166µmemissionlines,respectively(forcomparison,the 2014Feb06 1 0 -
radiusR∗ oftheprimarystarofηCaris∼100R(cid:12) ∼0.47au∼ 2014Feb07 1 0 -
0.20mas;however,R∗isnotknownwell;Hillieretal.2001). 2014Feb08 1 0 -
Smith et al. (2003) studied the stellar light reflected by the 2014Feb09 1 0 -
Homunculus nebula and found that the velocity of the primary 2014Feb13 1 0 -
stellarwindishighernearthesouthpolethanatthelatitudecor- 2014Feb14 1 0 -
2014Feb21 2 0 -
responding to our line-of-sight. Other studies suggest that the
2014Feb22 3 0 -
spectroscopicandinterferometricobservationscanbeexplained
2014Feb23 1 0 -
by the wind-wind collision zone (Groh et al. 2012b; Mehner
2014Feb24 1 0 -
etal.2012).
2014Feb25 1 0 -
Studies of the binary or binary wind-wind collision zone 2014Feb26 2 0 -
were reported by many authors (e.g., Damineli 1996; Damineli 2014Feb27 3 0 -
etal.1997,1998,2000,2008b;Davidson1997;Davidsonetal. 2014Feb28 2 0 -
2005, 2015; Smith & Gehrz 2000; Smith et al. 2004; Smith 2014Mar04 2 0 -
2010; Pittard & Corcoran 2002; Corcoran et al. 1997, 2001; 2014Mar05 3 3 HD46933
Corcoran 2005; Ishibashi et al. 1999; Soker 2003, 2007; Weis
Notes.aNumber of observations at different hour angles per night.
et al. 2005; Gull et al. 2006, 2009, 2011, 2016; Nielsen et al.
bNumber of visibilities calibrated with an interferometric calibrator
2007;Weigeltetal.2007;Kashi&Soker2007,2009a,b;Parkin
star.
et al. 2009; Groh et al. 2010, 2012b,a; Madura & Owocki
2010; Madura et al. 2012, 2013; Mehner et al. 2010a, 2011,
2012,2015; Richardsonetal.2010; Teodoroetal.2013, 2016; uv coverage
100
Clementel et al. 2015a,b; Hamaguchi et al. 2016). The X-rays
are believed to arise from a collision between the winds from
the LBV and a hotter O or WR-type star. The CHANDRA X-
m]
rcaoyllisspieocntroufmthceanprbimeeaxryplsatianrewdi(nPditt(aMr˙d=&2C.5or×co1r0a−n42M0(cid:12)0y2r)−b1y,ttehre- ate [ 50
minalvelocity∼ 500kms−1)andthewindofahotcompanion din
(M˙ = 10−5M(cid:12)yr−1, terminal velocity ∼ 3000 kms−1; probably oor 0
anextremeOforaWRstarperhapssimilartothethreeluminous c
y
WNLhstarsseenelsewhereintheCarinaNebula). e
n
The wind collision leads to a wind-wind collision cavity a
(e.g.,Pittard&Corcoran2002;Humphreysetal.2008;Okazaki v pl -50
u
et al. 2008; Parkin et al. 2009, 2011; Gull et al. 2009, 2011;
Madura&Owocki2010;Maduraetal.2012,2013;Grohetal.
2010,2012a,b).Radiativetransfermodelsofthewind-windcol- -100
lisionzonewerereportedbyClementeletal.(2015a,b).Three-
dimensional (3D) smoothed particle hydrodynamic simulations -100 -50 0 50 100
were used to determine the orientation of the binary’s orbit
uv plane x coordinate [m]
(semimajoraxislength∼15.4au)inspaceandonthesky(e.g.,
Okazaki et al. 2008; Madura et al. 2012). Madura et al. (2012) Fig.1.uvcoverageofallVLTI/AMBERobservationslistedinTable1.
derived an orbit inclination of 130 to 145◦ and a PA of the or-
bital axis projected on the sky of 302 to 327◦. Therefore, the
orbital axis of the binary is closely aligned with the Homuncu- thecentralstar(e.g.,Hillier&Allen1992;Weigeltetal.1995;
lussystemaxis.Theargumentofperiapsisplacesthesecondary Davidsonetal.1995,1997;Hillieretal.2001;Smithetal.2002,
star and wind-wind collision zone on the observer’s side of the 2004; Mehner et al. 2010a; Madura et al. 2013). To study this
systemwhenthecompanionstarisatapastron. occultingmaterial,Falckeetal.(1996)performedspecklemask-
Detailed HST/STIS imaging of η Car with a resolution of ingimagingpolarimetry(inHαpluscontinuum).Thepolarized
about 0.1(cid:48)(cid:48) allowed Gull et al. (2009, 2011), Mehner et al. flux image (Fig. 2b in the polarimetry paper) shows a bar with
(2010a), Teodoro et al. (2013), and Gull et al. (2016) to image alengthof∼ 0.4(cid:48)(cid:48) alongaPAof∼45and225◦,whichwasin-
thecircumstellarenvironmentofηCaracrossemissionlinesand terpretedasanobscuringequatorialdisk.Interestingly,thispo-
atseveralorbitalphases.Thediscoveredstructuresaretheresult larized45◦barhasapproximatelythesamelengthandPAasthe
ofthewind-windcollisionoverthepast∼20yr.Thesestructures fossilwindbardiscoveredbyGulletal.(2011,2016).
arecalledfossilwindstructures,becausetheywerecreatedover Inthispaper,wepresenthighspatialandhighspectralreso-
severalorbitalcyclesinthepast. lutionaperture-synthesisimagingofηCar’sinnermostwindre-
Variousobservationssuggestthatdisk-likeortoroidalmate- gion.WedescribetheobservationsinSection2,whileinSection
rial in our line-of-sight to the central object partially obscures 3,wepresentthevelocity-resolvedaperture-synthesisimagesre-
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G.Weigeltetal.:AMBER/VLTIobservationsofηCarinae
nits] 1.0 nits] 1.0
u 0.8 u 0.8
y y
ar 0.6 ar 0.6
bitr 0.4 bitr 0.4
ar ar
x [ 0.2 x [ 0.2
u u
fl 0.0 15.3 m / 85.0° fl 0.0 68.4 m / 67.0°
1.0 18.6 m / 38.5° 1.0 82.2 m / 4.3°
31.1 m / 59.3° 128.9 m / 32.5°
0.8 0.8
y y
bilit 0.6 bilit 0.6
si si
vi 0.4 vi 0.4
0.2 0.2
0.0 0.0
15.3 m / 85.0° 68.4 m / 67.0°
se []° 19305 1381..61 mm // 3589..53°° se []° 19305 12882.9.2 m m / /3 42..35°°
a a
ph 45 ph 45
al 0 al 0
enti -45 enti -45
er -90 er -90
diff -135 diff -135
-180 -180
e []° 19305 e []° 19305
as 45 as 45
h h
closure p --94005 closure p --94005
-135 -135
-180 -180
2.156 2.160 2.164 2.168 2.172 2.176 2.156 2.160 2.164 2.168 2.172 2.176
λ [µm] λ [µm]
Fig.2.IllustrativeexampleofanAMBERHR-mode(R=12000)ob- Fig.3. SameasFig.2,butdifferenttelescopeconfiguration.
servationofηCar(seeSect.2).Th1efigureshowsfromtoptobottom: 1
exampleinterferogram,spectrumoftheBrγregion(heliocentricwave-
lengths;thetelluriclinesarenotremoved),wavelengthdependenceof oftheobservations.Thespatial-filterfibersinAMBERlimitthe
thevisibilitiesatthreebaselines(PAsandprojectedbaselinelengthsare FOVtothediameterofthefibersonthesky(∼250masforthe
indicatedinthepanels),wavelength-differentialphases(thecurveofthe ATs).TheobservationswerecarriedoutaspartoftheOHANA
shortestbaselineisshiftedupby90◦andthelongestdownby90◦),and (Riviniusetal.2015)backuptargetproject(Observatorysurvey
closurephases. atHighANgularresolutionofActiveOBstars;Prog-ID:092.D-
0289).
Figures 2 and 3 present two examples of high spectral res-
constructed from theVLTI data. All images show the extended
olution η Car AMBER observations (the dates of observations
windregion,whichismoreextendedacrosstheBrγlinethanin
shown in Figs. 2 and 3 are 20 January 2014, 8:15 UT and 4
thecontinuum.InSection4,wediscusstheresults.Section5is
January 2014, 5:31 UT, respectively). The figures show, from
asummaryandconclusion.
top to bottom, an example of an AMBER interferogram, the
line profile, the visibilities obtained at three different baselines
2. Observationsanddataprocessing as a function of wavelength, the wavelength-differential phases
at three baselines, and the wavelength dependence of the clo-
2.1. Visibilities,wavelength-differentialphases,andclosure
surephase.Fortheobservations,theFINITOfringetracker(Le
phases
Bouquinetal.2008)wasemployed.Theobservationswereob-
TheobservationsofηCarlistedinTable1werecarriedoutwith tained between seven and five months before the 2014.6 peri-
the ESO Very Large Telescope Interferometer (VLTI; Schöller astron passage (see Table 1). The observing dates correspond
2007)andtheAMBERinterferometryinstrument(Petrovetal. to an average orbital phase of ∼ 0.90 if calculated as described
2007).Withprojectedbaselinelengthsuptoabout129mofthe in Mehner et al. (2011) and 0.91 if calculated as described in
Auxilliary1.8mTelescopes(ATs)used,anangularresolutionof Daminelietal.(2008b)orTeodoroetal.(2016).
∼ 6 mas (∼ 14 au) was obtained in the K band. Interferograms Weanalyzedwhethertheinterferometricobservables(spec-
wererecordedwiththehighspectralresolutionmode(HRmode; trum, visibilities, and phases of the object Fourier transform)
spectralresolutionR=12000).Figure1showstheuvcoverage changedoverthetwo-month-longobservingintervalandfound
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A&Aproofs:manuscriptno.Weigelt-ver6
negligible changes. Therefore, all observations were combined weusedtheminimizationalgorithmASA-CG(Hager&Zhang
to compute an averaged image. While the orbital eccentricity 2006), as described in (Hofmann et al. 2014). The images pre-
is ∼ 0.90 (Corcoran et al. 2001; Pittard & Corcoran 2002), the sentedinFigs.4,5,6,B.1,andB.2werereconstructedwiththis
changeinpositionofthecompanionrelativetotheprimarystar differential-phasemethod.
is small and the temporal evolution of the cavity is expected to Alternatively, we can reconstruct images from visibilities
be small over the observing interval. The latter is substantiated andclosurephases(Jennison1958).Thismethodisbrieflycalled
bysmallchangesin3-Dhydrodynamicmodelsofthewind-wind closure-phasemethodinthefollowingsections.WeusedtheIR-
cavitythatwerecomputedatorbitalphases0.50,0.90,and0.952 Bismethod(InfraredBispectrumimagereconstructionmethod)
(Maduraetal.2012,2013). for image reconstruction (Hofmann et al. 2014). The images
ThedatawerereducedwithourownAMBERdataprocess- presented in Fig. A.1 in Appendix A were reconstructed with
ing software package, which uses the pixel-to-visibility matrix the closure-phase method. Data processing is discussed in Ap-
algorithmP2VM(Tatullietal.2007;Chellietal.2009)inorder pendixAinmoredetail.
toextractvisibilities,differentialphases,andclosurephasesfor
eachspectralchannelofanAMBERinterferogram.Wederived
50 wavelength-dependent closure phases and 150 wavelength- 3. Velocity-resolvedaperture-synthesisimagesof
differential phases from the interferograms. For only 14 of the ηCar’sstellarwindandwind-windcollisionzone
observationsaresuitablecalibratorstarmeasurementsavailable acrosstheBrγline
to calibrate the interferometric transfer function and the visi-
bilities (see Table 1). The number of calibrator measurements We reconstructed images with both methods (differential- and
is small because the data were recorded within a backup-target closure-phasemethod)discussedinSect.2.Bothmethodswere
programme(Prog-ID:092.D-0289).Thewavelengthcalibration used to illustrate the similarities and differences of the recon-
wasperformedusingtelluriclines.Weusedthesamemethodas structions.
describedinAppendixAofWeigeltetal.(2007). Figure 4 presents the images reconstructed from the obser-
TheperformanceofthefringetrackerFINITOisusuallydif- vations listed in Table 1 using the differential-phase method
ferentduringtargetandcalibratorobservations,becausetheper- (Sect. 2). The images clearly show a strong wavelength depen-
formanceofFINITOdependsonatmosphericconditions,target denceacrosstheBrγ line.Oneofthemostremarkablefeatures
brightness, and target visibility. This can lead to visibility cali- oftheimagesisthefan-shapedstructureextendingtothesouth-
brationsoflowquality.Therefore,weusedtheVLTIReflective east (SE; from PA ∼ 90◦ to ∼ 180◦) at velocities between ap-
Memory Network Recorder (RMNrec) data (Le Bouquin et al. proximately −376 and −140 kms−1. Figure 5 shows the image
2009; Mérand et al. 2012) to improve the visibility calibration at −277 kms−1 to illustrate the asymmetric, fan-shaped struc-
by rejecting 46% of the exposures that showed a disagreement tureanditssize.Aswewilldiscussinthenextsection,thesize,
betweentheRMNrecvaluesofcalibratorandscienceobject. structure,andvelocitydependenceoftheseimagessuggestthat
The calibrated continuum visibilities were fit with a two- wehaveobtainedthefirstdirectimagesofthewallsofthewind-
dimensional exponential function in Fourier space, because the windcollisioncavity.Anotherinterestingandunexpectedstruc-
measured visibilities and the Fourier transform of the Hillier tureisthebar-likestructurethatextendstothesouthwest(SW)
continuum model curves are approximately exponential func- intheimagesatvelocitiesfrom−426to−339kms−1.
tions (van Boekel et al. 2003; Kervella 2007; Weigelt et al. The images presented in Fig. A.1 in Appendix A were re-
2007). From this visibility, we derived an axis ratio of 1.07 ± constructedwiththeclosure-phasemethod(seeSect.2).Forthe
0.14 and a PA of the major axis of 159.5 ± 47◦. The axis ratio reconstructionsinFig.A.1,weusedthesamedatasetasforthe
andthePAarenotwellconstrainedbecausethevisibilityerrors reconstructionsshowninFig.4.
arelargeandthenumberofcalibratedvisibilitiesissmall.This Figures 4 to 6 present images that consist of both contin-
modelvisibilityfunctionwasusedtocalibratethevisibilitiesof uumandBrγlinecomponents.Toassistintheinterpretationof
theobservationsmadewithoutacalibrator(seeTable1). the Brγ line images, we additionally computed the continuum-
subtracted Brγ line images shown in Figs. B.1 and B.2 in Ap-
pendix B (these two figures show images of different velocity
2.2. Differential-phaseandclosure-phaseimage
ranges). The white crosses are the centers of the continuum
reconstructionmethods
images. The images reconstructed with the differential-phase
We used two different image reconstruction methods, called method(Fig.4)seemtohaveaslightlyhighersignal-to-noisera-
below differential-phase method and closure-phase method. tiothantheclosure-phaseimages(Fig.A.1)becauseofthelarger
If spectrally dispersed interferograms are available, the amount of Fourier phase information (i.e., 150 Fourier phases
wavelength-differentialphasesinspectralchannelsacrossemis- instead of 50 closure phases). Therefore, the differential-phase
sionlinescanbeusedtoderivephasesoftheFouriertransform imageswereusedtocomputeallcontinuum-subtractedBrγline
of the object in spectral channels across emission lines (Petrov images.
et al. 2007; Schmitt et al. 2009; Millour et al. 2011; Ohnaka Thecontinuum-subtractedimagesinFigs.B.1andB.2show
et al. 2011, 2013; Mourard et al. 2015). This is possible if the the correct Brγ brightness of the images, because they are not
phaseoftheFouriertransformoftheobjectinthecontinuumis normalized to the peak brightness as the images in all previous
known(forexample,ifthecontinuumobjectisunresolvable)or figures.Thecomputationofthecontinuum-subtractedimagesis
canbederivedfromacontinuumimagereconstructedfromcon- discussedinAppendixB.
tinuum visibilities and continuum closure phases using a clo- Surprisingly, in the continuum-subtracted images at highest
sure phase method. The reconstruction of images from a set negative velocities (≤ −563 kms−1) in Fig. B.1, the center of
of visibilities and phases of the object Fourier transforms de- the Brγ line emitting region is clearly shifted about 1–2 mas
rivedfromdifferentialphasesisdiscussedinAppendixA.This to the northwest (NW) of the center of the continuum image.
methodiscalleddifferential-phasemethodinthefollowingsec- To study this unexpected offset in more detail, we computed
tions.Toreconstructimagesusingthedifferential-phasemethod, the continuum-subtracted images shown in Fig. B.2 (wider ve-
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G.Weigeltetal.:AMBER/VLTIobservationsofηCarinae
heliocentric radial velocity [km/s]
-2000 -1500 -1000 -500 0 500 1000 1500 2000
ux 3.5
d fl 3.0
e 2.5
z
ali 2.0
m 1.5
or 1.0
n
2.15 2.16 2.17 2.18
heliocentric wavelength [micrometer]
Fig.4.Aperture-synthesisimagesofηCarreconstructedwiththedifferential-phasemethod(seeSect.2;spectralresolutionR=12000).Top:
Continuum-normalizedBrγlineprofile.ThepresentedspectrumistheaverageofallspectraofthedatalistedinTable1.Thesmallcorrectiondue
tothesystemvelocityof−8kms−1(Smith2004)hasbeenneglected.Bottom:Wavelengthandvelocitydependence(velocitiesinunitsofkms−1
belowtheimages)ofthereconstructedηCarimagesacrosstheBrγline.TheFOVofthereconstructedimagesis50×50mas(118×118au;
1mascorrespondsto2.35au).Northisup,andeastistotheleft.Ina1llimages,thepeakbrightnessisnormalizedtounity.Atradialvelocities
betweenapproximately−339and−252kms−1,theimagesarefan-shapedandextendedtotheSEofthecenterofthecontinuumwindregion.
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locityrange)inadditiontothecontinuum-subtractedimagesin
Fig.B.1(noticethedifferentcolorcoderequiredbecauseofthe
faintnessofmostoftheimages).
The differential-phase (Fig. 4) and closure-phase (Fig. A.1)
images are similar, but there are also differences. Both types
of images present the discussed fan-shaped wind structure. To
show the differences, we derived contour plot images of the
differential-phase and closure-phase images at velocities be-
tween−376and−277kms−1showninFig.C.1inAppendixC.
FigureD.1inAppendixDpresentsthevelocitydependence
ofthephotocentershiftoftheimagesshowninFig.4(i.e.,con-
tinuum plus line flux images) with respect to the center of the
continuum images. Figure D.2 shows both the velocity depen-
dence of the size of the images in Fig. 4 (measured as 50%
encircled-intensity radius) and the velocity dependence of the
photocentershiftoftheimagesinFig.4withrespecttothecen-
terofthecontinuumimages.Thebiggestphotocentershiftsare
caused by the fan-shaped structure seen at many high negative
velocities.
InAppendixE,wecomparethemeasuredvisibilities,differ-
ential phases, and closure phases shown in Figs. 2 and 3 with
thevisibilitiesandphasesderivedfromtheimagesreconstructed
with the differential-phase method to check whether the inter-
ferometricquantitiesofthereconstructedimagesapproximately
agreewiththeobservations.
4. Discussion
The reconstructed images in Fig. 4 show a strong wavelength
dependenceoftheintensitydistributionacrosstheBrγline.
– Atwavelengthscorrespondingtoradialvelocitieslowerthan
about −426 kms−1 and higher than about +400 kms−1, the
continuumwindregionisdominant.
– Atvelocitiesbetweenapproximately−426and−339kms−1,
a bar-like extension appears in the southwest (SW) of the
continuumwind.
– Betweenapproximately−376and−140kms−1,afan-shaped
structure is visible in the SE of the continuum wind. This
unexpected fan-shaped structure is discussed in Figs. 5 and
6inmoredetail.
– Thewindintensitydistributionismoreextendedatmosthigh
negativevelocities(e.g.,−350to−250kms−1)thanathigh
positivevelocities(e.g.,+250to+350kms−1).
Fig.5.Top:Fan-shapedηCarimageatvelocityof−277kms−1(image
– At all velocities, the continuum wind region is shining
from Fig. 4). The FOV of the image is 50 × 50 mas (118 × 118 au;
through the Brγ emitting wind region, suggesting that the
thecenterofthecontinuumregionisatcoordinatezero).Contourlines
line-emittingregionisopticallythin.
areplottedat8,16,32,and64%ofthepeakintensity.ThePAofthe
symmetryaxisofthefanis∼126◦(whitearrow).Thefan-shapedstruc- – Thecontinuum-subtractedimagesinFigs.B.1andB.2show
tureextends∼ 18.8au(8.0mas;bluearrow)totheSEand∼ 13.6au thatafaintline-emittingregionisvisibleathighnegativeand
(5.8 mas) to the NW, measured with respect to the center of the con- positive velocities. At some high negative velocities (e.g.,
tinuumwindalongthefansymmetryaxisatthe16%intensitycontour. about−563to−825kms−1inFig.B.2),thecenteroftheline
Forcomparison,theradiusR∗oftheprimarystarofηCaris∼100R(cid:12)∼ emissionisoffsettotheNWofthecenterofthecontinuum
0.47au∼0.20mas(R isnotwellknown; Hillieretal.2001).There-
∗ wind(seeSect.4.1).
fore, thehuge wind extensionto t1heSE isabout 40 timeslarger than
R .Bottom:Overlayofasketchoftheorbitofthesecondarystarrel-
∗ Figure 5 illustrates the geometry of the fan-shaped image
ativetotheprimarystaronthesky(yellow)andthefan-shapedimage
at −277 kms−1. Figure 6 (second row) has the goal to high-
at−277kms−1.TheorbitisadoptedfromTeodoroetal.2016(phase
lighttheevolutionofthefan-shapedstructurebetween−364and
0.91computedasdescribedinthispaper).Thetworeddotsarethepri-
maryandthesecondarystaratthetimeofourobservations.Thebinary −277 kms−1 and (third row) compare images obtained at neg-
separationandPAwereapproximately4.9mas(11.5au)and315◦,re- ative and positive velocities to illustrate the structure and size
spectively,atthetimeofobservation(Sect.4).Thebluearrowindicates differences. For example, the images at velocities of −277 and
the motion direction and the bar at the bottom right the length of the +271kms−1clearlyshowabigsizedifference.Thiscomparison
majoraxisoftheorbit(30.9au;Maduraetal.2012). andthecavitymodelsketchatthebottomofFig.6areneededin
thefollowingdiscussions.
Articlenumber,page6of20
G.Weigeltetal.:AMBER/VLTIobservationsofηCarinae
heliocentric radial velocity [km/s]
-2000 -1500 -1000 -500 0 500 1000 1500 2000
ux 3.5
d fl 3.0
ze 2.5
ali 2.0
m 1.5
nor 1.0
2.15 2.16 2.17 2.18
heliocentric wavelength [micrometer]

2200 2200 2200 2200
s]s] s]s] s]s] s]s]
aa 1100 aa 1100 aa 1100 aa 1100
mm mm mm mm
n [n [ n [n [ n [n [ n [n [
sitiositio 00 sitiositio 00 sitiositio 00 sitiositio 00
oo oo oo oo
pp pp pp pp
el. el. --1100 el. el. --1100 el. el. --1100 el. el. --1100
rr rr rr rr
--2200 --2200 --2200 --2200

--22 00 --11 00 00 11 00 22 00 --22 00 --11 00 00 11 00 22 00 --22 00 --11 00 00 11 00 22 00 --22 00 --11 00 00 11 00 22 00
rreell.. ppoossiittii oonn [[mmaass]] rreell.. ppoossiittii oonn [[mmaass]] rreell.. ppoossiittii oonn [[mmaass]] rreell.. ppoossiittii oonn [[mmaass]]
Fig.6.ComparisonofηCarimageswithmodeldensitydistributions.Top:Continuum-normalizedBrγlineprofile(sameasinFig.4).Second
row:SWbarstructureatvelocitiesof−364to−352kms−1andfan-shapedSEstructureatvelocitiesofabout−339to−277kms−1(imagesfrom
Fig.4;theFOVis50×50masor118×118au;1mascorrespondsto2.35au;northisup,andeastistotheleft).Contourlinesareplottedat8,16,
32,and64%ofthepeakintensity.Thefan-shapedSEstructuressuggestthatthereconstructedimagesaredirectimagesofthewind-windcollision
zone.Thirdrow:Fromlefttoright,imagesatvelocitiesof−464kms−1 (continuum),−277kms−1 (fan-shapedSEextension),+271kms−1,and
+458kms−1 (continuum).Atvelocityof−277kms−1,theimageisasymmetric,fan-shaped,andmoreextendedthantheimageatvelocityof
+271kms−1.Bottomleft:IllustrationoftheV-shapedwind-windcollisionmodel,ourviewingdirectionatthetimeofobservation,andtheradial
velocitiesseenbytheobserver.Theiso-densitycontoursareextractedfromthemodelpresentedinFig.B1(phase0.90)inMaduraetal.(2013).
Two density contours (10−16 g cm−3 and 10−14 g cm−3) of the model density distribution are shown. The binary orbit (semimajor axis length
∼15.45au)issketchedasablueline.Theprojectedellipticalorbitisa1lineintheshownplaneperpendiculartotheorbitalplane(Maduraetal.
2012,2013).Thetworeddotsmarkthepositionsoftheprimaryandsecondarystaratthetimeofobservation(binaryseparation∼4.9masor
11.5au;seeSect.4).Bottomright:Line-of-sightviewofathree-dimensionaliso-densitysurface(density10−16 gcm−3)ofthewindatthetime
ofobservation(northisup,andeastistotheleft;FOVis∼50×50mas).Thisiso-densitydistributionisasmoothedversionofmodeliso-density
derivedbyMaduraetal.(2013).AsmallpartoftheBrγprimarywindisvisibletotheSE.Thecontinuumprimarywindisnotvisiblebecauseits
diameterissmallerthantheBrγprimarywind.ThebinaryseparationandPAwereapproximately4.9mas(11.5au)and315◦,respectively,atthe
timeofobservation.Thesecondarystarisnotvisibleintheimages,becauseitisatleastabout100timesfainterthantheprimarystar,whichis
alsonotvisibleduetohighextinction.
Articlenumber,page7of20
A&Aproofs:manuscriptno.Weigelt-ver6
At velocity of −277 kms−1, the reconstructed image has a 4.2. Toroidaloccultinggasanddustintheline-of-sighttothe
fan-shapedintensitydistribution(seeFig.5).ThePAofthesym- centralobject
metryaxisofthefanis∼126◦.Thefan-shapedstructureextends
∼8.0mas(18.8au)totheSEalongaPAof∼126◦and∼5.8mas Occultinggasintheline-of-sighttothecentralobjectmayinflu-
(13.6au)totheNWalongaPAof∼306◦measuredwithrespect ence the observed intensity distributions for the following rea-
tothecenterofthecontinuumwindimageatthe16%intensity sons.ThethreeejectedobjectsηCarB,C,andDatseparations
contour. between 0.1(cid:48)(cid:48)(235 au) and 0.3(cid:48)(cid:48) (Weigelt & Ebersberger 1986;
Hofmann&Weigelt1988)wereonlyabout12timesfainterthan
TheobservedimageasymmetriesindicatethattheLBVwind
thecentralobjectin1985atthewavelengthofabout900nm.In
isnotsphericalorthatthereisanexternalinfluenceonthewind.
theUV(at∼190nm),whereobscuringmaterialleadstomuch
For the discussions in the next sections, we need to know the
moreextinction,theseobjectsBtoDwereonlyaboutthreetimes
positionofthesecondarystarontheskyatthetimeofobserva-
fainterthanthecentralobjectin1991(Weigeltetal.1995).
tion.The3-DorientationofηCar’sorbitwasderivedbyMadura
etal.(2012).Usingtheseresultsandassumingthelatestorien- This seems to be puzzling because the objects B, C, and D
tationparametersandorbit,asshowninFig.17inTeodoroetal. are objects illuminated by the central object (and only a very
(2016), we derived the projected binary separation of approxi- smallfractionofthestarfluxhitstheseobjects).Therefore,sev-
mately 4.9 mas (11.5 au) and PA of 315◦ at the time of obser- eralauthorssuggestedthatdisk-likeortoroidaloccultingmate-
vation.Thesecondarystarisnotvisibleinourimages,because rial in our line-of-sight to the central object may partially ob-
it is at least about 100 times fainter than the primary star (e.g., scurethecentralstar,butnottheobjectsB,C,andD,justlikea
Hillieretal.2006;Weigeltetal.2007).Theprimaryisalsonot naturalcoronagraph(Hillier&Allen1992;Weigeltetal.1995;
visibleduetohighextinction. Davidsonetal.1995,1997;Hillieretal.2001;Smithetal.2002,
2004;Mehneretal.2010a;Maduraetal.2013).
In the next sections, we discuss the high-velocity gas
Tostudythisoccultingtoroidalmaterial,Falckeetal.(1996)
(Sect. 4.1), the toroidal gas (Sect. 4.2), the fossil wind
performedspecklemaskingimagingpolarimetryintheHαline.
(Sect.4.3),thepolarwind(Sect.4.4),andthewind-windcolli-
The image obtained in polarized light (see Fig. 2b in the po-
sion(Sect.4.5),becauseallofthemmayinfluencetheobserved
larimetry paper) shows a bar with a length of ∼ 0.4(cid:48)(cid:48)(940 au)
images.
along a PA of ∼ 45◦ and ∼ 225◦ (about equal to the PA of the
equatorial plane of the Homunculus). It was interpreted as an
obscuring equatorial disk. Interestingly, this polarized bar has
4.1. High-velocitystructureinthecontinuum-subtracted
approximatelythesamelengthandPAasthefossilwindbardis-
images
cussedinthenextsection.
In the continuum-subtracted images presented in Fig. B.2, the Furthermore,inFig.4,abar-orarc-likestructurealongPA
centerofmostoftheBrγlineimagesisatthecenterofthecon- of∼225◦canbeseenatvelocitiesbetweenapproximately−426
tinuum images (marked by white crosses). However, in the ve- and−339kms−1 thatmaybecausedbygasintheinnerregion
locityrangeofabout−613to−825kms−1,thecenteroftheBrγ oftheequatorialdiskortorus.
lineemittingregionisclearlyshiftedabout1–2mastotheNW
ofthecenterofthecontinuumimage.At−663kms−1,theNW
offset is 1.7 ± 0.4 mas along a PA of 292 ± 20◦. At velocities 4.3. Fossilwind
smaller than about −837 kms−1, the high-velocity structure is
stillvisible,buttheNWoffsetdisappeared. Thebar-likeSWextensionatPAof∼225◦inFigs.4to6andB.1
ismainlyvisibleatvelocitiesbetweenapproximately−426and
IntheBrγlineprofile,theouterhigh-velocityregionisdom- −339kms−1.ItsPAof∼225◦approximatelyagreeswiththePA
inatedbytheelectronscatteringwings(e.g.,Hillieretal.2001; ofthefossilwindstructurereportedbyGulletal.(2011,2016)
Grohetal.2010).However,inthebluelinewing,theremayalso andthepolarisationbar(Falckeetal.1996).Theimagesofthe
be additional line contributions from (1) fast moving primary fossil wind structure (at ∼ 0.1–0.5(cid:48)(cid:48)) in Gull et al. (2011) were
windgas(inthewindcavity)acceleratedbythefastsecondary madebyintegratingtheHST/STISobservationsovertheveloc-
wind(Daminelietal.2008a;Grohetal.2010),and(2)theweak ity range of −400 to −200 kms−1 (see also the wind studies in
Hei2.16127µmline(thecenterofthislineisapproximatelyat Gulletal.2009;Madura&Owocki2010;Teodoroetal.2013).
velocity of about −550 kms−1 in the Brγ line profile). Model Thefossilwindmayslightlyinfluencetheintensitydistribution
spectra of η Car computed with the radiative transfer model of of the dominant inner wind region. However, the fossil wind is
Hillier & Miller (1998) and Hillier et al. (2001) show that the faintcomparedtotheinnermostwind-windcollisionzone.
peakbrightnessofthisHei2.16127µmlineisabout50to100
Interestingly, both the intensity contours of the blueshifted
timesweakerthanthepeakbrightnessoftheBrγline.
Feiii 0.4659 µm HST/STIS map in Mehner et al. (2010a) and
Thecauseoftheobserved∼1–2mas(2.35–4.7au)NWoff- theaforementionedblue-wingimagesinGulletal.(2011)show
setofthelineimagesatabout−613to−825kms−1isunknown, asimilarasymmetricstructureassomeofourfan-shapedVLTI
but the high negative velocity and location of the high-velocity images, although of course HST’s angular resolution was infe-
structure suggest that the emitting region (of both the above rior.Mehneretal.(2010a)discussvariousoriginsofthisstruc-
discussed high-velocity Brγ emission and the Hei 2.16127 µm ture, for example, the near side of a latitude-dependent struc-
emission) is located in the innermost region of the wind-wind tureintheouterstellarwind,theinnerpartsoftheHomunculus
collision cavity (Damineli et al. 2008a; Groh et al. 2010), be- ejecta, or the obscuring material that causes about two magni-
cause the innermost region of the cavity was in the NW of the tudesmoretheextinctioninfrontoftheprimarystarthaninfront
centerofthecontinuumimageatthephaseofourobservations oftheejectaatseparationsbetween0.1and0.3(cid:48)(cid:48).Thefan-shaped
(binaryseparationandPAwere4.9masor11.5auand315◦ at imagesalsolooklikeinclinedbipolarnebulaewithasymmetry
thetimeofobservation). axisparalleltotheHomunculusaxis.
Articlenumber,page8of20
G.Weigeltetal.:AMBER/VLTIobservationsofηCarinae
4.4. High-densityfastpolarwind wind model (Madura et al. 2013) at the time of observation. In
thenextparagraphs,wewilldiscussthesizeandstructureofthe
In the SE of the continuum center, we are looking at the south imagesatdifferentvelocities.
polarregionofthestellarwind(Smithetal.2003),becausethe
inclinationofthepolaraxisoftheHomunculusnebulawiththe
(1) High positive radial velocities. At high positive radial
line-of-sight is ∼ 41◦ (Davidson et al. 2001; Smith 2006) and velocities,forexample+200to+400kms−1,theobservedBrγ
the PA of the projected Homunculus axis is ∼ 132◦ (Davidson
windintensitydistributionsareonlyslightlywiderthanthecon-
& Humphreys 1997; Davidson et al. 2001; Smith 2006). Smith tinuumwindregion(seeFigs.4,A.1,B.1,andthe+271kms−1
etal.(2003)studiedthestellarlightreflectedbydifferentregions
image in the third row of Fig. 6). The sketch at the bottom of
oftheHomunculusnebulaandfoundthatthewindvelocityand
Fig.6illustratesthattheemissionregioncorrespondingtothese
densityofthemassiveprimarywindofηCarishigheratthepo-
highpositiveradialvelocitiesispredictedtobeonthebackside
lar region than at latitudes corresponding to our line-of-sight,
oftheprimarywind(i.e.,onthefarsidebehindtheprimarystar)
and the polar wind axis is approximately parallel to the Ho-
and to have a smaller angular diameter than, for example, the
munculus axis. Therefore, the SE extension of our images may
emissionregionwithverysmallradialvelocity(e.g.,zero).The
at least partially be caused by an increased wind density at the
primarywindonthebacksideisnotdisturbedbythewind-wind
southpole.Ontheotherhand,otherstudiessuggestthatthespec-
collisionzoneattheorbitalphaseofourobservations.Wenote
troscopic and interferometric observations can be explained by
that in all images in Figs. 4 to 6 and A.1, the peak brightness
the wind-wind collision zone (Groh et al. 2012b; Mehner et al.
is normalized to unity. This explains why the compact wind at
2012).
high positive velocities is not more visible in spite of the high
linefluxatthesewavelengths.
4.5. Wind-windcollisionzone
(2) Low radial velocities. The sketch in Fig. 6 shows that
3-DsmoothedparticlehydrodynamicsimulationsofηCar’scol- the model region of small positive radial velocities (marked as
lidingwindspredictabowshockwithanenhanceddensityand "low, positive radial velocities" at the top of the sketch) is the
an extended wind-wind collision cavity or bore hole (Okazaki upper cavity region. This upper cavity region is in the NW of
et al. 2008; Gull et al. 2009, 2011; Groh et al. 2010; Madura the continuum center because of the orientation of the orbit
& Owocki 2010; Madura et al. 2012, 2013; Clementel et al. (Madura et al. 2012). This NW extension is only weakly vis-
2015b,a) that has a density distribution more extended than the ible in the reconstructed images in Fig. 4 (e.g., at velocities
undisturbed primary wind region, as in our observed images. of +22 to +77 kms−1). The reason may be the small cross-
This large extension of the wind-wind collision zone can, for section of the outer region in our viewing direction (i.e., we
example, be seen in the model density distributions presented arelookingthroughonlyasmallamountofmaterial).However,
in Fig. 18 (see insets) in Gull et al. (2009), Fig. 6 in Madura thefaintlow-velocityNWextensionoftheintensitydistribution
etal.(2012),Figs.B1(orangeregion),1,and10inMaduraetal. of the wind collision zone is clearly visible in the continuum-
(2013), Fig. 2 in Clementel et al. (2015b), and in the sketch in subtractedimages(seeFig.B.1).
Fig.6.
Becauseoftheorbitorientationinspace,thesecondarystar (3) High negative radial velocities in the range of approx-
of η Car spends most of the time in front of the primary star imately −140 to −376 kms−1. The sketch in Fig. 6 suggests
(e.g., Madura et al. 2012, 2013). Only for a few weeks around that the wind collision cavity was opened up into our line-of-
theperiastronpassagearethesecondarystarandthewind-wind sight at the time of observation, and that the emission from the
collisionzonebehindtheprimarystar(onthebackorfarsideof lowerarmofthecavityhashighnegativeradialvelocities.The
the primary wind). According to aforementioned hydrodynam- lower arm is in the SE of the center of the continuum wind
ical simulations, at the time of observation, the secondary star because of the orientation of the orbit (Madura et al. 2012).
wasinfrontoftheprimarystar,andwewerelookingattheex- At radial velocities between −215 and −376 kms−1, the recon-
tendedwind-windcollisioncavity(e.g.,Maduraetal.2012).In structed images (Fig. 4) clearly show a fan-shaped SE exten-
other words, the wind collision cavity was opened up into our sion as predicted by the wind-wind collision model (sketch in
line-of-sightatthetimeofobservation. Fig. 6). This suggests that our images are direct images of the
Unfortunately, no η Car model images of the Brγ intensity inner wind-wind collision cavity. Our observations provide de-
distribution(e.g.,3-Dsmoothedparticlehydrodynamicsimula- tailed velocity-dependent image structures that can be used to
tions combined with radiative transfer modeling) are available. test 3-D hydrodynamical, radiative transfer models of massive
Therefore,weusethedensitydistributionof3-Dsmoothedpar- interactingwinds.
ticle hydrodynamic simulations (see above references) for the
followingdiscussion. Thiswind-windcavityinterpretationofourimagesatnega-
Figure6(bottomleft)illustratesthewind-windcollisioncav- tivevelocitiesappearstobemorelikelythanthehigh-densitypo-
ityandourviewingdirectionatorbitalphaseof0.90.Twoden- larwindinterpretationbecauseahigh-densitypolarwindwould
sitycontourlines(density10−16gcm−3and10−14gcm−3)ofthe beexpectedtoshowablue-shiftedextensiontotheSE(nearthe
windcollisioncavitymodelpresentedinFig.B1inMaduraetal. south pole) and potentially a red-shifted extension to the NW
(2013)areshown(bottomleft).Thelowerarmofthewind-wind (north pole). Instead we see a blue-shifted extension to the SE
collisioncavitydepictedinthesketchinFig.6(bottomleft)can (at approximately −140 and −376 kms−1) and no red-shifted
beseeninthereconstructedimages(Figs.4and6)totheSEof extension to the NW at similar positive velocities (see Fig. 4).
the center of the continuum wind, given the orientation of the However,wecannotexcludethattheadditionalemissionsources
orbit(Maduraetal.2012). discussedinSect.4.1(e.g.,thefaintHei2.16127µmemission
Toillustratethecomplicated3-Dwindstructure,Fig.6(bot- line),thefossilwind,toroidaloccultingmaterialinline-of-sight,
tom right) shows a line-of-sight view of a smoothed three- and a high-density polar wind slightly influence the observed
dimensional iso-density surface (density 10−16 g cm−3) of the windcavityintensitydistributions.
Articlenumber,page9of20
A&Aproofs:manuscriptno.Weigelt-ver6
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acknowledges support from an STFC Rutherford Fellowship (ST/J004030/1) Ohnaka,K.,Hofmann,K.-H.,Schertl,D.,etal.2013,A&A,555,A24
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