detection of highvelocity_material_from_the_win_wind_collision_zone_of_eta_carinae

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arXiv:1003.4527v1 [astro-ph.SR] 23 Mar 2010 Astronomy & Astrophysics manuscript no. groh˙etacar˙highvel˙accepted˙version˙astroph c ESO 2010 March 25, 2010 Detection of high-velocity material from the wind-wind collision zone of Eta Carinae across the 2009.0 periastron passage J. H. Groh 1 ,⋆⋆ , K. E. Nielsen 2,3 , A. Damineli 4 , T. R. Gull 2 , T. I. Madura 5 , D. J. Hillier 6 , M. Teodoro 4 , T. Driebe 1 , G. Weigelt 1 , H. Hartman 8 , F. Kerber 9 , A. T. Okazaki 7 , S. P. Owocki 5 , F. Millour 1 , K. Murakawa 1 , S. Kraus 10 , K.-H. Hofmann 1 , and D. Schertl 1 1 Max-Planck-Institut f¨ ur Radioastronomie, Auf dem H¨ ugel 69, D-53121 Bonn, Germany 2 Astrophysics Science Division, Code 667, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA 3 Department of Physics, IACS, Catholic University of America, Washington DC 20064, USA 4 Instituto de Astronomia, Geof´ ısica e Ciˆ encias Atmosf´ ericas, Universidade de S˜ ao Paulo, Rua do Mat˜ ao 1226, Cidade Universit´ aria, 05508-090, S˜ ao Paulo, SP, Brazil 5 Bartol Research Institute, University of Delaware, Newark, DE 19716, USA 6 Department of Physics and Astronomy, University of Pittsburgh, 3941 O’Hara Street, Pittsburgh, PA 15260, USA 7 Faculty of Engineering, Hokkai-Gakuen University, Toyohira-ku, Sapporo 062-8605, Japan 8 Lund Observatory, Lund University, Box 43, 221 00 Lund, Sweden 9 ESO, Karl-Schwarzschild-Strasse 2, 85748 Garching, Germany 10 Department of Astronomy, University of Michigan, 500 Church Street, Ann Arbor, MI 48103, USA Received / Accepted ABSTRACT We report near-infrared spectroscopic observations of the Eta Carinae massive binary system during 2008–2009 using the CRIRES spectrograph mounted on the 8m UT1 Very Large Telescope (VLT Antu). We detect a strong, broad absorption wing in He i λ10833 extending up to 1900 km s 1 across the 2009.0 spectroscopic event. Analysis of archival Hubble Space Telescope/Space Telescope Imaging Spectrograph ultraviolet and optical data reveals the presence of a similar high-velocity absorption (up to 2100 km s 1 ) in the ultraviolet resonance lines of Si iv λλ1394, 1403 across the 2003.5 event. Ultraviolet resonance lines from low-ionization species, such as Si ii λλ1527, 1533 and C ii λλ1334, 1335, show absorption only up to 1200 km s 1 , indicating that the absorption with velocities 1200 to 2100 km s 1 originates in a region markedly faster and more ionized than the nominal wind of the primary star. Seeing-limited observations obtained at the 1.6 m OPD/LNA telescope during the last four spectroscopic cycles of Eta Carinae (1989– 2009) also show high-velocity absorption in He i λ10833 during periastron. Based on the large OPD/LNA dataset, we determine that material with velocities more negative than 900 km s 1 is present in the phase range 0.976 φ 1.023 of the spectroscopic cycle, but absent in spectra taken at φ 0.947 and φ 1.049. Therefore, we constrain the duration of the high-velocity absorption to be 95 to 206 days (or 0.047 to 0.102 in phase). We suggest that the high-velocity absorption component originates from shocked gas in the wind-wind collision zone, at distances of 15 to 45 AU in the line-of-sight to the primary star. With the aid of three-dimensional hydrodynamical simulations of the wind-wind collision zone, we find that the dense high-velocity gas is in the line-of-sight to the primary star only if the binary system is oriented in the sky so that the companion is behind the primary star during periastron, corresponding to a longitude of periastron of ω 240 270 . We study a possible tilt of the orbital plane relative to the Homunculus equatorial plane and conclude that our data are broadly consistent with orbital inclinations in the range i = 40 60 . Key words. stars: winds — stars: early-type — stars: individual (η Carinae) — stars: mass loss —near-infrared: stars — ultraviolet: stars 1. Introduction Eta Carinae oers a unique opportunity to study the evolution of the most massive stars and their violent, giant outbursts during the Luminous Blue Variable (LBV) phase. Eta Car is located at a distance of 2.3 ± 0.1 kpc (Walborn 1973; Hillier & Allen 1992; Davidson & Humphreys 1997; Smith 2006) in the Trumpler 16 OB cluster in Carina and presents itself as a very luminous cen- tral object (L 5 × 10 6 L , Davidson & Humphreys 1997) en- Based on observations made with ESO Telescopes at the La Silla Paranal Observatory under programme IDs 381.D-0262, 282.D- 5043, and 383.D-0240; with the Hubble Space Telescope Imaging Spectrograph (HST/STIS) under programs 9420 and 9973; and with the 1.6m telescope of the OPD/LNA (Brazil). ⋆⋆ e-mail: [email protected] shrouded in massive ejecta of 12 20 M (the Homunculus nebula; Smith et al. 2003b). The optical and near-infrared spectra of Eta Car present low- ionization permitted and forbidden lines mainly of H i, Fe ii, N ii, [Fe ii], and [Ni ii] (Thackeray 1953). Since 1944, high- ionization forbidden lines, such as [Fe iii], [Ar iii], and [Ne iii], appeared in the optical spectrum of Eta Car (Gaviola 1953; Humphreys et al. 2008; Feast et al. 2001). High-spatial resolu- tion imaging and spectroscopy have shown that the narrow emission component of the low- and high-ionization forbid- den lines arise in the ejecta (Davidson et al. 1995; Nielsen et al. 2005; Gull et al. 2009), in condensations known as the Weigelt blobs (Weigelt & Ebersberger 1986; Hofmann & Weigelt 1988; Zethson 2001; Hartman et al. 2005). It was noticed that at some epochs the high-ionization forbidden lines become weaker and eventually vanish, and then recover their normal strength

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  • 1. Astronomy & Astrophysics manuscript no. grohetacarhighvelacceptedversionastrophc ESO 2010 March 25, 2010 Detection of high-velocity material from the wind-wind collision zone of Eta Carinae across the 2009.0 periastron passage J. H. Groh1, , K. E. Nielsen2,3 , A. Damineli4 , T. R. Gull2 , T. I. Madura5 , D. J. Hillier6 , M. Teodoro4 , T. Driebe1 , G.Weigelt1 , H. Hartman8 , F. Kerber9 , A. T. Okazaki7 , S. P. Owocki5 , F. Millour1 , K. Murakawa1 , S. Kraus10 , K.-H. Hofmann1 , and D. Schertl11Max-Planck-Institut f r Radioastronomie, Auf dem H gel 69, D-53121 Bonn, Germanyu u2Astrophysics Science Division, Code 667, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USAarXiv:1003.4527v1 [astro-ph.SR] 23 Mar 20103Department of Physics, IACS, Catholic University of America, Washington DC 20064, USA4Instituto de Astronomia, Geofsica e Ci ncias Atmosf ricas, Universidade de S o Paulo, Rua do Mat o 1226, Cidade Universit ria,e e aa a05508-090, S o Paulo, SP, Brazila5Bartol Research Institute, University of Delaware, Newark, DE 19716, USA6Department of Physics and Astronomy, University of Pittsburgh, 3941 OHara Street, Pittsburgh, PA 15260, USA7Faculty of Engineering, Hokkai-Gakuen University, Toyohira-ku, Sapporo 062-8605, Japan8Lund Observatory, Lund University, Box 43, 221 00 Lund, Sweden9ESO, Karl-Schwarzschild-Strasse 2, 85748 Garching, Germany 10Department of Astronomy, University of Michigan, 500 Church Street, Ann Arbor, MI 48103, USA Received / Accepted ABSTRACT We report near-infrared spectroscopic observations of the Eta Carinae massive binary system during 20082009 using the CRIRES spectrograph mounted on the 8 m UT 1 Very Large Telescope (VLT Antu). We detect a strong, broad absorption wing in He i 10833 extending up to 1900 km s1 across the 2009.0 spectroscopic event. Analysis of archival Hubble Space Telescope/Space Telescope Imaging Spectrograph ultraviolet and optical data reveals the presence of a similar high-velocity absorption (up to 2100 km s1 ) in the ultraviolet resonance lines of Si iv 1394, 1403 across the 2003.5 event. Ultraviolet resonance lines from low-ionization species, such as Si ii 1527, 1533 and C ii 1334, 1335, show absorption only up to 1200 km s1 , indicating that the absorption with velocities 1200 to 2100 km s1 originates in a region markedly faster and more ionized than the nominal wind of the primary star. Seeing-limited observations obtained at the 1.6 m OPD/LNA telescope during the last four spectroscopic cycles of Eta Carinae (1989 2009) also show high-velocity absorption in He i 10833 during periastron. Based on the large OPD/LNA dataset, we determine that material with velocities more negative than 900 km s1 is present in the phase range 0.976 1.023 of the spectroscopic cycle, but absent in spectra taken at 0.947 and 1.049. Therefore, we constrain the duration of the high-velocity absorption to be 95 to 206 days (or 0.047 to 0.102 in phase). We suggest that the high-velocity absorption component originates from shocked gas in the wind-wind collision zone, at distances of 15 to 45 AU in the line-of-sight to the primary star. With the aid of three-dimensional hydrodynamical simulations of the wind-wind collision zone, we nd that the dense high-velocity gas is in the line-of-sight to the primary star only if the binary system is oriented in the sky so that the companion is behind the primary star during periastron, corresponding to a longitude of periastron of 240 270 . We study a possible tilt of the orbital plane relative to the Homunculus equatorial plane and conclude that our data are broadly consistent with orbital inclinations in the range i = 40 60 . Key words. stars: winds stars: early-type stars: individual ( Carinae) stars: mass loss near-infrared: stars ultraviolet: stars1. Introduction shrouded in massive ejecta of 12 20 M (the Homunculusnebula; Smith et al. 2003b).Eta Carinae oers a unique opportunity to study the evolution of The optical and near-infrared spectra of Eta Car present low-the most massive stars and their violent, giant outbursts duringionization permitted and forbidden lines mainly of H i, Fe ii,the Luminous Blue Variable (LBV) phase. Eta Car is located at N ii, [Fe ii], and [Ni ii] (Thackeray 1953). Since 1944, high-a distance of 2.3 0.1 kpc (Walborn 1973; Hillier & Allen 1992;ionization forbidden lines, such as [Fe iii], [Ar iii], and [Ne iii],Davidson & Humphreys 1997; Smith 2006) in the Trumpler 16 appeared in the optical spectrum of Eta Car (Gaviola 1953;OB cluster in Carina and presents itself as a very luminous cen-Humphreys et al. 2008; Feast et al. 2001). High-spatial resolu-tral object (L 5 106 L , Davidson & Humphreys 1997) en- tion imaging and spectroscopy have shown that the narrowemission component of the low- and high-ionization forbid- Based on observations made with ESO Telescopes at the La den lines arise in the ejecta (Davidson et al. 1995; Nielsen et al.Silla Paranal Observatory under programme IDs 381.D-0262, 282.D-2005; Gull et al. 2009), in condensations known as the Weigelt5043, and 383.D-0240; with the Hubble Space Telescope Imaging blobs (Weigelt & Ebersberger 1986; Hofmann & Weigelt 1988;Spectrograph (HST/STIS) under programs 9420 and 9973; and with theZethson 2001; Hartman et al. 2005). It was noticed that at1.6m telescope of the OPD/LNA (Brazil). some epochs the high-ionization forbidden lines become weaker e-mail: [email protected] and eventually vanish, and then recover their normal strength

2. 2 Groh et al.: High-velocity material in Eta Car across periastronafter several months (Gaviola 1953; Whitelock et al. 1983; able wind-wind collision zone is present between Eta Car AZanella et al. 1984; Damineli 1996; Damineli et al. 1998, 2000,and Eta Car B (Hamaguchi et al. 2007; Henley et al. 2008). The2008b,a). Throughout this paper, we refer to these epochs when wind of Eta Car B likely inuences geometry and ionizationthe high-ionization lines disappear as a spectroscopic event, andof the dense wind of Eta Car A, since numerical simulationsto the time interval between consecutive spectroscopic events as have suggested that Eta Car B creates a cavity in the winda spectroscopic cycle. of Eta Car A (Pittard & Corcoran 2002; Okazaki et al. 2008; Extensive monitoring of the optical spectrum of Eta Car Parkin et al. 2009). The extended outer interacting wind struc-showed that the spectroscopic events repeat periodically ev- ture has been shown to produce broad ( 400 km s1 ), time-ery 5.54 yr (Damineli 1996; Damineli et al. 2000, 2008b,a),variable forbidden line emission (Gull et al. 2009).leading to the suggestion that Eta Car is a binary systemSpectroscopic observations of the near-infrared He i 108331(Damineli et al. 1997) consisting of two very massive stars, Eta line in Eta Car have shown evidence for a brief appear-Car A (primary) and Eta Car B (secondary), amounting to at ance of fast-moving material up to 1500 km s1 duringleast 110 M (Hillier et al. 2001). The spectroscopic events are previous periastron passages (Damineli et al. 1998; Groh et al.related to the periastron passage of Eta Car B, and the bi-2007; Damineli et al. 2008a). Ultraviolet observations with thenary scenario is supported by numerous multi-wavelength ob-International Ultraviolet Explorer (IUE) obtained during theservations from X-ray (Corcoran et al. 1997; Pittard et al. 1998;1980 spectroscopic event suggested that the resonance linesPittard & Corcoran 2002; Ishibashi et al. 1999; Corcoran et al.of Si iv 1393,1403 show absorptions up to 1240 km s12001; Corcoran 2005; Hamaguchi et al. 2007; Henley et al.(Viotti et al. 1989). X-rays observations of high-ionization sil-2008), ultraviolet (Smith et al. 2004; Iping et al. 2005), op- icon and sulfur emission lines detected increased line widthstical (van Genderen et al. 2003, 2006; Fern ndez-Laj s et al.a u(1000 to 1500 km s1 ) during the 2003.5 spectroscopic event,2003, 2009, 2010; Steiner & Damineli 2004; Nielsen et al.suggesting that these lines come from the inner, hotter part of2007; Damineli et al. 2008b,a), near-infrared (Feast et al. 2001;the wind-wind collision zone (Henley et al. 2008) or from jetsWhitelock et al. 2004), and radio wavelengths (Duncan & White(Behar et al. 2007). A clear detection of very high-velocity ma-2003; Abraham et al. 2005). Although most orbital parameters terial (v > 1500 km s1 ) coming directly from Eta Car B or fromof Eta Car are uncertain, the wealth of multi-wavelength works the wind-wind collision zone has been elusive so far in the ul-mentioned above supports a high eccentricity (e 0.9) and traviolet, optical, and infrared, in particular because the ux ofan orbital period of 2022.7 1.3 d (Damineli et al. 2008b;Eta Car A is several orders of magnitude larger than Eta Car BFern ndez-Laj s et al. 2010).a u(Hillier et al. 2006). The relative ux between Eta Car B and Eta Signicant advancement on obtaining the properties of Eta Car A increases towards the ultraviolet, but the absorptions ofCar A has been achieved in recent years, conrming that it is an the wind of Eta Car A likely modies and masks the UVLBV star with a high mass-loss rate 2.5 104 to 103 M yr1 ) spectrum of Eta Car B.and a wind terminal velocity in the range 500 to 600 km s1While previous spectroscopic observations of He i 10833(Davidson et al. 1995; Hillier et al. 2001; Pittard & Corcoran used only moderate spectral resolving power (R 7000) and2002; Smith et al. 2003a; Hillier et al. 2006). Several observa- seeinglimited spatial resolution (1. 5), we monitored Eta Cartional works have suggested that the wind of Eta Car A isacross the 2009.0 spectroscopic event at much higher spectrallatitude-dependent (Smith et al. 2003a; van Boekel et al. 2003; (R 90 000) and spatial (0. 3) resolutions in the near-infrared,Weigelt et al. 2007), with the polar wind presenting the higherwhere the strong, unblended He i 10833 line is present. Wedensities and faster velocities. Smith et al. (2003a), based onobtained spectroscopic observations with the highest spectralH absorption proles obtained with HST/STIS during 1998and spatial resolutions obtained so far for longslit spectroscopy2000, found evidence for material with velocities of up to of Eta Car in the near-infrared. We combined our data with1200 km s1 during most of the spectroscopic cycle but only multi-wavelength diagnostics from the ultraviolet to the near-in the polar wind of Eta Car A. Smith et al. (2003a) also sug- infrared. The goal of this paper is to characterize the origin,gested that the wind of Eta Car A became roughly spherical formation region, physical conditions, and the phase intervalacross the 1998.0 spectroscopic event, with a terminal velocitywhen the high-velocity material can be detected in He i 10833,around 600 km s1 . Extremely massive material moving faster to possibly constrain the orbital parameters of the Eta Car binarythan 3000 km s1 was discovered by Smith (2008) in distant system. In particular, we aim at constraining the longitude ofejecta far from the Homunculus nebula, and is thought to beperiastron , given that some previous studies have determinedrelated to the Giant Eruption and not (at least directly) to Eta around 240 270, i.e., Eta Car B is behind Eta Car ACar B. during periastron (e.g., Damineli et al. 1997; Pittard & Corcoran Little is known about Eta Car B, since it has never been2002; Nielsen et al. 2007; Corcoran 2005; Iping et al. 2005;observed directly. The role of Eta Car B in the giant outburstsHenley et al. 2008; Okazaki et al. 2008; Parkin et al. 2009),and on the long-term evolution of Eta Car A is not yet under-while others have found 50 90 , i.e., Eta Car B is instood. Earlier analysis of the ionization of the ejecta around front of Eta Car A during periastron (e.g., Abraham et al.Eta Car have suggested an O-type or WR nature for Eta Car B2005;Abraham & Falceta-Goncalves2007, 2009;(Verner et al. 2005; Teodoro et al. 2008). Mehner et al. (2010)Falceta-Goncalves et al. 2005; Falceta-Goncalves & Abraham showed that a broad range of luminosities (105 to 106 L ) and 2009; Kashi & Soker 2007, 2008b, 2009). Sideways orbitaleective temperatures (36 000 to 41 000 K) of Eta Car B are able orientations (i.e., = 0 or 180 ) have also been suggested into account for the relatively high ionization stage of the ejecta, the literature (e.g., Ishibashi et al. 1999; Smith et al. 2004).adding further uncertainty to the evolutionary status and exactThis paper is organized as follows. Sect. 2 describes theposition of Eta Car B in the HR diagram. near-infrared spectroscopic data obtained at the 8 m Very Large X-ray observations require that Eta Car B must havea wind terminal velocity on the order of 3000 km s1 1 Vacuum wavelengths and heliocentric velocities are adopted in this(Pittard & Corcoran 2002; Okazaki et al. 2008; Parkin et al. paper. The spectra presented here have not been corrected by the sys-2009). X-ray studies have also shown that a strong and vari- temic velocity of 8 km s1 of Eta Car (Smith 2004). 3. Groh et al.: High-velocity material in Eta Car across periastron3Telescope (VLT) during 20082009, the archival ultraviolet and Table 1. Summary of VLT/CRIRES Eta Car spectroscopic ob-optical HST/STIS from 20022003, and the archival and newservations used in this papernear-infrared observations recorded at the 1.6 m OPD/LNA tele-scope. Section 3 reports the detection of a strong high-velocity Date JDHigh-velocity absorption(1900 km s1 ), broad absorption wing in He i 10833 dur- 2008 May 05 11.8752454591.53Noing the 2009.0 periastron passage. A similar high-velocity ab- 2008 Dec 26 11.9912454826.82Yessorption component is also seen in the ultraviolet resonance 2009 Jan 08 11.9982454839.80Yeslines in spectra obtained with HST/STIS during 2003.5 and in 2009 Feb 09 12.0142454871.77Yesthe OPD/LNA data. In Sect. 4 we derive the timescale for the 2009 Apr 03 12.0402454924.61Nopresence of the high-velocity absorption component. In Sect. 5we discuss possible scenarios to explain the presence of high-velocity gas in Eta Car during periastron, and argue that ourThe spectra were reduced in the standard way using IRAFobservations provide direct detection of high-velocity materialand other custom IDL data reduction routines specic forowing from the wind-wind collision zone in the Eta Car binary CRIRES developed by one of us (JHG). The spectral resolvingsystem.power is estimated to be R90 000 using unresolved calibration lamp lines. The spectra were extracted by averaging 3 pixels in the spatial direction, centered on the central source of Eta Car,2. Observationsthus corresponding to a 0. 26 0. 20 spatial region. The spec- tra were corrected to the heliocentric rest frame and normalizedThe multi-wavelength spectroscopic observations used to inves- to continuum. The error in the continuum normalization is esti-tigate the presence of high-velocity gas are summarized in Table mated to be less than 5%. Using as reference the Th-Ar spectral1, 2, and 3. Since we are interested in studying material with ve- line database from Kerber et al. (2008), an error in the wave-locities well above the terminal speed of the wind of Eta Car A, length calibration of 0.5 km s1 was achieved.in this paper the designation high-velocity means velocitiesmore negative than 900 km s1 . 2.2. OPD/LNA medium-resolution near-infraredThroughout this paper we use the ephemeris obtained byspectroscopyDamineli et al. (2008b)2 to calculate the phases across agiven spectroscopic cycle E: JD(phase zero)=2 452 819.8 +We use a large amount of archival near-infrared medium-2022.7(E 11). Note that phase zero is dened according toresolution spectroscopic observations, obtained during 1992the disappearance of the narrow component of He i 6678 and2004 (Damineli et al. 1998, 2008a; Groh et al. 2007), and gath-does not necessarily coincide exactly with the time of periastronered additional spectra of Eta Car during 20042009 withpassage. However, given the high orbital eccentricity of the Eta the 1.6-m telescope of the Brazilian Observatorio Pico dosCar system, the periastron passage is expected to occur close to Dias/Laboratorio Nacional de Astrosica (OPD/LNA). The datathe phase zero of the spectroscopic cycle. We adopt the nomen- reduction has been performed using standard near-infrared spec-clature described by Groh & Damineli (2004) to label the spec- troscopic techniques as described in Groh et al. (2007) andtroscopic cycles of Eta Car, so that cycle #1 started after theDamineli et al. (2008a). These spectra cover the He i 10833event observed in 1948 by Gaviola. Therefore, phase zero of theline with a moderate spectral resolving power (R 7000) and1992.5, 1998.0, 2003.5, and 2009.0 spectroscopic events corre- seeinglimited spatial resolution ( 1. 5), but have excellentsponds to = 9.0, = 10.0, = 11.0, and = 12.0, respec- time-sampling of the order of days around the 2003.5 spectro-tively.scopic event. The 2009.0 event was covered with a time sam- pling of the order of a month. The time evolution of the emission component of the He i 10833 line in the OPD/LNA dataset from2.1. VLT/CRIRES high-resolution near-infrared spectroscopy the 2003.5 event has been extensively discussed in Groh et al.Spatially resolved spectra of Eta Car were recorded with (2007) and Damineli et al. (2008a). Table 2 lists the OPD/LNAthe CRyogenic high-resolution InfraRed Echelle Spectrographdataset used in this paper.(CRIRES; Kaeu et al. 2004) mounted on the 8-m VLT UnitTelescope 1. Spectra were obtained in 2008 May 05 (=11.875),2.3. HST/STIS ultraviolet and optical spectroscopy2008 Dec 26 (=11.991), 2009 Jan 08 (=11.998), 2009 Feb09 (=12.014) and 2009 Apr 03 (=12.040), and are summa- Spectroscopic observations of Eta Car were obtained with therized in Table 1. The air-mass ( 10000 km s1 ) from component becomes noticeably stronger between = 10.984the rest wavelength of the line of interest. Our purpose for scal-and = 10.995 (Fig. 7, left panel). The reality of the high-ing the spectra was to separate changes in the line prole due to velocity absorption is conrmed by its presence in both ofthe high-velocity absorption from shifts in the continuum level,the Si iv doublet lines. The weaker C iv 1550 line is severelywhich otherwise could be misinterpreted as increases in absorp- blended with the stronger C iv 1548 line, and it is impossibletion strength. We veried that the control regions, which are to unambiguously judge whether the high-velocity absorption isalso aected by Fe ii lines, do not change appreciably in the present in C iv 1550 or not.scaled spectra as a function of phase before = 11.0. Thus,Therefore, ultraviolet resonance lines from low-ionizationwe investigate the temporal behavior of the remaining relativespecies, such as Si ii 1527, 1533, C ii 1334, 1335, andchanges in the scaled line proles, which are intrinsic to the UV Al ii 1671, show absorption up to 800 km s1 , possiblyresonance lines.1200 km s1 , but the UV resonance lines from high-ionizationThe scaled line proles of the strongest, more isolated UVspecies, specically Si iv, show absorption from 1200 toresonance lines in Eta Car are presented in Figure 7, where the 2100 km s1 . Thus, the high-velocity absorption originatesgrey region shows the dierence between the spectrum taken at from a region that is markedly more ionized than the wind of = 10.820 and at = 10.995, corresponding to the excess Eta Car A. 8. 8 Groh et al.: High-velocity material in Eta Car across periastronFig. 7. Montage of proles of resonance lines seen in ultraviolet spectra of Eta Car obtained with HST/STIS across the 2003.5 eventat = 10.820 (blue line), = 10.930 (green), = 10.984 (black), and = 10.995 (red). The continuum level of the spectra takenat = 10.820, = 10.930, and = 10.984 were scaled to approximately match the continuum level of the spectrum taken at = 10.995. The grey region shows the dierence between the spectrum taken at = 10.820 and at = 10.995, corresponding tothe excess absorption due to the high-velocity material in Eta Car. Left panel: High-ionization lines. From top to bottom, resonancelines of Si iv 1394, Si iv 1403, C iv 1548, and a control region around 1483 are shown. Notice that little changes are seenin the control region as a function of phase, indicating that the relative variability seen in the UV resonance lines are intrinsicto these lines, and not due to blending. Right panel: Low-ionization lines. From top to bottom, C ii 1334, 1335, Si ii 1526, Si ii1533, and Al ii 1671 are displayed. Note that part of the Si ii 1533 line prole, from 1200 to 2100 km s1 , is contaminated bySi ii 1526.3.3.2. Optical He i linessorption in He i 10833. Figure 9 presents the maximum velocity of the He i 10833 absorption component as a function of phase,Analysis of optical He i singlet and triplet line proles was done combining data from all cycles available folded around = 1.0.by Nielsen et al. (2007). Proles from the same observations are The timescale depends on the velocity of the material, with thereproduced in Figure 8. A noticeable high-velocity absorption is highest velocities likely appearing for the briefest time intervals.apparent in the two triplet line proles, He i 3888 and 5876,However, due to the S/N of the observations and normalizationextending to 900, possibly 1000 km s1 , at = 10.995 (2003 errors, it is not possible to derive quantitatively the variation ofJul 05). The proles of singlet lines, such as He i 6680, showthe timescale as a function of velocity for the OPD dataset. Forless pronounced absorptions up to 800 km s1 (note that the that purpose, one needs a much larger amount of high spatialHe i 4714 line is much weaker and contaminated by [Fe iii]and spectral resolution VLT/CRIRES data during periastron than4702 emission), as noted by Nielsen et al. (2005). Stahl et al. what is presented in Section 3.1. Thus, we are unable to compare2005 also detected similar absorption up to 750 km s1 for He i the duration of the absorption at 2000 km s1 relatively to the6680 from the polar spectrum reected in the Homunculus. Theabsorption at 900 km s1 , for instance. Henceforth, we optedoptical depths of all of these optical He i lines are substantiallyfor determining the timescale when gas with velocities moreless than even that of the He i 20587 line. Hence, the likelihood negative than 900 km s1 is present, since such velocity is wellof seeing absorptions up to 2000 km s1 in the He i optical lines above the terminal speed of the wind of Eta Car A and shouldis low.give a characteristic value for the timescale of the high-velocity absorption component.4. Duration of the high-velocity absorptionAbsorptions with velocities bluer than 900 km s1 are de- component tected across 47 d t +46 d (0.976 1.023) while the high-velocity absorption is absent in spectra taken atWe use the ground-based OPD/LNA data from 1992 to 2009,t 106 d ( 0.947) and t +100 d ( 1.049). Hence,which are a homogeneous dataset and have a ne time-sampling,based upon the large OPD/LNA dataset, we constrain the dura-to estimate the timescale for presence of the high-velocity ab-tion of the high-velocity absorption component to be 95 to 206 d. 9. Groh et al.: High-velocity material in Eta Car across periastron9 Fig. 9. Top: Maximum absorption velocity (vedge ) observed in the He i 10833 line prole as a function of phase of the spec- troscopic cycle for the OPD/LNA dataset from 1992 to 2009, encompassing four cycles, with the data folded around = 1.0. For clarity, errorbars are presented only in the bottom panel. Bottom: Zoom-in around the spectroscopic event. To help the reader, in both panels, the horizontal red dashed line denotes the vedge = 900 km s1 , while the vertical blue dashed lines repre- sent the range where the high-velocity absorption is detected in He i 10833. 5. Discussion: origin of the high-velocity material inEta Car In the following subsections, we discuss three distinct possibil- ities that might explain the origin of the high-velocity absorp- tion in the spectrum of Eta Car. Although many exotic scenar- ios could be envisioned, the high-velocity material is most likely due to either a transient episode of high-velocity material ejectedFig. 8. Montage of continuum-normalized He i line proles ob-by Eta Car A (Sect. 5.2), directly from the wind of Eta Car Btained with HST/STIS across the 2003.5 event. From top to bot- crossing the line-of-sight (Sect. 5.3), or formed in a dense, high-tom, we present He i 3889 (highly-contaminated by H i 3890,velocity part of the wind-wind collision zone (Sect. 5.4). Sincein particular at velocities v > 800 km s1 ), He i 4714 (contam- we are analyzing absorption lines, the high-velocity absorptioninated by [Fe iii] 4702 emission), He i 5877, and He i 6680 region must be between the continuum source and the observer.line proles. The bottom panel displays optical He i line pro- Therefore, the source of the continuum emission is crucial forles observed with HST/STIS at = 10.995 (2003 Jun 22)the interpretation of the origin of the high-velocity gas, and iswith the near-infrared He i 10833 line prole observed with briey discussed below.VLT/CRIRES at = 11.998 (2009 Jan 08). 5.1. Source of the continuum emission at 1.0 mDuring most of the spectroscopic cycle, the maximum absorp-The radiative transfer models of Hillier et al. (2001) have showntion velocity is 650 km s1 (Fig. 9). that Eta Car A has an extended photosphere in the near-Since a very limited amount of high spatial resolution obser-infrared due to the presence of its dense wind (see their Fig.vations with VLT/CRIRES and HST/STIS are available, only a 8). This causes a huge amount of extended free-free and bound-lower limit on the timescale of the high-velocity absorption can free emission that is able to explain the quiescent continuumbe obtained, but this estimate agrees well with the value obtained emission at 1.0 m from the inner regions, as measured byabove from the OPD/LNA dataset. The VLT/CRIRES data (Fig.HST/STIS (see Fig. 4 of Hillier et al. 2001). The extended near-1) from the 2009.0 event is consistent with the He i 10833 high-infrared continuum emitting region at 2 m has been directly re-velocity absorption (1000 to 2000 km s1 ) appearing between solved by interferometric measurements (van Boekel et al. 2003; = 11.875 (2008 May 05) and = 11.991 (2008 Dec 26), and Weigelt et al. 2007), conrming that the observed size of thedisappearing before = 12.041 (2009 Apr 03). The UV reso- 2 m continuum emission (50% encircled-energy radius of 4.8nance line of Si iv 1394 present in the HST/STIS data indicates AU) is well reproduced by the Hillier et al. (2001) wind modelthat the high-velocity absorption appears between = 10.984 of Eta Car A. This implies that most, if not all, of the quiescent(2003 Jun 01) and = 10.995 (2003 Jun 22). The analysis ofK-band emission is indeed due to free-free and bound-free emis-the ultraviolet absorption is hampered by severe blending with sion from the wind of Eta Car A, and that the contribution fromFe ii lines after = 11.0; consequently, the presence of the high-hot dust to the K-band emission is negligible within 70 milli-velocity absorption and the timescale are less accurately deter- arcseconds of Eta Car A. Note that the amount of emission frommined. hot dust would be even smaller at 1.08 m than in the K-band. Of 10. 10Groh et al.: High-velocity material in Eta Car across periastroncourse, hot dust is well-known to be present in Eta Car on spatial form directly in the wind of Eta Car B? In the next two subsec-scales larger than 70 milli-arcseconds (see, e.g., Chesneau et al. tions, we investigate that possibility.2005), and will certainly contaminate measurements using largerapertures.Several studies have proposed that dust forms in Eta Car 5.3.1. The wind of Eta Car B absorbs its own continuumduring periastron (Falceta-Goncalves et al. 2005; Kashi & Soker radiation2008a), although Smith (2010) showed that the dust formation Such a hypothesis would correspond to the classical detectionis cycle-dependent, occurring preferentially in the earlier docu- of a companion in normal massive binary systems, such as inmented spectroscopic events of 1981.4 and 1992.5. Signicant WR+OB binaries. However, the ux of Eta Car B is severaldust formation is uncertain during the 2003.5 event Smith orders of magnitude lower than that of Eta Car A in the near-(2010), and no near-infrared photometry has been reported for infrared continuum around the He i 10833 (Hillier et al. 2006).the 2009.0 event. The J-band ux increased by ca. 25% just be- Therefore, even if the wind of Eta Car B could produce a satu-fore the 2003.5 event (Whitelock et al. 2004), which has been in- rated He i 10833 absorption prole when observed in isolation,terpreted as due to free-free (Whitelock et al. 2004) or hot dust an undetectable amount of absorption ( 0.5 1%) would beemission (Kashi & Soker 2008a). More importantly, interfero- seen in the combined spectrum of Eta Car A and B.metric observations in the K-band during the 2009.0 spectro-scopic event, obtained simultaneously to our VLT/CRIRES mea-surements, do not show a signicant change in the size of the5.3.2. The wind of Eta Car B absorbs the continuumK-band emitting region (Weigelt et al. 2010, in preparation), ar- radiation from Eta Car Aguing against signicant emission from hot dust in the inner 70milli-arcseconds of Eta Car. One possible way to observe the wind of Eta Car B, should itTherefore, a photospheric radius of Eta Car A at 1.08 m ofcontain signicant amounts of neutral He, would be if its He i2.2 AU is hereafter assumed as the size of the continuum emis- 10833 absorption zone is extended and dense enough to absorbsion, based on the direct interferometric measurements in thecontinuum radiation from Eta Car A, in a wind-eclipse sce-K-band (van Boekel et al. 2003; Weigelt et al. 2007) scaled to nario. In order to detect the wind of Eta Car B only during a1.08 m and on the value that we computed using the CMFGEN brief period, at phases 0.976 1.023 (i.e., 95 d), the binaryradiative transfer model of Eta Car A (Hillier et al. 2001). system would again have to be oriented in the sky with a longi- tude of periastron of 90 , since Eta Car B would need to be located between the observer and the continuum source for5.2. Transient fast material in the wind of Eta Car A? only a brief period around periastron passage. In addition, the material in the wind of Eta Car B must have a suciently highIf a binary companion is evoked, the periodicity might be ex-column density of neutral He to absorb enough He i 10833 pho-plained as due to brief ejections of high-velocity material by Eta tons, but this is not predicted by the Hillier et al. (2006) radiativeCar A triggered during each periastron passage. However, this transfer model either. A much larger M and/or lower Te wouldscenario presents several diculties, given that previous spec-again be needed to produce enough optical depth in the wind oftroscopic observations suggested that the wind of Eta Car A be-Eta Car B for He i 10833. In principle, this would argue for thecomes roughly spherical during periastron (Smith et al. 2003a).presence of a Wolf-Rayet (WR) instead of an O-type star com-It would also imply that, during periastron, material from Eta panion, since WRs have a higher wind density than O-type stars.Car A at 2000 km s1 (instead of the usual 500600 km s1 )collides with the shock front. This increased velocity from Eta Even in the unlikely possibility that the wind of Eta CarCar A would produce a much higher X-ray luminosity thanB could signicantly absorb He i 10833 photons, the wind-what is currently observed. Both issues could be circumventedeclipse scenario also fails to reproduce the observed durationif the density and volume-lling factor of the 2000 km s1 of the high-velocity absorption component for the assumed or-transient wind are suciently low so as not to aect the X-ray bital parameters, even allowing for signicant uncertainties inhardness luminosity and the H absorption proles measured bythese parameters. We show in Figure 10a a pole-on view of theSmith et al. (2003a). However, it is unlikely that such a thin windgeometry of the orbit for the wind-eclipse scenario, assumingwould produce detectable absorption in He i 10833.masses of 90 and 30 M for Eta Car A and B, respectively, or- bital period of P = 2022.7 d, semi-major axis of a = 15.4 AU,The existence of a brief high-velocity wind from Eta Car A eccentricity of e = 0.9, and = 90 . A photospheric radiuswould be very unlikely in a single star scenario, although weat 1.08 m of 2.2 AU is assumed for Eta Car A, as discussedcannot rule out that possibility based on our present data. In par-in Section 5.1. In this subsection, we assume an inclination an-ticular, a single-star scenario would have to invoke a yet un- gle of i = 90 to derive an upper limit for the timescale of theknown mechanism that would produce a periodic episode of wind-eclipse. For (more realistic) lower inclination angles, anhigh-velocity wind like clockwork every 2022.7 1.3 days, aseven shorter timescale will be obtained.measured by Damineli et al. (2008b).From Figure 10a it is apparent that, with the parameters de- scribed above, the wind of Eta Car B is in front of the continuum5.3. Direct observation of the wind of Eta Car B?source due to Eta Car A during a much shorter (by a factor of 4) time interval (0.994 1.006, green line) than observedThe edge velocity of the high-velocity absorption compo- (at least 0.976 1.023, red line). Unrealistic values for thenent seen in He i 10833 and Si iv 1394, 1403 appears to eccentricity (e = 0, Fig. 10b), combined mass of the stars (max-approach the velocity expected of the wind of Eta Car B, imum of 8 M , Fig. 10c), or a larger photospheric radius of Eta3000 km s1 , based upon X-ray spectroscopic modeling by Car A (5.5 AU, Fig. 10d) would be required in order for this sce-Pittard & Corcoran (2002). To date, Eta Car B has not been ob- nario to work. Alternatively, unrealistically large amounts of hotserved directly. Could the high-velocity absorption componentdust emission located conveniently behind the wind of Eta Car 11. Groh et al.: High-velocity material in Eta Car across periastron 11 ing which orbital orientation is more consistent with our data. Specically, we aim at obtaining for which inclination angles and orbital orientations there is high-velocity gas, with veloci- ties between 800 and 2000 km s1 , in our line-of-sight to Eta Car A across periastron, and at which distances that gas is lo- cated. The presence of high-velocity gas in our-line-of-sight is a necessary, but not sucient condition for the presence of high- velocity absorption in a given spectral line. The amount of ab- sorption of a spectral line will depend on the population of the lower energy level related to that line, which is regulated by the ionization stage of the gas. The 3-D hydrodynamical simulations allow us to analyze the hydrodynamics of the material owing from the wind-wind collision zone with a much higher preci- sion than in the analytical models of Kashi & Soker (2009), in particular for epochs across periastron, when the high-velocityFig. 10. Illustration of the pole-on orbital geometry of the Eta material has been detected. For these epochs, the structure of theCar binary system for dierent orbital parameters, assumingwind-wind collision zone is severely distorted, with the arms ofP = 2022.7 d, = 90 and i = 90 (the observer is located the bowshock being wrapped around Eta Car A (Okazaki et al.to the right, along the x axis). The part of the orbit highlighted in2008; Parkin et al. 2009).green corresponds to phases when the wind of Eta Car B could We use 3-D simulations which are similar to and have thebe able to absorb continuum photons from Eta Car A (the wind- same parameters as those presented in Okazaki et al. (2008),eclipse scenario). The red highlighted part of the orbit repre- with the exception that adiabatic cooling has been included4 .sents the phases when the high-velocity absorption component The simulations assume the following parameters: for Eta Car A,is observed in the He i 10833 line assuming, for simplication, a mass of 90 M , radius of 90 R , mass-loss rate of 2.5 that the phase zero of the spectroscopic cycle coincides with the104 M yr1 , and wind terminal velocity of 500 km s1 ; for Etaperiastron passage. In a) we assume masses of 90 and 30 M Car B, a mass of 30 M , radius of 30 R , mass-loss rate offor Eta Car A and B, respectively, a photospheric radius of Eta105 M yr1 , and wind terminal velocity of 3000 km s1 ; orbitalCar A at 1.08 m of 2.2 AU, and e = 0.9. In b) we vary the period of P = 2024 d, eccentricity of e = 0.9, and semi-majoreccentricity to e = 0; c) assumes unrealistic masses of 4 M for axis of a = 15.4 AU. We refer the reader to Okazaki et al. (2008)both Eta Car A and B; and d) assumes a photospheric radius offor further details. Figure 11 presents 2-D slices of the wind-Eta Car A at 1.08 m of 5.5 AU.wind collision zone geometry based on the 3-D hydrodynamical simulations of Eta Car to help better visualize the geometry of the binary system.B, which seems unlikely and is not supported by the availableTime-dependent, multi-dimensional radiative transfer mod-observations, would be required to make this scenario to work. eling of the outowing material from the wind-wind collisionAn additional issue regards the amount of absorption ob- zone is needed to obtain the physical conditions of the high-served in He i 10833. Since the observed high-velocity absorp-velocity gas. That is well beyond the scope of this paper, andtion spans velocities from 800 up to 2000 km s1 , the ab- we will defer such analysis for future work. Since we did notsorption necessarily has to occur in the acceleration zone of Etacompute a multi-dimensional radiative transfer model, we areCar B, before the wind reaches the supposed terminal velocity of able to obtain only total column densities from the SPH simula-3000 km s1 . Based on the CMFGEN radiative transfer model oftions, but not the column density of the population of the lowerthe wind of Eta Car B, the acceleration zone of the wind of Etaenergy level of He i 10833 (2s 3 S). The total column densityCar B is relatively compact compared to the size of the photo- computed here (hereafter referred to as column density) pro-sphere of Eta Car A. Consequently, if the wind of Eta Car B is tovides an upper limit for the amount of absorption. Thus, a lowabsorb continuum photons from Eta Car A, the coverage of suchcolumn density at higher velocities implies that no high-velocitya continuum source would be very small.absorption will be present. However, a high column density doesWe conclude that it is unlikely that the high-velocity absorp- not necessarily mean that a strong absorption line will be de-tion component originates in the wind of Eta Car B.tected. In particular, the current 3-D simulations that we use do not account for radiative cooling, which makes it dicult to5.4. High-velocity, shocked material from the wind-windestimate the actual temperature and ionization structure of the collision zonehigh-velocity material in the wind-wind collision zone. In this Section, we assume that this material is able to eciently coolThe high-velocity absorption may originate in shocked mate-and to produce the observed high-velocity absorption if the col-rial from the wind-wind collision zone that crosses our line-umn density and velocity in the line-of-sight to Eta Car A areof-sight to Eta Car briey across periastron. Such a hypothe-high enough.sis has been already suggested by Damineli et al. (2008a) to ex- Hereafter, for simplicity we assume that the phase zero of theplain the behavior of He i 10833, assuming = 270 , and byspectroscopic cycle (derived from the disappearance of the nar-Kashi & Soker (2009), who instead derived = 90 from their row emission component of He i 6678) coincides with phaseanalytical modeling. zero of the orbital cycle (periastron passage). Note that in aHere we use the aid of three-dimensional (3-D) hydrody-highly-eccentric binary system like Eta Car the two values arenamical simulations of the Eta Car binary system to investigatenot expected to be shifted by more than a few weeks. Such timewhere high-velocity material can be found in the system, and atwhich epochs. We qualitatively compare our observations with 4 3-D simulations from Parkin et al. (2009) show similar hydrody-the 3-D hydrodynamical simulations with the goal of constrain- namics as in the Okazaki et al. (2008) simulations. 12. 12Groh et al.: High-velocity material in Eta Car across periastronpole onequator onmaterial in the range of 800 to 2000 km s1 (Fig. 12i). For a lower value of = 50 , part of the wind of Eta Car B crosses our line-of-sight to Eta Car A after periastron, producing a consid- erable amount of column density of high-velocity material up to 1300 km s1 (Fig. 12df). This is exactly the wind-eclipseApastron scenario described above (Sect. 5.3.2) and, as one can see in Figure 12f, the duration of the high-column density of high- velocity material in our line-of-sight to Eta Car A is very brief. Therefore, based on the hydrodynamics predicted from detailed 3-D hydrodynamical simulations, we can rule out that the Eta Car system has orbital orientations around 50 90 if the high-velocity gas originates in shocked material from the wind- wind collision zone. We also obtained that orbital orientations with = 0 (Fig. 12ac) and = 180 (Fig. 12jl) do not provide high-velocity gas with sucient column density in the line-of-sight to Eta Car A during the observed duration of thePeriastron high-velocity absorption.We nd that the 3-D hydrodynamical simulations require an orbital orientation with in the range 240 270 to produce high-velocity gas with enough density in the line-of-sight to- wards Eta Car A (Figs. 12mo, 12pr). For these orbital ori- entations, the density of the high-velocity gas is approximately an order of magnitude higher than expected from the wind of Eta Car B, while the velocities are signicantly lower compared to the value expected for the wind of Eta Car B. These physi-Fig. 11. Illustration of the wind-wind collision and orbital ge- cal conditions show that the dense, high-velocity gas which isometry from dierent vantage points of the Eta Car system, in our line-of-sight to Eta Car A originates from shocked mate-based on snapshots from the 3-D hydrodynamic simulations sim- rial from the wind-wind collision zone. For = 243, the high-ilar to those of Okazaki et al. (2008), but including adiabatic velocity material is located between 1 and 3 semi-major axis (15cooling. The color scale refers to the 2-D density structure, and to 45 AU; Figs. 12mo). The radial dependence of the densitythe spatial scales are in units of the semi-major axis a of the orbit follows roughly an r2 law, which closely resembles that of a(a = 15AU). The top row shows the system conguration dur- stellar wind and might explain why the observed high-velocitying apastron, while the bottom row refers to periastron. The wind absorption proles are rather smooth and broad.from Eta Car A, the wind-wind collision zone, and the wind fromEta Car B are seen from a pole-on view (left panels) and from For = 270 , the increase in column density around =the equator (right). The white arrow corresponds to = 270 , 11.991 11.998 occurs at a velocity region around 1200 towhile the green arrow corresponds to = 90 . 1800 km s1 , and the column density of the gas with velocities 800 to 1200 km s1 actually decreases before periastron and increases after periastron (Fig. 12pr). Qualitatively, we would expect the opposite behavior to explain the increase in high-shift would only cause a small change of 10 20 in the best velocity absorption. However, ionization and radiative transfervalue of , which will not aect our conclusions.eects might also play a role in determining the amount of ab-As discussed in Section 5.1, the main source of continuumsorption.radiation at 1.0 m is the free-free and bound-free emission from A better qualitative agreement between our data and thethe wind of Eta Car A, and we analyze the physical conditionsmodels is obtained for i = 41 and = 243, which isof the gas between the observer and Eta Car A. in line with the values obtained by Okazaki et al. (2008) and (Parkin et al. 2009) to t the X-ray lightcurve of Eta Car. For5.4.1. Orbital plane is aligned with the Homunculusthis orientation, there is an overall increase of the column den- equatorial planesity of the gas with velocities between 800 to 2000 km s1 from = 11.875 to = 11.991 11.998 (Fig. 12mo), whichIn this Section we investigate the hydrodynamics of the mate-corresponds to the phase range when the high-velocity compo-rial in line-of-sight to Eta Car A viewing the system from dif-nent appears in the observations (Section 4). There is still sig-ferent , assuming that the orbital plane has is aligned with thenicant column density at = 11.875 at some velocities, inHomunculus equatorial plane and, thus, the inclination of theparticular around 1600 km s1 , which is due to a blob expand-orbit is i = 41 (Smith 2006). Since we are analyzing an ab- ing at that velocity. A steep overall decrease occurs at phasessorption line, Figure 12 presents the one-dimensional density, = 12.014 12.040, agreeing qualitatively with the disappear-velocity, and column density structure of the gas for dierent ance of high-velocity absorption. Therefore, it is very likely thatlines-of-sight to Eta Car A at orbital phases when VLT/CRIRESthe huge changes in the column density of the high-velocity ma-observations were available. terial from the wind-wind collision zone, which occurs acrossFor = 90 , the line-of-sight to Eta Car A contains only periastron, are one of the main explanations for the brief appear-high-density material from the wind of Eta Car A, with v ance of the high-velocity absorption component.500 km s1 , before periastron (Fig. 12gh). After periastron, For some velocity ranges (e.g., 1100 to 1400 km s1 ), thea patch of shocked material crosses the line-of-sight, but it does column density is higher at = 11.991 than at = 11.998,not possess material faster than 800 km s1 and, as a conse-which is opposite to what one would naively expect if the to-quence, produces a negligible column density of high-velocitytal column densities computed here corresponded directly to a 13. Groh et al.: High-velocity material in Eta Car across periastron 13Fig. 12. Left: Log density of material as a function of line-of-sight distance to Eta Car A (in units of the semi-major axis a =15.4 AU) for selected orbital phases. Middle: Line-of-sight velocity of material along the same assumed line-of-sight to Eta Car A.Right: Log of the column density (in units of cm2 per 50 km s1 bin) along the same assumed line-of-sight to the primary star. Allpanels assume i = 41 and, from top to bottom, = 0 , 50 , 90 , 180 , 243 , and 270, respectively. The grey region correspondsto the observed range of high-velocity absorption. 14. 14 Groh et al.: High-velocity material in Eta Car across periastron x/ax/a -10 -5 15. 05 16. 10 -10 -5 17. 05 18. 10 certain amount of line absorption. If ionization eects occur, the 1010 t=0.875 t=0.875behavior of the column density of the population of the lower en-ergy level of He i 10833 (2s 3 S) as a function of velocity would 55 19. dier from that of the total column density. Since the distanceof Eta Car B to the high-velocity material and its optical depth y/ay/a 00change signicantly across periastron due to the high orbitaleccentricity, ionization eects are indeed expected to happen. -5-5 20. 21. These probably play a role in the observed duration of the high-velocity absorption as well as to explain why no high-velocity -10-10 -20-18 -16log density [g/cm3]-14-12 -2500-1500 -500velocity [km/s]500 material is detected with velocities from 2000 to 3000 km s1 , x/ax/a even if a high total column density is predicted by the SPH mod- -10 -5 05 10-10 -50 5 10 22. els. Note that, with the qualitative comparison done here, we do 1010 t=0.991 t=0.991not aim at explaining the exact behavior of the column densityas a function of velocity, nor to claim that the column density 55 23. derived from the SPH simulations is able to explain the amount y/ay/aof absorption at each velocity. In order to do that, a proper radia- 00 tive transfer model including the ionizing ux of Eta Car B and -5-5 24. 25. of the wind-wind collision-zone would be needed, which is wellbeyond the scope of this paper. -10-10 -20-18 -16 -14-12 -2500-1500 -500500 The hydrodynamical structure of the wind-wind interactionlog density [g/cm3] velocity [km/s]zone is extremely complex, in particular across periastron when x/ax/a -10 -5 26. 05 27. 10 -10 -5 28. 05 29. 10 the high-velocity absorption is observed. To illustrate the com- 1010 t=0.998 t=0.998plex geometry and dynamics of the wind-wind interaction in EtaCar, we present in Figure 13 2-D slices of density and velocity in 5 30. 5 31. the plane containing the observers line-of-sight and Eta Car Afor our best orientation of i = 41 and = 243. We notice that y/ay/a00 the region responsible for the high-velocity absorption, locatedbetween 1 and 3 semi-major axis (15 to 45 AU), is clumpy and -5-5 32. 33. increases in density across periastron. However, after periastron,there is a major decrease in the wind velocity in the line-of-sight -10-10 -20-18 -16log density [g/cm3]-14-12 -2500-1500 -500velocity [km/s]500 to the primary star, explaining the quick disappearance of the x/ax/a high-velocity absorption. -10-5 34. 05 35. 10-10-5 36. 0 5 37. 10Using relatively simple analytical models for the wind-wind 1010 t=1.014 t=1.014collision zone, Kashi & Soker (2009) obtained a dierent value 38. of = 90 as their best-t toy model. We suggest that the com- 5 39. 5plex hydrodynamics of the Eta Car system across periastron, y/ay/a which was not included in the Kashi & Soker (2009) calcula- 00tions, and the dierent assumption for the source of the contin- -5uum emission (extended hot dust emission on scales of 30-5 40. 41. AU) are the main reasons for the very dierent value of found -10-10 -20-18 -16 -14-12 -2500-1500 -500500by these authors. We show in Figure 14 2-D slices of densitylog density [g/cm3] velocity [km/s] and velocity in the plane containing the observers line-of-sight -10-5 42. x/a05 43. 10-10 -5x/a 0 5 10and Eta Car A for i = 60 and = 90 . As discussed above, 44. 1010there is no high-velocity material from the wind-wind collision t=1.041 t=1.041zone in line-of-sight to Eta Car A, and only high-velocity ma- 45. terial from the wind of the Eta Car Car B is in line-of sight (as 5 46. 5in the wind-eclipse scenario described in Section 5.3.2, which y/ay/a 00does not explain the observations). Since Kashi & Soker (2009)assumed that the absorption region was compact ( a few AU), -5-5 47. 48. it is also unclear how such a compact region would cover a sig-nicant fraction of their extended continuum source ( 30 AU) -10-10 -20-18 -16log density [g/cm3]-14-12 -2500-1500 -500velocity [km/s]500 in order to reproduce the signicant high fraction of continuumcoverage (3050%) inferred from the amount of high-velocityFig. 13. 2-D slices in the plane containing the observers line-absorption reported in our present paper.of-sight and Eta Car A for our preferred orbital orientation ofi = 41 and a longitude of periastron of = 243 , based on 3-D hydrodynamic simulations of the Eta Car binary system sim-5.4.2. A tilted orbital plane relative to the Homunculusilar to those presented by Okazaki et al. (2008). The observer equatorial plane?is located on the right, along the abscissa axis (indicated byRecent works have suggested that the orbital plane ofthe arrows in each panel). The semi-major axis of the orbit isthe Eta Car binary system might not be aligned with thethe arrow labeled x, the semi-minor axis the arrow y, and the Homunculus equatorial plane (Abraham & Falceta-Goncalves orbital axis the arrow z. From top to bottom, each row corre- 2007, 2009; Okazaki et al. 2008; Henley et al. 2008;sponds to orbital phases of = 11.875, 11.991, 11.998, 12.014, Falceta-Goncalves & Abraham 2009). To verify whetherand 12.041, respectively. Left: Density in logarithmic scale as a our observations support a tilted orbital axis scenario, here wefunction of line-of-sight distance from Eta Car A, in units of thesemi=major axis a = 15.4 AU. Right: Line-of-sight velocity ofmaterial through the same plane. Material is color coded to line-of-sight velocity towards (blue) or away (red) from the observer. 49. Groh et al.: High-velocity material in Eta Car across periastron15Fig. 15. Similar to Fig. 12 but, from top to bottom, the following line-of-sights are shown: i = 60 and = 50 , i = 90 and = 50 , i = 60 and = 243, and i = 90 and = 243 , respectively.investigate whether the 3-D SPH models show a signicant col- with our observations for the same reasons discussed in Sect.umn density of high-velocity gas at other orbital inclination and 5.3.2. Hence, our data are not in agreement with the orbital pa-at the orbital phases corresponding to when the high-velocity rameters derived by Abraham & Falceta-Goncalves (2007) and absorption was observed. Figure 15 presents, similar to FigureFalceta-Goncalves & Abraham (2009) if the high-velocity ab- 12, the one-dimensional density, velocity, and column density sorption originates in the wind-wind collision zone.sstructure of the gas for dierent lines-of-sight to Eta Car A, butinclined at i = 60 and 90 from the plane of the sky.If the orbital plane is tilted relative to the Homunculus, thehydrodynamics of the 3-D simulations still require that the bi-For longitudes of periastron 50 90 , similar to the nary system is seen under 240 270 , otherwise no dense,aligned case, most of the material in the line-of-sight towards extended, high-velocity material from the wind-wind collisionEta Car A is from its own wind at roughly 500 km s1 , exceptzone is in line-of-sight during the observed amount of time. Theduring a brief period after periastron when the wind-eclipsetemporal behavior of the column density of the high-velocity gasscenario occurs (Fig. 15af). For inclination angles higher thanfor i = 60 and = 243 (Fig. 15jl) is very similar to thati = 41 , such as 60 and 90 , a higher fraction of the wind of Etaobtained for i = 41 and = 243 (Fig. 12gi). Indeed, forCar B crosses the line-of-sight to Eta Car A and, thus, higher col- = 243 , the overall relative increase (decrease) in the col-umn densities are obtained at higher velocities after periastron. umn density of the high-velocity material before (after) perias-However, that occurs only briey and it is still in disagreementtron is even higher in the case of i = 60 than for i = 41 . For 50. 16Groh et al.: High-velocity material in Eta Car across periastron x/a x/a -10 -5 0510-10 -50510 simulations of the wind-wind collision zone of Eta Car, we found1010 that dense high-velocity gas is in the line-of-sight to the primary t=0.998t=0.998star only if the binary system is oriented in the sky such that the companion is behind the primary star during periastron, cor-55 responding to a longitude of periastron of 240 270 .y/ay/a Our data is broadly consistent with an orbital inclination in the00 range i = 40 60 . We derived that the high-velocity gas is-5-5 located at distances of 15 to 45 AU in the line-of-sight to Eta Car A. More importantly, we can rule out orbital orientations in-10-10 the range 0 180 for all inclination angles, since these do -20-18 -16 -14 -12 -2000-100001000log density [g/cm3]velocity [km/s] not produce a signicant column density of high-velocity gas in our line-of-sight to match our observations of the high-velocity absorption component.Fig. 14. 2-D slices in the plane containing the observers line- The current 3-D SPH simulations used in this paper do notof-sight and Eta Car A, similar to Fig. 13, but for an orientation account for radiative cooling, which makes it dicult to estimateof i = 60 , longitude of periastron of = 90 , and orbital phase the ionization stage of the high-velocity material in the wind- = 11.998. The observer is located on the right, along the ab- wind collision zone. In addition to the increase in column den-scissa axis (indicated by the arrows in each panel). Left: Density sity of the high-velocity gas, ionization eects due to the closein logarithmic scale as a function of line-of-sight distance from presence of Eta Car B likely play an important role in explainingEta Car A, in units of the semi=major axis a = 15.4 AU. the amount of high-velocity absorption seen during periastron.Right: Line-of-sight velocity of material through the same plane. Time-dependent, multi-dimensional radiative transfer modelingMaterial is color coded to line-of-sight velocity towards (blue) or of the outowing gas from the wind-wind collision zone of theaway (red) from the observer. The insets show a zoom-in around Eta Car binary system is urgently warranted, and will allow us tothe inner 1a region. better understand the inuence of Eta Car B on the wind of Eta Car A across periastron. Ultimately, this will provide constraints on the masses of the stars and on the wind parameters of the Etai = 90 , there is no signicant increase in the column densityCar binary system.of the high-velocity material before periastron and, actually, astrong decrease is seen at = 11.998 (Fig. 15gi). Strong ion-Acknowledgements. We wish to thank the kind allocation of ESO Directorization eects would be necessary to explain the high-velocity Discretionary Time that was crucial for the completion of this project, andcomponent if indeed i = 90 , which seems unlikely. We can the ESO sta at Garching and Paranal, in particular Hugues Sana and Andreaalso argue that relatively less strong ionization eects would beAhumada, for carrying out the VLT/CRIRES observations. We appreciate many discussions and comments on the manuscript from Michael Corcoran and Nathanneeded to explain the appearance and disappearance of high-Smith. We thank an anonymous referee for the suggestions to improve the orig-velocity material during periastron if i = 60 compared to the inal manuscript. JHG thanks the Max-Planck-Gesellschaft for nancial supporti = 41 case. However, since the exact amount of ionizationfor this work. AD and MT thanks the FAPESP foundation for continuous sup-eects due to Eta Car B is unclear, we conclude that our ob- port. TIM is supported by a NASA GSRP fellowship. The HST observations were accomplished through STIS GTO, HST GO and HST Eta Car Treasuryservations can be explained by the 3-D hydrodynamical models Team programmes. HST/STIS data were obtained through the HST Eta Carwith both i = 41 and i = 60 for = 243, which is consistentTreasury archive hosted at University of Minnesota.with previous X-ray analyses (Okazaki et al. 2008; Henley et al.2008; Parkin et al. 2009). References Abraham, Z. & Falceta-Gonc alves, D. 2007, MNRAS, 378, 3096. 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Groh et al.: High-velocity material in Eta Car across periastron, Online Material p 1 1.0 1.0 0.8Orbital Phase, 0.7 0.8 0.5 0.3 0.6 0.2 0.0 -2000 -1000010002000 Velocity (km s-1) 1.0 1.0 0.8Orbital Phase, 0.7 0.8 0.5 0.3 0.6 0.2 0.0 -2000 -1000010002000 Velocity (km s-1) 1.0 1.0 0.8Orbital Phase, 0.7 0.8 0.5 0.3 0.6 0.2 0.0 -2000 -1000010002000 Velocity (km s-1)Fig. 6. Similar to Fig. 5, but for C iv 1548, 1551 (upper panel),C ii 1334, 1335 (middle panel), Si ii 1526, 1533 (bottompanel).