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Nebulae from Eruptions of Luminous Evolved Stars: Eta Carinae, RY Scuti, and the Lbvs

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Nebulae from Eruptions of Luminous Evolved Stars: Eta Carinae, RY Scuti, and the Lbvs Active OB Stars: Laboratories for Stellar and Circumstellar Physics ASP Conference Series, Vol. 361, 2007 S. Stefl,ˇ S. P. Owocki and A. T. Okazaki Nebulae from Eruptions of Luminous Evolved Stars: Eta Carinae, RY Scuti, and the LBVs N. Smith CASA, University of Colorado, 389 UCB, Boulder, CO 80309 Abstract. The most prodigious mass loss for luminous hot stars occurs during the Luminous Blue Variable (LBV) phase in transition to a Wolf-Rayet star. Most of the mass loss is a result of a few brief eruptions, rather than a steady wind. For the most luminous stars, these eruptions eject several M⊙ at once, accounting for a large fraction of their total post–main-sequence mass loss. The geometry of their nebulae in the young free expansion phase traces the roles of rotation and binary interactions. Our most observable example is the nebula around η Car, while nebulae around the eclipsing binary RY Sct and other LBVs share similar but less extreme properties. Both η Car and RY Sct have nebulae less than 200 yrs old with pronounced axial symmetry. 1. Introduction: High Luminosities and Circumstellar Nebulae In this contribution, I would like to explore the results of stellar activity in stars with temperatures similar to Be stars, but with much higher luminosities. Various types of stellar activity and non-spherical mass loss discussed at this meeting are present in Be stars largely as a consequence of lower geff induced by very rapid rotation rates. In high-luminosity B supergiants, especially the luminous blue variables (LBVs), extreme mass-loss and the effects of rotation follow from lower geff due to the proximity to the Eddington limit and near- critical rotation, even if the stars do not have high rotational velocities. The most spectacular manifestation of this is the short-duration outbursts of LBVs, which may dominate the post-MS mass loss in luminous stars. While we cannot study rotation rates directly (i.e. broadened lines) in these stars due to their dense winds, we can infer the importance of rotation from their circumstellar geometry. However, we must catch these nebulae soon after ejection, while still in the free-expansion phase. The most well-studied case is the 160-yr old bipolar nebula around η Car, but I will also mention a few other relevant objects. 2. Eta Carinae and the Homunculus The Homunculus Nebula around η Car is perhaps the most recognizable bipo- lar nebula seen by HST (Morse et al. 1998), and its 19th century eruption is arguably the most extreme example of stellar activity, barring the final destruc- tion of a massive star. That outburst ejected more than 10 M⊙ of material and released more kinetic energy than SN1054 (Smith et al. 2003b). The mass loss rate was so high that it could not have been a conventional line-driven wind because the material was optically thick, and so the mass-loss was either a super- 200 Nebulae from Eruptions 201 Figure 1. Dust structure in the Homunculus. (a) 18 µm image. (b) Color temperature. (c) Warm dust column density at 8.8 µm. (d) Cool dust column density at 18 µm (see Smith et al. 2003b). Figure 2. Spatio-kinematic structure of (a) [Fe ii] λ16435 and (b) H2 2.122 µm, with the long-slit aperture along the major axis of the Homunculus, but offset slightly from the star; from Smith (2002). 202 Smith Figure 3. High resolution spectrum of [Fe ii] λ16435 in P Cygniprofiles’s nebula, with the long-slit aperture oriented north-south (Smith & Hartigan 2006). Eddington continuum-driven wind (e.g., Owocki et al. 2004) or a hydrodynamic explosion (Arnett et al. 2005). The huge amount of mass ejected suggests that these outbursts may be important in stellar evolution. Recent study of the Homunculus has revealed a double-shell excitation structure (see Figs. 1 and 2), with a thin outer shell seen in cool dust and H2 emission, and a thicker inner shell seen in warmer dust and [Fe ii] emission (Smith et al. 2003b; Smith 2002, 2005). Most of the mass is contained in the thin and cool H2-emitting shell, whose bipolar shape and pinched waist point toward rapid rotation of the erupting star as the shaping mechanism (Smith 2002). The nebula is too dense and massive to have been shaped by a pre-existing torus, and proper motions of the expanding nebula are ballistic. Subsequent mass ejections have followed the same bipolar symmetry axis: for example, the 1890 eruption that formed the “Little Homunculus” (Ishibashi et al. 2003; Smith 2005) and the present-day stellar wind (Smith et al. 2003a). Eta Car is probably a binary system, but the specific role of the companion star is unclear and continues to be investigated. It is unlikely, however, that the putative companion star played a major role in shaping the ejecta of the 19th century eruption, since the kinetic energy of the Homunculus is greater than the gravitational binding energy of the orbit (Smith et al. 2003b). Of course, recurring close periastron passages in this eccentric system may have spun up the outer layers of the star (e.g., Smith et al. 2003a). 3. P Cygniprofiles: A Counter-Example P Cygniprofiles is the only other LBV in our Galaxy that has been observed during a giant outburst, back in 1600 A.D. (Humphreys et al. 1999). Its nebula ejected during that eruption (Smith & Hartigan 2006) is older and less massive than η Car’s, and so is more prone to shaping by the ambient medium. Unlike the Homunculus of η Car, the nebula around P Cygniprofiles appears to be spherical. Nebulae from Eruptions 203 It is a challenge to image this nebula because of the bright central star, but the spherical shape can be seen in high-resolution spectra, like the [Fe ii] data in Figure 3 (Smith & Hartigan 2006) or in similar [Ni ii] or [N ii] spectra (Barlow et al. 1994). The spherical shape is in stark contrast to the bipolar Homunculus of η Car. This implies that the effects of rotation in shaping outflows will be more severe at higher luminosities closer to the Eddington limit, since P Cyg is far 5.8 less luminous than η Car (P Cygniprofiles is ∼10 L⊙ at D=1.7 kpc (Najarro et al. 1997). The mildly bipolar or elliptical shape of AG Car’s nebula would support this view, since its luminosity is intermediate between η Car and P Cyg. The ejection speed and radiative luminosity during P Cygni’s outburst were less than for η Car. Recently, (Smith & Hartigan 2006) estimated the mass of P Cygni’s nebula to be ∼0.1 M⊙, which shows that the total mass ejected, the mass loss rate, and the kinetic energy during this eruption were also far less extreme than for η Car (although interestingly similar to η Car’s 1890 outburst which created the Little Homunculus (Smith 2005; Ishibashi et al. 2003). This illustrates the likely range of mass ejection in LBV outbursts, since P Cyg is at the lower luminosity end of LBVs that are not post-RSGs, relevant to the distribution of LBVs on the HR diagram (e.g., Smith et al. 2004). a) HST/WFPC2 b) VLA Hα 15 GHz (1997) (1992) 1" 1" c) Keck/LWS d) Flux ratio 11.7 µm 11.7 µm (1999) Hα 2" 2" Figure 4. RY Scuti’s nebula. (a) HST/WFPC2 image in Hα from (Smith et al. 2002). (b) 2 cm free-free continuum image taken with the VLA (Smith et al. 2001a; Gehrz et al. 1995)). (c) Dust emission at 11.7 µm (Gehrz et al. 2001); note the difference in size compared to Panel a. (d) Flux ratio of panels a and c, showing the spatial separation between ionized gas and dust. 204 Smith Figure 5. Left: Long-slit HST/STIS spectrum of [N ii] λ6583 in RY Scuti’s nebula, with the slit along the equatorial (major) axis. This shows the uneven brightness distribution around the ring, indicating severe azimuthal asymme- try (Smith et al. 2002). Right: Proper motions of RY Scuti’s nebula, showing the increase in the diameter of the nebula (Smith et al. 2001a). 4. RY Scuti, Equatorial Rings, and SN1987a RY Scuti is a luminous WR+OB eclipsing binary with an 11 day period and a separation of about 0.4 AU (Smith et al. 2002; Gehrz et al. 1995). What makes it interesting for trying to understand how binary stars shape outflows is its unusual circumstellar nebula (see Fig. 4). RY Scuti’s nebula is an equatorial dust torus, with the inner edge of the torus ionized by the central star (1995 Gehrz et al. 2001; Smith et al. 2002). The ionized nebula is especially weird, as it seems to show a pair of plane-parallel ionized rings with the same radius above and below the equator, while kinematics in spectra (Fig. 5) reveal severe azimuthal asymmetry (Smith et al. 2002). This geometry is likely to be a direct imprint of the mass-ejection geometry, since the expanding nebula was ejected sometime in the 19th century (Smith et al. 2001a, Fig. 5). The total mass of ionized gas in RY Scuti’s nebula is about 0.003 M⊙ (Smith et al. 2002). Thus, −1 the mass-loss rate to create the ionized nebula is 0.003/(∆tyr) M⊙ yr . For −1 ∆tyr∼<11 days, that’s M˙ ∼>0.1 M⊙ yr , which is extremely high. The mass in RY Scuti’s nebula is concentrated near the equatorial plane, implying that mass loss through the outer Lagrangian points in this over-contact system is the dominant mechanism shaping the outflow.
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