Latitude-dependent Effects in the Stellar Wind of η Carinae

By N. Smith, K. Davidson, T. Gull, K. Ishibashi, John Hillier. 2003

157 citations recorded by the source corpus.

The Homunculus reflection nebula around η Carinae provides a rare opportunity to observe the spectrum of a star from more than one direction. In the case of η Car, the nebula's geometry is known well enough to infer how line profiles vary with latitude. We present Space Telescope Imaging Spectrograph (STIS) spectra of several positions in the Homunculus, showing directly that η Car has an aspherical stellar wind. P Cygni absorption in Balmer lines depends on latitude, with relatively high velocities and strong absorption near the polar axis. Stronger absorption at high latitudes is surprising, and it suggests higher mass flux toward the poles, perhaps resulting from radiative driving with equatorial gravity darkening on a rotating star. Reflected profiles of He I lines are more puzzling, offering clues to the wind geometry and ionization structure. During η Car's high-excitation state in 2000 March, the wind was fast and dense at the poles, with higher ionization at low latitudes. Older STIS data obtained since 1998 reveal that this global stellar wind geometry changes during η Car's 5.5 yr cycle and may suggest that this star's spectroscopic events are shell ejections. Whether or not a companion star triggers these outbursts remains ambiguous. The most dramatic changes in the wind occur at low latitudes, while the dense polar wind remains relatively undisturbed during an event. The apparent stability of the polar wind also supports the inferred bipolar geometry. The wind geometry and its variability have critical implications for understanding the 5.5 yr cycle and long-term variability but do not provide a clear alternative to the binary hypothesis for generating η Car's X-rays.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/a4fb43fe0dd953bb51f66628ab245ef1fb6d346b

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In Astronomy and Astrophysics

1. N. Smith
2. S. Smartt
3. N. Langer
4. A. Filippenko
5. J. Vink
6. Weidong Li
7. Stan Woosley
8. P. Crowther

Citations in the corpus

• The Structure of the Homunculus. I. Shape and Latitude Dependence from H2 and [Fe II] Velocity Maps of η Carinae (2006, methodology): “This is because the H2 emission traces the cool and dense outer skin of theHomunculus, which thermal-IR observations indicate contains the majority of the nebula’s mass and kinetic energy (Smith et al. 2003b).”
• The empirical metallicity dependence of the mass-loss rate of O- and early B-type stars (2007, background): “…on mass loss appears important only for stars close to the Ω-Γ limit (e.g. Maeder & Meynet 2000; and Lamers 2004; and, e.g., van Boekel et al. 2003; and Smith et al. 2003, for the extreme case of ηCarinae), therefore, for relatively “normal” early-type stars one may consider it less critical.”
• The Birth of High-Mass Stars: Accretion and/or Mergers? (2005, background): “Radiation, and the resulting line-driven stellar winds, will tend to emerge from the poles of the rotationally flattened object (von Zeipel 1924; Owocki et al. 1998; Smith et al. 2003).”
• Luminous blue variables are antisocial: their isolation implies that they are kicked mass gainers in binary evolution (2014, background): “As seen in the case of η Car, these eruptions can eject 10-20 M⊙ in a single event (Smith et al. 2003a).”
• On the Role of Continuum-driven Eruptions in the Evolution of Very Massive Stars and Population III Stars (2006, background): “…mass leave no room for such substantial mass loss if there is an upper limit of 150 M⊙ to the initial masses of stars (Figer 2005; Kroups 2005). in 1890 (Smith 2005; Ishibashi et al. 2003), and it may suffer smaller ejections every 5.5 years (Davidson 1999; Smith et al. 2003a; Martin et al. 2006).”
• Luminous blue variable eruptions and related transients: diversity of progenitors and outburst properties (2010, background): “Following another smaller eruption in ∼1890, the star is apparently still slowly recovering from the upheaval of its 19th century eruptions (e.g., Smith et al. 2003a; Davidson et al. 2005).”