* Astronomy

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RE: eta Carinae

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Giant Star Could Explode Any Day Now

When the sun finally dies some 5 billion years from now, the end will come quietly, the conclusion of a long, uneventful life. Our star will, in a sense, go flabby, swelling first, releasing its outer layers into space and finally shrinking into the stellar corpse known as a white dwarf.
Things will play out quite differently for a supermassive star like Eta Carinae, which lies 7,500 light-years from Earth.
For Eta Carinae, that violent end might not be long in coming, according to a report in the latest Nature.
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Astronomers Watch Delayed Broadcast of a Powerful Stellar Eruption

Astronomers are watching a delayed broadcast of a spectacular outburst from the unstable, behemoth double-star system Eta Carinae, an event initially seen on Earth nearly 170 years ago. Dubbed the "Great Eruption," the outburst first caught the attention of sky watchers in 1837 and was observed through 1858. But astronomers didn't have sophisticated science instruments to accurately record the star system's petulant activity. Luckily for today's astronomers, some of the light from the eruption took an indirect path to Earth and is just arriving now, providing an opportunity to analyze the outburst in detail. The wayward light was heading in a different direction, away from our planet, when it bounced off dust clouds lingering far from the turbulent stars and was rerouted to Earth, an effect called a "light echo." Because of its longer path, the light reached Earth 170 years later than the light that arrived directly.
The astronomers' study involved a mix of visible-light and spectroscopic observations from ground-based telescopes. The team's paper will appear Feb. 16 in a letter to the journal Nature.

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Title: Constraining the Absolute Orientation of Eta Carinae's Binary Orbit: A 3-D Dynamical Model for the Broad [Fe III] Emission
Authors: Thomas I. Madura, Theodore R. Gull, Stanley P. Owocki, Jose H. Groh, Atsuo T. Okazaki, Christopher M. P. Russell

We present a three-dimensional (3-D) dynamical model for the broad [Fe III] emission observed in Eta Carinae using the Hubble Space Telescope/Space Telescope Imaging Spectrograph (HST/STIS). This model is based on full 3-D Smoothed Particle Hydrodynamics (SPH) simulations of Eta Car's binary colliding winds. Radiative transfer codes are used to generate synthetic spectro-images of [Fe III] emission line structures at various observed orbital phases and STIS slit position angles (PAs). Through a parameter study that varies the orbital inclination i, the PA {\theta} that the orbital plane projection of the line-of-sight makes with the apastron side of the semi-major axis, and the PA on the sky of the orbital axis, we are able, for the first time, to tightly constrain the absolute 3-D orientation of the binary orbit. To simultaneously reproduce the blue-shifted emission arcs observed at orbital phase 0.976, STIS slit PA = +38 degrees, and the temporal variations in emission seen at negative slit PAs, the binary needs to have an i ~ 130 to 145 degrees, {\theta} ~ -15 to +30 degrees, and an orbital axis projected on the sky at a PA ~ 302 to 327 degrees east of north. This represents a system with an orbital axis that is closely aligned with the inferred polar axis of the Homunculus nebula, in 3-D. The companion star, Eta B, thus orbits clockwise on the sky and is on the observer's side of the system at apastron. This orientation has important implications for theories for the formation of the Homunculus and helps lay the groundwork for orbital modelling to determine the stellar masses.

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Title: The Interaction of the Eta Carinae Primary Wind with a Century Old Slow Equatorial Ejecta
Authors: Noam Soker (Technion), Amit Kashi (UNLV)

We argue that the asymmetric morphology of the blue and red shifted components of the outflow at hundreds of AU from the massive binary system eta Carinae can be understood form the collision of the primary stellar wind with the slowly expanding dense equatorial gas. Recent high spatial observations of some forbidden lines, e.g. [Fe III] lambda 4659, reveal the outflowing gas within about one arcsecond (2300 AU) from eta Car. The distribution of the blue and red shifted components are not symmetric about the center, and are quite different from each other. The morphologies of the blue and red shifted components correlate with the location of a dense slowly moving equatorial gas (termed the Weigelt blobs environment; WBE), that is thought to have been ejected during the 1887 -- 1895 Lesser Eruption (LE). In our model the division to the blue and red shifted components is caused by the postshock flow of the primary wind on the two sides of the equatorial plane after it collides with the WBE. The fast wind from the secondary star plays no role in our model for these components, and it is a freely expanding primary wind that collides with the WBE. Because the line of sight is inclined to the binary axis, the two components are not symmetric. We show that the postshock gas can also account for the observed intensity in the [Fe III] lambda 4659 line.

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Title: Imaging with HST the time evolution of Eta Carinae's colliding winds
Authors: Theodore R. Gull, Thomas I. Madura, Jose H. Groh, Michael F. Corcoran

We report new HST/STIS observations that map the high-ionisation forbidden line emission in the inner arcsecond of Eta Car, the first that fully image the extended wind-wind interaction region of the massive colliding wind binary. These observations were obtained after the 2009.0 periastron at orbital phases 0.084, 0.163, and 0.323 of the 5.54-year spectroscopic cycle. We analyse the variations in brightness and morphology of the emission, and find that blue-shifted emission (-400 to -200 km s-1) is symmetric and elongated along the northeast-southwest axis, while the red-shifted emission (+100 to +200 km s-1) is asymmetric and extends to the north- northwest. Comparison to synthetic images generated from a 3-D dynamical model strengthens the 3-D orbital orientation found by Madura et al. (2011), with an inclination i ~138°, argument of periapsis {\omega} ~ 270°, and an orbital axis that is aligned at the same PA on the sky as the symmetry axis of the Homunculus, 312°. We discuss the potential that these and future mappings have for constraining the stellar parameters of the companion star and the long-term variability of the system.

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Title: Accretion of Dense Clumps in the Periastron Passage of Eta Carinae
Authors: Muhammad Akashi, Amit Kashi, Noam Soker

We perform 3D hydrodynamical numerical simulations of the winds interaction process in the massive binary system Eta Carinae, and find the secondary star to accrete mass from the dense primary wind close to periastron passage. This accretion is thought to result in the spectroscopic event and X-ray minimum observed in the system every revolution. We included the gravity of the secondary star and the orbital motion starting 19 days (90 degrees) before periastron passage. The accretion process is triggered by dense clumps that cannot be decelerated by the ram pressure of the secondary wind. The dense clumps are formed by instabilities in the thin dense shell formed by the shocked primary wind gas. We explore the role of the numerical viscosity and some physical parameters on the initiation of the accretion process, and explain the unique properties of Eta Carinae that allow for the periastron accretion process to occur. The accretion starts about a week before periastron passage, as is required to explain the several weeks long X-ray minimum.

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Title: An Aboriginal Australian Record of the Great Eruption of Eta Carinae
Authors: Duane W. Hamacher, David J. Frew

We present evidence that the Boorong Aboriginal people of northwestern Victoria observed the Great Eruption of Eta Carinae in the nineteenth century and incorporated the event into their oral traditions. We identify this star, as well as others not specifically identified by name, using descriptive material presented in the 1858 paper by William Edward Stanbridge in conjunction with early southern star catalogues. This identification of a transient astronomical event supports the assertion that Aboriginal oral traditions are dynamic and evolving, and not static. This is the only definitive indigenous record of Eta Carinae's outburst identified in the literature to date.

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Title: Is Eta Carinae a fast rotator, and how much does the companion influence the inner wind structure?
Authors: Jose H. Groh (Max-Planck-Institute for Radioastronomy), Thomas I. Madura (U Delaware), Stanley P. Owocki (U Delaware), D. John Hillier (U Pittsburgh), Gerd Weigelt (MPIfR)

We analyse interferometric measurements of the Luminous Blue Variable Eta Carinae with the goal of constraining the rotational velocity of the primary star and probing the influence of the companion. Using 2-D radiative transfer models of latitude-dependent stellar winds, we find that prolate wind models with a ratio of the rotational velocity (vrot) to the critical velocity (vcrit) of W=0.77-0.92, inclination angle of i=60-90 degrees, and position angle PA=108-142 degrees reproduce simultaneously K-band continuum visibilities from VLTI/VINCI and closure phase measurements from VLTI/AMBER. Interestingly, oblate models with W=0.73-0.90 and i=80-90 degrees produce similar fits to the interferometric data, but require PA=210-230 degrees. Therefore, both prolate and oblate models suggest that the rotation axis of the primary star is not aligned with the Homunculus polar axis. We also compute radiative transfer models of the primary star allowing for the presence of a cavity and dense wind-wind interaction region created by the companion star. We find that the wind-wind interaction has a significant effect on the K-band image mainly via free-free emission from the compressed walls and, for reasonable model parameters, can reproduce the VLTI/VINCI visibilities taken at phase 0.92-0.93. We conclude that the density structure of the primary wind can be sufficiently disturbed by the companion, thus mimicking the effects of fast rotation in the interferometric observables. Therefore, fast rotation may not be the only explanation for the interferometric observations. Intense temporal monitoring and 3-D modelling are needed to resolve these issues.

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Enigmatic star could emerge from its gassy cocoon

The star Eta Carinae (Eta Car), once one of the brightest in the southern sky, has long been shrouded in mystery. After a huge outburst of gas that occurred more than 150 years ago, it has largely been hidden by a dense cloud of dust - a strong indicator of sporadic eruptions.
Now, Eta Car, which sits in our part of the Milky Way some 2,300 parsecs (7,500 light years) from the Sun, is puzzling researchers and theorists all over again. A US-based team has recorded a steep, inexplicable drop in its stellar wind - the outflow of gas from the star - measured as a change in the emission lines, or lines in the spectrum caused by the emission of light at particular wavelengths.
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