Nature News - 15 June 2010
A dramatic drop in Eta Carinae's stellar winds could herald a bright future.
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.
Researchers from the University of Minnesota in Minneapolis recorded the dip from the eruptive, luminous variable star using data from the Gemini South Telescope in Chile and the Hubble Space Telescope. Their results are detailed in a paper accepted for publication in Astrophysical Journal Letters.
Huge stars like Eta Car can erupt for reasons that Davidson says are about 95% "mysterious". One theory is that once these massive stars near the end of their short lives — after some 2 million to 3 million years — perturbations in their cores may set off sporadic 'supernova impostor' eruptions.
During its mid-nineteenth-century eruption, Eta Car lost about ten times the mass of the Sun. Although this loss was equivalent to only 10% of its mass, the star's outer 50% was ejected. As a result, the star has still not returned to thermal and rotational equilibrium. Since 1858, it has been observed only as a massive gaseous outflow, losing the equivalent of one Jupiter-mass in gas per year.
Before 1700, Davidson notes that Eta Car was a fourth-magnitude star, recorded by astronomer Edmond Halley and only a few others. By 1843, it was almost as bright as Sirius before suddenly fading in 1858.
Meanwhile, if Eta Car's current trend of decreasing winds continues, in a decade it will have very nearly emerged from its cocoon of dense gaseous outflow. That would return it to the state observed by Halley some 300 years ago, when it was seen as a hot, blue star, now known to have been of spectral class O.
A Sea Change in Eta Carinae
- arXiv.org > astro-ph > arXiv:1004.3529
< 20 Apr 2010 (v1), 01 Jun 2010 (v3) >
Astrophysical Journal Letters (accepted)
Major stellar-wind emission features in the spectrum of Eta Car have recently decreased by factors of order 2 relative to the continuum. This is unprecedented in the modern observational record. The simplest, but unproven, explanation is a rapid decrease in the wind density.