In the news this month: a drop in eta Carinae's stellar wind
A huge, billowing pair of gas and dust clouds are captured in this stunning NASA Hubble Space Telescope image of the supermassive star Eta Carinae. CREDIT: NASA, ESA and Jon Morse (University of Colorado)
Supernovae are the violent explosions of massive stars, so bright that the events can be seen in distant galaxies. But not all apparent explosions are genuine supernovae. Some fall into the category of supernova impostors, the sudden increase in brightness of a star without the terminal explosion. One such impostor event was the great eruption of eta Carinae, a star which is amongst the most massive known in the Milky Way. Located 7,500 light years away in the constellation of Carina, the star is five million times more luminous than the Sun and an estimated 100 times as massive. Eta Carinae underwent a massive but non-terminal explosion 150 years ago, allowing the close-up study of a supernova impostor. During the eruption, the star lost about ten percent of its mass, throwing off the outer layers in the surrounding nebula. Since then, the star has been enshrouded in a thick cloud of dusty debris and has been losing material at the rate of one Jupiter-mass per year in a strong stellar wind.
Now, a team of researchers, led by Andrea Mehner at the University of Minnesota, have observed dramatic changes in the star's spectrum. Observations over the last decade have shown an increase in the star's magnitude, but with no major long-term changes in its spectrum, something that might be expected following an event causing a major change in brightness. New observations carried out with the Space Telescope Imaging Spectrograph on the Hubble Space Telescope by Mehner's team show a substantial change in the emission lines, caused by specific elements in the star's atmosphere. According to the team, the sudden rapid decrease in the brightness of the emission lines (dropping to a third of their original strength in just ten years) suggests a decrease in the strength of the stellar wind, possibly signifying a much more rapid return to the pre-explosion state than was previously anticipated. With the wind density decreasing, the nebula should begin to thin and the star itself may become visible to modern telescopes for the first time, possibly within the next decade.
There are other explanations which may account for the unusual spectroscopic developments, including a change in the latitude dependence of the wind, but the complicated nature of the surrounding nebula and the difficulties constructing accurate models make an accurate assessment problematic.
This blog post is a news story from the Jodcast, aired in the July 2010 edition.
Mehner, A., Davidson, K., Humphreys, R., Martin, J., Ishibashi, K., Ferland, G., & Walborn, N. (2010). A SEA CHANGE IN ETA CARINAEThe Astrophysical Journal, 717 (1) DOI: 10.1088/2041-8205/717/1/L22