In the news this month: coronal loops imaged around another star
A comparison of the sizes of Bellatrix (left), Algol B (right), the Sun (centre) CREDIT: Paul Stansifer, wikicommons
Most of our knowledge of the processes and morphology of stellar coronae comes from observing our nearest star, the Sun. Coronal loops are associated with sunspot groups which affect the streams of charged particles leaving the Sun as the solar wind, so an understanding of the processes in these loops has implications for space weather predictions which can impact on satellite operations and the safety of astronauts.Studying the same processes in other stars is difficult due to the distances involved and the high resolution required to see any detail. Some of the highest resolution observations possible in astronomy are made using arrays of radio telescopes linked together in a process known as very long baseline interferometry; the more widely separated the telescopes in the array, the higher the resolution of the final images.Using this technique, a team led by William Peterson, a graduate student at the University of Iowa, have detected a large coronal loop on another star.
Using a very sensitive array of radio telescopes which included the ten antennas of the Very Long Baseline Array in the US, the 100-m Green Bank Telescope in West Virginia, the Very Large Array in New Mexico and the 100-m Effelsberg telescope in Germany, the astronomers imaged the variable star Algol in Perseus. Algol is an eclipsing binary system consisting of a large main sequence B-class star and a cooler K-class sub-giant in orbit around each other. The two stars are very close, just 6% of the distance between the Earth and our own Sun, and orbit each other every 2.86 days. The results show a gigantic coronal loop stretching out from the surface of Algol B, the K-class sub-giant star, towards its companion Algol A, with the two ends of the loop located at the magnetic poles of the sub-giant star. Throughout the orbit, this loop continues to point towards Algol A.
The researchers say that Algol B's coronal loop is similar to those seen on the Sun, but is much larger, and the magnetic field at Algol is about 1,000 times more powerful. The size of the coronal loop is larger than predicted by stellar models, and the suggestion is that this is probably due to the tidal effects of the companion star distorting the loop and stretching it.
The results, the first time a coronal loop has been imaged on another star, were published in the journal Nature on February 14th.
Peterson, W., Mutel, R., Güdel, M., & Goss, W. (2010). A large coronal loop in the Algol system Nature, 463 (7278), 207-209 DOI: 10.1038/nature08643