The total solar eclipse in June this year provoked a 3,000-person rave in Africa and for solar scientists it provided a chance to observe the Sun's usually invisible corona. The million-degree X-ray-emitting envelope is outshone by the cooler, brighter solar surface until the Moon slides in front of the yellow disc allowing the coronal ring to appear around the lunar edge.
Luckily for solar scientists, coronagraphs make it possible to replicate an eclipse in the laboratory. But the corona of other low-mass stars have only been observed by X-ray telescopes, until now. In an article published in the latest issue of the journal Nature, two scientists from Universitat Hamburg report observing the corona of the star CN Leonis in the visible spectra.
"This is a very fundamental feature of low-mass stars, whose physics is not very well understood so far," said Rainer Wichmann, one of the Hamburg researchers. "The study of stellar coronae is important for the understanding of phenomenon such as stellar activity that in the case of our own Sun can have a significant impact on Earth."
Detecting the corona of other stars is more problematic than detecting the corona of the Sun. The small size of even the closest star makes it impossible to block the photosphere with an eclipse or a coronagraph. This means stellar coronal emissions have to be detected against the full photospheric backgrounds. But Wichmann and his colleague Juergen Schmitt took advantage of two things to do just that.
The benefits of red
The first trick used by the scientists was to rely on the fact that red dwarf stars exhibit unique coronal emission wavelengths. The million-degree temperatures of the corona ionizes iron atoms many times more than the mere 3,000-degree surface of the star. The line emissions from the photosphere and the corona are therefore different, making it possible to separate coronal emissions data from the photospheric emissions.
"CN Leo is a very red star, which is helpful, because most of the photospheric light is emitted in the red part of the spectrum, while in the blue, where the coronal Iron-13 line is located, the photospheric luminosity is very low," Wichmann said.
For their second trick, the Hamburg scientists detected emissions of CN Leonis with the ultra sensitive Ultraviolet-Visual Echelle Spectrograph (UVES) mounted on the European Southern Observatory's Kueyen unit of the Very Large Telescope at the Paranal Observatory in Chile. The spectral resolution of the UVES spectrograph, said Wichmann, is more than a magnitude stronger than that of the next generation of X-ray spectrographs. UVES' resolution, available to the scientific community for just a little over a year now, is able to detect stellar coronal emissions from the ground for the first time.
This is quite an advance since coronal observations in the past had to be made above our X-ray-filtering atmosphere with telescopes on board the Einstein and Rosat satellites, both of which now are defunct.
"Space observations are much more expensive than ground-based ones," Wichmann said. "Telescope time on space observatories is limited more severely, which makes programs that required large amounts of observing time quite difficult."
Soon, measurements will be made to record changes in stellar coronae over time. These planned studies will help the stellar scientists determine coronal structures, Wichmann said, and will further the understanding of the cyclic stellar and solar weather.
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