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King of Spin

posted Jul 26, 2016, 4:11 AM by Jeremy Drake
In 1947, legendary Mount Wilson Observatory stellar spectroscopist Paul Merrill noticed some peculiar features in photographic spectrograms of an otherwise fairly normal looking yellow giant, HD 117555.  Spectral lines appeared very broad compared with other giants, and there was a strong emission line of hydrogen - Halpha - that is not usually present in giant star spectra.  Merrill deduced that the lines were broad because of the Doppler effect across the surface of the spinning star - and that the giant must be an extremely rapid rotator.  The equatorial rotation speed is in fact about 185 km/s, and the rotation period was subsequently discovered from the light modulation caused by large star spots rotating in and out of view.  It is 2.4 days - more than ten times shorter than the Sun's.  The hydrogen emission line is caused by surface magnetic activity driven to a frenzy by the fast rotation.  

Later given the variable star name FK Comae, HD 117555 is thought to be rotating so fast because it is the  result of a "binary merger" - a rare object formed from two stars orbiting so close to each other that they eventually merge into a single rapidly spinning star.  Latter day legendary University of Colorado astrophysicist, Tom Ayres, is leading a project to use FK Com as a way of studying the processes of stellar magnetic activity at their extremes.  The "COordinated Campaign of Observations and Analysis, Photosphere to Upper Atmosphere, of a Fast-rotating Star"  - COCOA-PUFS for short, of course - is using the Hubble Space Telescope (HST) and Chandra X-ray Observatory to study UV and X-ray emission, together with supporting photometry and spectropolarimetry from the ground to monitor the visible light signatures of star spots and other fingerprints of magnetic activity.  The figure shows a series of HST spectra obtained at different rotational phases of two lines of ionized magnesium that are formed in the stellar chromosphere.  The lines are extremely broad, but sliced open by narrow absorption features at the line centres. The initial data analysis, published in the 2016 March edition of the Astrophysical Journal Supplement Series, has revealed a highly extended, dynamic, 10 million degree K coronal magnetosphere around the star, threaded by cooler structures that might be similar to solar prominences.