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Starspots on ultra-fast rotating stars

posted Apr 27, 2013, 7:16 AM by Jeremy Drake   [ updated Jun 3, 2013, 11:58 AM ]
A few million years after stars like the Sun are born and have passed through the initial, still puffed-up, "T Tauri" stage, they contract onto the "main sequence" of stellar evolution during which they are powered by steady hydrogen burning.  As they contract, they spin-up from rotation periods of several days to as little as a day or less. HD199143 is just such a spun-up star.  

It is a member of the Beta Pictoris moving group - a collection stars at a distance of about 36pc formed about 12 Million years ago from the same molecular gas cloud and consequently all moving in the same general direction compared with the Galactic flow. It is an F7 dwarf, about 50% more massive than the Sun. 

Rapid rotation also generates strong magnetic fields by means of a dynamo that operates in the stellar interior.  In the Sun, we see the consequences of such a dynamo in magnetic concentrations at the solar surface that appear as sunspots, and in the ultraviolet and X-ray emission from the dissipation of this magnetic energy in the solar corona.  Stars like HD199143 are also expected to have "starpots", although spots in rapidly rotating stars tend to be much larger than those on the Sun and can appear at higher latitudes.  

We have mapped the spots on HD199143, and on another rapidly rotating companion in the Beta Pic moving group, a K8 dwarf CD -68 1208,  through photometric observations obtained at the South African Astronomical Observatory, and at the Australian Siding Springs Observatory. The image here shows how the light from the star is modulated by the presence of what we infer to be a large spot running from low to mid-latitudes.  The modulation is only at a level of 2%, but is enough to be able to map out the location and shape of the spot.  We can also measure the rotation rate of the star and find the rotation period of HD199243 is only 0.36 days - seventy times shorter than for the Sun. This ultra-fast rotation makes HD199143 one of the most oblate late-type stars known. CD -68 1208 is similarly fast, with a period of 0.35 days.  From a repeat series of observations twenty days later than the first, we find that the spot area on HD199243 changed by about 7%, likely as a result of the continuous emergence of magnetic field at the stellar surface.  This work was lead by ex-SAO postdoc Dr. David Garcia-Alvarez, now at the Instituto de Astrofisica de Canarias, in Tenerife, Spain, and was published in Astronomy & Astrophysics, volume 533 in 2011