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Download ring - The Evergreen State College
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Ringing Magnetic Stars A colloquium at the University College of the Cariboo, Kamloops, BC Many stars - including the sun - have magnetic fields, and many - including the sun - ring with sound waves. Why do stars ring? What, if anything, do magnetic fields have to do with the ringing? What light does this shed on the lives of stars and life on Earth? Dr. E.J. Zita, The Evergreen State College, Olympia, WA http://192.211.16.13/individuals/zita/home.htm Outline: • Which stars ring? • How can we tell? • What do sound waves look like in stars? • Why do stars ring? • How do magnetic fields affect ringing? • Who is studying these effects? Which stars ring? The Sun “5-minute oscillations” Peculiar A stars “mHz p-modes” Diagrams courtesy of NASA and SIMBAD (DO Eri), sounds from Stanford How can we tell they ring? Doppler shifts on the surface of the Sun ... reveal expanding and contracting gas. Diagrams courtesy of JCD and D.W.Kurtz, 1990 Luminosity fluctuations in peculiar A stars ... reveal changes in shape of slowly rotating stars. What do sound waves look like? Short-wavelength p-modes checker the Sun’s surface and interior Movies courtesy of Hathaway and Kawaler (WET) Long-wavelength p-modes distort the surface of peculiar A stars Why do stars ring? ? Convection compresses the photosphere and rings the Sun’s surface Diagram courtesy of JCD Peculiar A stars ring similarly, but not due to convection! Magnetosonic waves in roAp stars B(-) vr E B(+) The twisted magnetic field B oscillates and creates a perpendicular electric field E... … an ExB drift compresses the plasma driving sound waves vr. How do magnetic fields affect ringing stars? We have seen that magnetic fields may DRIVE ringing in peculiar A stars. But in the Sun ... When sound waves bounce into magnetic sunspots... Diagram courtesy of JCD … energy is lost to Alfven waves: magnetic fields can DESTROY sound waves. Sunspots and the solar dynamo Differential rotation twists magnetic field lines ... Diagrams courtesy of NASA … and magnetic buoyancy lifts kinks to the surface Helioseismology gives insight into the Sun’s structure and evolution Solar core rotates slower than equatorial surface. Understanding flows helps test dynamo models. Diagrams courtesy of JCD and HAO Solar dynamo cycle magnetic activity increased radiation to Earth. Summary Helioseismology of p-modes reveals inner details of Sun Ringing p-modes reveal shape of stars surfaces. Magnetic fields can absorb ringing in the Sun Magnetic fields may drive ringing in peculiar A stars Diagram courtesy of HAO and WET Who studies helioseismology & stellar seismology? Global Oscillation Network Group http://helios.tuc.noao.edu/ Bogdan & Rosenthal, 94+ D.W. Kurtz, 1985 + E.J. Zita 97, 98, 00, 01 Dziembowski & Goode 96 Gough & Cunha, 2000 Shibahashi et al, 1985 + JCD=http://www.obs.aau.dk/helio_outreach/ http://wet.iitap.iastate.edu/ http://www.astro.ubc.ca/MOST/
