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Transcript
Asteroseismology and the Solar-Stellar Connection Travis Metcalfe (NCAR) Collaborators: W. A. Dziembowski, P. G. Judge, M. Snow Seeing with sound • Sun-like stars have their own internal source of “ultrasound” waves • Turbulent convection creates acoustic noise in a broad frequency range • Some of the frequencies are resonant inside the star’s spherical cavity Helioseismology Larger Surface spatial scale Smaller Shallower Interior sampled Deeper http://sohowww.nascom.nasa.gov/ • Millions of independent oscillation frequencies excited simultaneously • Each mode samples the interior in a different and complementary way • Only the lowest degree modes are detectable in distant stars (l < 3) Asteroseismology • Oscillations decomposed into spherical harmonics, small features cancel out • Low-degree modes probe deepest into the interior, several dozen detectable • Such data will allow lowresolution inversions of the inner 30% of radius Gough & Kosovichev (1993) Observing techniques Bouchy et al. (2004) Velocity variation (ground) Fletcher et al. (2006) Light variation (space) Ground-based: a Cen A+B Butler et al. (2004) a Cen A Frohlich et al. (1997) Sun Kjeldsen et al. (2005) a Cen B • Nearest stellar system, masses slightly above and below solar mass • The range of excited frequencies scales inversely with radius • Amplitudes and mode lifetimes mostly agree with our expectations MOST: differential rotation • Three seasons of precise photometry for the young solar-type star k1 Ceti • Detailed spot modeling from 30, 20, and 15 days of uninterrupted data Ca HK period Walker et al. (2007) • Latitudinal differential rotation pattern has same functional form as Sun CoRoT: CZ depth • Expected seismic signal from 5-month observation of the star HD 49933 • Second differences (d2n) measure deviations from even frequency spacing • Base of the convection zone and He ionization create oscillatory signals Baglin et al. (2006) Kepler: 105 solar-type stars • 105 square degrees for 4-6 years, slightly above galactic plane in Cygnus • Intensity measurements and rotation profiles for 100,000 solar-type stars • Stellar radii and ages down to 12-14 magnitude from 1-year seismic data Christensen-Dalsgaard et al. (2007) Solar activity • Active regions on the Sun are bright in Mg II (UV) and Ca II (optical) • Measure ratio of total emission in line cores to flux in the wings • Use disk-integrated time series measurements to track magnetic cycles http://spacescience.spaceref.com/ Stellar activity cycles Frohlich & Lean (2004) • Sun-as-a-star data show 10% variation in the Mg II index through solar cycle • Similar magnetic activity cycles can be observed in other solar-type stars Dravins et al. (1993) • Mean activity level and cycle period scale with Rossby number (Prot / tc ) Solar p-mode shifts Salabert et al. (2004) • Solar p-mode shifts first detected in 1990, depend on frequency and degree • Even the lowest degree solar p-modes are shifted by the solar cycle • Unique constraints on the mechanism could come from solar-type stars Libbrecht & Woodard (1990) Theoretical interpretation • Magnetic perturbations modify the near-surface propagation speed Goldreich et al. (1991) Dziembowski & Goode (2005) • Also leads to decreased convective velocity and change in temperature • Distinct behavior for solar f-modes and p-modes confirms these sources Observations: b Hydri Dravins et al. (1993) • Only star with a known activity cycle and multiepoch asteroseismic data • Widely studied as “future Sun” (age ~ 7 Gyr) with a relatively low activity level • Reanalysis of archival IUE data, including more recent observations Metcalfe et al. (2007) Scaling from the Sun • Parameterize shifts with Dn ~ A0 (R / M) Qj(Dc) and fit the MDI p-mode data • A0 ~ activity level, while the depth of the source Dc ~ Hp ~ L1/4 R3/2 / M • A0 captures most of the variation if depth fixed at Dc = 0.3 Mm for the Sun Metcalfe et al. (2007) Frequency dependence • Solar p-mode shifts show spread with degree and frequency dependence • Normalizing with our expression removes most of both dependencies • Rise at low frequencies due to fixed Dc but largest shifts at higher frequency Metcalfe et al. (2007) Results: b Hydri Bedding et al. (2007) MODEL OBSERVATIONS • Asteroseismic data from 2000 (just past maximum) and 2005 (near minimum) • Cross-correlation of data sets yields a systematic shift of 0.1 + 0.4 mHz • Single radial mode (n=18) in both data sets shifted by 0.17 + 0.62 mHz Outlook • Space-based asteroseismology will extend the calibration of stellar structure/evolution models to many new sets of physical conditions and ages. • Measurements of latitudinal differential rotation and convection zone depths will provide new constraints for solar/stellar dynamo models. • Though we will observe each star in less detail, we will have hundreds of stars to provide a broad context for our understanding of the Sun.