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Hearing the Stars! Lars Bildsten Kavli Institute for Theoretical Physics University of California Santa Barbara Happy Birthday David!! KITP Residence: Opens January 2017 Though we know how stars evolve, we are only just beginning to probe rotation, interior states, hydrodynamics and magnetism in a meaningful way. Most observational progress is from the asteroseismic data from the Kepler and CoRoT satellites, while theoretical progress is driven by people and new computational tools. Matteo Cantiello (KITP), Joergen ChristensenDalsgaard (Aarhus Univ.), Jim Fuller (Caltech/KITP), Phil Macias (UCSB=>UCSC), Chris Mankovich (UCSB=>UCSC), Kevin Moore (UCSB=>Claremont), Bill Paxton (KITP), Dennis Stello (U. Sydney) & Rich Townsend (U. Wisconsin) After the Main Sequence: Red Giant Branch and Clump Stars Clump stars Paxton et al. ‘11 • M< 2 M develop degenerate Helium cores that increase in mass with time until ignition in a flash => lifting degeneracy => stable He burning in core (Thomas 1967). Space-Based Photometry CoRoT 27 cm diameter Launched December 2006 Kepler 95 cm diameter Launched March 2009 Non-Radial Stellar Oscillations • P-modes (acoustic waves): In the high wavenumber limit=> Evenly spaced in Frequency (in Envelope!) • G-modes (gravity waves): In the high wavenumber limit=> Evenly spaced in Period (in Core!) Only Acoustic Waves seen in the Sun Christensen-Dalsgaard Acoustic Waves (p-modes) in Giants • Persistent convection in the outer parts of the giant excites standing acoustic waves (i.e. modes with n radial nodes for each l) =>Measures mean density • The pulsation amplitudes were estimated (e.g. Christensen-Dalsgaard; Kjeldsen & Bedding ‘05) based on earlier solar work (c.f. Goldreich & Keeley ’77), but ground based tests were a challenge. . . as amplitudes were low. . . CoRoT finds p-modes ! Kepler Observations Bedding et al. ‘10 (Kepler) Kepler Bedding et al. ‘10 (Kepler) • Large frequency spacing is well measured and collapsing these allows for identification of l=0, 1, 2 and often l=3 acoustic modes • These give mean density measurements straight away! • n~10-15. . . WKB nearly valid Highest Observed Frequency Huber et al. 2011 • Highest observed frequency is at the acoustic cutoff of the photosphere. • Higher frequency waves have large damping due to wave escape • Combined with frequency spacing, M and R inferred! • However, internal state not probed. . . Revealing the Population (CoRoT) Miglio et al. 2013 Highlights so Far • Acoustic waves seen in nearly all evolved stars with amplitudes of 3-200 parts per million. • Measured frequency spacing and maximum observed frequency give R, M and D for >10,000 stars across the galaxy. Great test for GAIA and new galactic science enabled. • Useful diagnostics for extra-solar planets. Where tested, distance estimates are ~10% accurate (Silva Aguirre+ ’13) Degenerate Core => Burning Core Bildsten et al. ‘12 • Time spent on the Red Giant Branch (RGB) at L>30L is comparable to that spent on the Red Clump. • Hard to distinguish a clump star from an RGB star in the field, but let’s see what seismology can do . . • More modes are present than allowed for by acoustic waves! • Most prevalent plethora is near the l=1 modes, which scatter away from simple expectations. Bedding et al. ‘10 (Kepler) Inferring the State of the Stellar Core Propagation Diagrams and Mixed Modes • Scuflaire ’74; Osaki ’75 and Aizenman et al. ’77 noted that the acoustic waves couple to the non-radial g-modes, which are uniformly spaced in period at: • Coupling is strongest for l=1, and many g-modes exist between each successive acoustic mode Burning vs. Degenerate Cores RGB Clump Internal Gravity Waves in the Stellar Core then Detected The g-mode spectrum is very dense in the core, but the modes couple to the envelope well enough to emerge and be detected as oscillations evenly spaced in period. Very stable and long-lived CLOCK! A=p-dominated mode (np,ng=8, 476) B=g-dominated mode (np,ng=7, 505) Star near the RGB bump. Bedding et al. 2011 Luminosity Mosser et al. 2011 • Distinction of stars on the Red Giant branch from those doing He burning in the core (clump stars)!! • Discovered now in nearly all RGB stars (Stello et al. 2013) Solar Model “Trivially” Evolved Courtesy C. Mankovich See also Tayar & Pinsonneault ‘13 • Rotation rate in the core can impact the later evolution of the star, especially for massive stars • Certainly sets rotation for the He burning core and eventually the white dwarf RGB Power Spectrum: Rotation! G G Beck et al. ‘12 Stello et al. 2013 P G Inferred Core Rotation Mosser et al. 2012 Core loses 95% of its Angular Momentum after Leaving MS Calculations with Magnetic Dynamos Cantiello et al. ‘14, Still not enough angular momentum loss from the shrinking core! Conclusions • New astronomical tools are revealing the interiors of stars in ways previously impossible. • Rotation and Magnetism can now be explored, an important player in how stars finally collapse. • Theory remains key in at least three ways: – Reliable physical modeling (MESA+GYRE) needed to fully interpret the data – Novel analytics and mechanisms (Cantiello et al. 2014; Fuller et al. 2014) to transport J – Recent theoretical (Fuller et al. 2015) work showed how dipole suppression arises from strong B in core (Stello et al. ‘16, Cantiello, Fuller, LB’16) We now have a New way to Hear! The width of 10 nuclei over the earth’s diameter! The width of one atom between earth and sun!