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Angular momentum evolution of low-mass stars

Angular momentum evolution of low-mass stars. The critical role of the magnetic field. Jérôme Bouvier. Stellar rotation : a window into fundamental physical processes . Star formation : initial angular momentum distribution (collapse, fragmentation)

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Angular momentum evolution of low-mass stars

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  1. Angularmomentumevolution of low-mass stars The critical role of the magnetic field JérômeBouvier

  2. Stellar rotation : a window into fundamental physical processes • Star formation : initial angularmomentum distribution (collapse, fragmentation) • Star-disk interactionduring the PMS • Rotationalbraking by magnetizedwinds • AM transfer in stellarinteriors • Binary system evolution, stellar dynamos and magneticactivity, chemicalmixing, etc.

  3. 3 major physical processes The evolution of surface rotation from the PMS to the late-MS (1 Myr – 10 Gyr) is dictated by : • Star-disk interaction (early PMS) : magnetospheric accretion/ejection • Wind braking (late PMS, MS) : magnetized stellar winds • Core-envelope decoupling (late PMS, MS) : internal magnetic fields ?

  4. Camenzind 1990 Magnetic star-disk interaction • Young, low-mass stars rotate at 10% of the break-up velocity • How to get stellar spin down from the star-disk interaction ? • Accretion-driven winds (Matt & Pudritz) • Propeller regime (Romanova et al.) • Magnetospheric ejections (Zanni & Ferreira)

  5. Star-disk magnetic coupling 2D MHD simulation of disk accretion onto an aligned dipole Zanni et al. 2009 Bessolaz et al. 2008 Mstar = 0.8Mo; Rstar=2Ro Bdipole = 800 G; dMacc/dt = 10-8 Mo/yr

  6. Magnetized wind braking Once the disk has disappeared (~5 Myr), wind braking is the dominant process to counteract PMS contraction and later on for MS spin down : • Kawaler’s (1988) semi-empirical prescription • Magnetized stellar winds (Matt & Pudritz 2008) • PMS wind braking (Vidotto et al. 2010) How does (dJ/dt)wind vary in time ?

  7. Core-envelope decoupling Surface velocity measured at the top at the convective envelope while radiative core’s velocity unknown (except for the Sun) How much angular momentum is exchanged ? On what timescale ? • Turbulence, circulation (Denissenkov et al. 2010) • Magnetic coupling (Eggenberger et al. 2011) • Internal gravity waves (Talon & Charbonnel 2008) How rigidly is a star rotating ?

  8. Observational constraints Tremendous progress in the last years…

  9. Observational constraints Wichmann et al. 1998

  10. Observational constraints Irwin & Bouvier 2009 0.9-1.1 Mo

  11. Observational constraints Today’s update… 0.9-1.1 Mo Gallet & Bouvier, in prep.

  12. Irwin et al. (2010) MS PMS

  13. Observational constraints • Several thousands of rotational periods now available for solar-type and low-mass stars from ~1 Myr to a ~10Gyr (0.2-1.2 Msun) • Kepler still expected to yield many more rotational periods for field stars • Several tens of vsini measurements available for VLM stars and brown dwarfs

  14. Models vs. observations What have we learnt so far ?

  15. AM evolution : model assumptions • AccretingPMS stars are braked by magneticstar-disk interaction (~fixedangularvelocity) • Non-accreting PMS stars are free to spin up as theycontracttowards the ZAMS • Low mass main sequence stars are braked by magnetizedwinds (saturated dynamo) • Radiative core / convective envelope exchange AM on a timescaleτc(core-envelopedecoupling)

  16. ZAMS PMS MS Wind braking PMS spin up Disk locking Grids of rotationalevolutionmodels Surface rotationis dictated by the initial velocity + disk lifetime + magnetic winds (+ core-envelope decoupling)

  17. Radiative core Convective envelope Core-envelope decoupling (τc) τc: core-envelope coupling timescale Differential rotation between the inner radiative core and the outer convective

  18. Angular momentum loss: I. Solar-type winds • Most modellers use the Kawaler (1988) formulation with n = 3/2 to reproduce the Skumanich (1972) t-1/2 law • Introduce saturation for ω > ωsat to allow for “ultra-fast rotators” on the ZAMS • Weak, starts to dominate only at the end of PMS contraction

  19. Wind braking • Modified Kawaler’s prescription Suitable for solar-type stars But fails for VLM stars 1 Mo 0.25 Mo Irwin et al. (2010) Irwin & Bouvier (2009)

  20. Wind braking • Matt & Pudritz’s (2008) prescription • Calibrated onto numerical simulations of stellar winds Mass-loss : Cranmer & Saar 2011 Dynamo : Reiners et al. 2009

  21. Wind braking 1Mo M&P08 braking Gallet & Bouvier, in prep.

  22. Core-envelope decoupling • Models with a constant coupling timescale between the core and the envelope cannot reproduce the observations τc=106yr for fast rot τc=108yr for slow rot Bouvier 2008

  23. Core-envelope decoupling • Models with a constant coupling timescale between the core and the envelope cannot reproduce the observations • Need for a rotation-dependent core-envelope coupling timescale : weak coupling in slow rotators, strong coupling in fast rotators • Still need to identify the physical origin of this rotation-dependent coupling (hydro ? B ? waves ?)

  24. Long et al. 2007

  25. Star-disk interaction On-going work… • C. Zanni’smagnetospheric ejection model Numerical simulations

  26. Star-disk interaction Gallet, Zanni & Bouvier, in prep.

  27. Star-disk interaction • Strong observational evidence that accreting stars are prevented from spinning up in the first few Myr • Still no fully consistent PMS stellar spin down from star disk interaction models (e.g. Matt et al. 2010) • Angular momentum has to be removed from the star, and not only from the disk

  28. How to further constrain the angular momentum evolution models ? Investigate magnetic field evolution

  29. “The magnetic Sun in time” (on-going project, TBL/NARVAL, CFHT/ESPADONS) • Investigate the magnetic field topology of young stars in open clusters in the age range from 30 to 600 Myr • Expectations : the topology of the surface magnetic field depends on the shear at the tachocline • Goal : use surface magnetic field as a proxy for internal rotation and test the model predictions (e.g., core-envelope decoupling)

  30. “The magnetic Sun in time”(J. Bouvier, F. Gallet, P. Petit, J.-F. Donati, A. Morgenthaler, E. Moraux) • Targets : G-K stars in young open clusters • Clusters : • Coma Ber (600 Myr) • Pleiades (120 Myr) • Alpha Per (80 Myr) • IC 4665 (30 Myr) • Preliminary results (2009-2011), on-going analysis

  31. “The magnetic Sun in time” Young open clusters Marsden et al. Donati et al. Petit, Morin, et al.

  32. Conclusion • Still need to identify the physical process(es) by which internal angular momentum is transported (core-envelope coupling) • Still need to understand the origin of the long-lived dispersion of rotation rates in VLM stars (dynamos bifurcation?) • Still awaiting a fully consistent physical description of PMS stellar spin down from the star-disk interaction : (dJ/dt)net < 0 ! • Still lacking constraints on the internal rotation profile (e.g. tachocline) and its evolution

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