Jérôme Bouvier (IPAG Grenoble, France)!
Sean Matt ! (U. Exeter, UK)!
Subu Mohanty (Imp. Col. London, UK)!
Aleks Scholz ! (U. St. Andrews, UK)!
Keivan Stassun ! (Vanderbilt U., USA)!
Claudio Zanni (Oss. Astro. di Torino, Italy)!
Protostars & Planets VI, Heidelberg!July 17, 2013!
Angular Momentum Evolution of Young Low-Mass Stars & Brown Dwarfs: Observations & Theory!
Stellar Rotation!
Importance of Stellar Rotation!
- Fundamental property: influences structure, mixing, evolution!
- Evolution of light elements (Be, Li)!
- Magnetic activity (UV, X-ray, spots, flares, mass loss)!
- Interaction between star and disk/planets!
- Studying rotational evolution complements knowledge:!!- stellar ages (gyrochronology)!!- history/future of activity!!- probes accretion/formation history, interior processes,
!mass loss rates!
Rotation & convection produces magnetic activity"magnetic fields, winds, spots, high energy radiation, …!
Rotation-activity relationship!
Log Rossby # = P / #conv!
Log
L x
/ L b
ol !
Wright et al. (2011)!
Non-saturated!
Saturated!
Magnetic properties depend on mass, rotation, & age.!
Magnetic field measurements (e.g., Johns-Krull 2007; Donati & Landstreet 2009; Gregory et al. 2010; Morin et al. 2011)!
Gregory et al. 2008!
V2129 Oph!
BP Tau!
Outline!
1. Observations of Stellar Rotation!
2. Physical Torque Mechanisms !
3. Models of Spin Evolution!
Observations of Stellar Rotation!
> 5000 new photometric periods since 2007 (in ~30 publications)!
Observations of Stellar Rotation!Solar Mass (0.9 – 1.1 Msun)!
Bouvier (2008)!
Then!
Gallet & Bouvier (2013)!
Now!
- Even sampling from 1 Myr ! 1 Gyr!
- Approaching statistically robust samples!
Age (Myr)!
Observations of Stellar Rotation!
Gallet & Bouvier (2013)!
Solar Mass (0.9 – 1.1 Msun)!
Power law spin down !(e.g., Skumanich 1972): !* " t-1/2 !Implies, torque " !*
3!Mag. activity: Non-saturated!
Pre-main-sequence:!- “Initial” distribution?!- distribution broadens to ZAMS!
Exponential spin down: !!* " exp(-t/#sd)!#sd ~ 100 Myr!Implies, torque " !*!Mag. activity: Saturated!
- Similar behavior down to the convective limit (~0.3 Msun)!
Observations of Stellar Rotation!Very Low Mass (<0.3 Msun)!
Gallet & Bouvier (2013)!
Solar mass!
Irwin et al. (2011)!
- “Initial” distribution faster, & all stars spin-up toward ZAMS!- All stars on exponential spin-down, #sd ~ Gyrs!- Bifurcation? (Irwin et al. 2011; Morin et al. 2011; Gastine et al. 2013)!
Very low mass!
Irwin et al. (2011)!
~200 rotation periods!most in PMS!
- Mean ~2 days at PMS!(e.g., Cody & Hillenbrand 2010)!
- All rapid rotators at 1 Gyr!(e.g., Heinze et al. ’13; Girardin et al. ‘13; ! Gillon et al. ‘13)!
Not much angular momentum loss, #sd > 5 Gyr?!
Observations of Sub-Stellar Rotation!Brown Dwarfs (<0.08 Msun)!
Brown Dwarfs!
Outline!
1. Observations of Stellar Rotation!
2. Physical Torque Mechanisms !
3. Models of Spin Evolution!
Stellar Winds (non-accreting phase)!
Matt & Pudritz (2008), et al. (2012)!
New formula from simulations:!
Most models to date (e.g., Kawaler 1988) !!dJ/dt = Tw " B*2 $*!
Tw " Mw0.56 B*
0.87 $*!.!
Use “dynamo relationship:” B* R*2 " $*
a !Saturated: a = 0 ! exponential spin-down!Unsaturated: a = 1 ! Skumanich spin-down!
Stellar Winds, Key Developments!
!- Physical torque relates spin, B*, and mass loss rate (Matt et al. ‘08, ‘12)!
!- Mass loss rate determinations:!
! !- measurements (Wood et al. 2002, 2005)!
! !- steady wind theory (Cranmer & Saar 2011; Suzuki et al. 2013)!! !- CME Mdot inferred from flares (Aarnio et al. ‘12; Drake et al. ’12)!
!- Non-dipole B* models (Matt et al. ‘08, Cohen et al. ‘09, Vidotto et al. ‘10; Pinto et al. ’11)!
Accreting Phase!During PMS:!to counteract contraction &
accretion,!need ~106 % solar wind torque !
Gallet & Bouvier (2013)!
J conserved!
“Disk locking”!Can the star-disk interaction
provide these torques?!
Cieza & Baliber (2007)!
Accreting stars are (on average) slower rotators than non-accretors (maybe not for VLM stars)!
Accreting Phase!Accretion-Powered Stellar Winds!(Hartmann & Stauffer 1989; Matt & Pudritz 2005)!
Ghosh & Lamb (1978); Matt & Pudritz (2005)!
“propeller” regime?!D’Angelo & Spruit (2011); Romanova et al. (2009); Zanni & Ferreira (2013)!
Magnetic Star-Disk Connection!
Magnetospheric Ejections, Conical Winds!
Zanni & Ferreira (2013); Romanova et al. (2009)!
Disk “Viscosity” ! ! (Popham & Narayan 1991)!Gravitational Torques (Lin et al. 2011)!
Shu et al. (1994...);!Mohanty & Shu (2008)!
Obs. Support: Cauley et al. (2012)!
X-Wind!
Star-Disk Interaction, Key Developments!(still exploring mechanisms)!
!- APSW development (Meliani et al. ’06; Matt & Pudritz ‘07, ’08a,b; Cranmer ’08, ’09;
! ! ! Fendt ‘09; Zanni & Ferreira ’11; Sauty et al. ’11; Matt et al. ‘12) !
!- New mechanism, MEs (Zanni & Ferreira 2013)!
!- Effects of non-dipole fields:!
! !- X-wind (Mohanty & Shu 2008; Cauley et al. 2012; Ferreira & Casse 2013)!! !- disk truncation (Gregory et al. 2008)!! !- MHD simulations (Romanova et al. 2011; Long et al. 2007, 2008, 2009, 2011)!
!- Radiative transfer/high-res spectroscopy (Kurosawa et al., ‘06, ‘11, ‘12; !! ! ! !Bouvier et al. ‘07; Kwan et al. ’07; Fisher et al. ‘08; Alencar et al. ‘12)!
Gregory et al. ‘08!
BP Tau!
Outline!
1. Observations of Stellar Rotation!
2. Physical Torque Mechanisms !
3. Models of Spin Evolution!
Anatomy of (Most) Spin Evolution Models!Goal: get $*(t)!
(1) 1D evolution model (structure, moment of inertia)!!+ solve dJ/dt equation!
(2) External torques: !!Stellar winds: Kawaler88 + dynamo + “saturation”!!accretion-phase: not treated physically ! “disk locking”!
(3) Internal transport of angular momentum:!!solid body or 2-zone with “decoupling”!!some solve transport in radiative zone!
Spin Evolution Models, Highlights!
Gallet & Bouvier (2013)!
Solar mass stars!
- Realistic Tw (Matt et al. 2012) and mass loss rates (Cranmer & Saar 2011)!
- Slow rotators less “coupled” (#c-e ~ 30 Myr versus 12 Myr)!
Spin Evolution Models, Highlights!
Irwin et al. (2011)!
- Internal transport: circulations, instabilities, gravity waves, B-fields! (e.g., Decressin et al. 2009; Denissenkov et al. 2010; Turck-Chièze et al. 2010; Strugarek et al.! 2011; Marques et al. 2013; Charbonnel et al. 2013) (Asteroseismology: e.g., Beck et al. 2011)!
APSW; B* = 2 kG!
Matt et al. (2012)!
- Accretion phase! spin evolution ! (Matt et al. 2010, 2012)!
- Mass dependence of spin-down (Reiners & Mohanty 2012) !
Tw " M*-2/3 R*
16/3 (?) !
Reiners & Mohanty (2012)!
B* R*2 " $*
a!
Open Q’s, Near Future Prospects!- What gives rise to “initial” spin rates and distributions?!
- What is most important mechanism(s) for PMS ang. mom. loss?!
- Explanation for range of spin rates at given mass/age (formation conditions, !accretion history, coupling timescales, dynamo modes/bistability…)?!
- Will see large increase in observed magnetic field maps; need more models !considering realistic fields!
- Are magnetic fields strong enough for existing models to work!
- Improved mass loss rates (measurements & models, accreting & non-accreting !phases)!
- Improved physics in spin evolution models (accretion phase, internal transport, !better torque models…)!
- More spin rate measurements, especially VLM field stars, asteroseismology!
- Improved/cleaner sampling in PMS & better ages/masses/acc. rates, etc.!
“And in the end, it’s only round and round and round...” - Pink Floyd, DSOTM !
