1. Stellar Evolution and the Habitable Zone
Marc Pinsonneault

2. Outline
Stellar properties are crucial for establishing the formation and evolution of planets
Main ingredients for evolution are explained by well-understood physics
Phenomena neglected in standard models (rotation and magnetism) significantly impact evolution and are tied to formation
Implications for detection and habitability

3. Evolutionary Stages Assembly Phase (t ~ 0.1 Myr)
Star acquires its mass rapidly
L governed by D burning of accreted matter
Disk Accretion Phase (t ~ Myr)
Star-Disk interactions set stellar rotation
Planet assembly phase
Pre-Main Sequence (t 30 Myr)
Star contracts and heats up until nuclear fuel ignited
Main Sequence (t 10 Gyr)
Core H burning
Gradual brightening with age
Stars spin down from magnetized winds

4. Assembly Phase
Spitzer C2D: Stars assemble at a wider range of rates than previously believed (Evans et al. 2009)

5. Pre-Main Sequence Steep drop in IR excess fraction by 6 Myr
Rotation is determined by star-disk interactions
Subsequent contraction, spin-up, heating, and ignition of nuclear reactions
tKH ~ GM2/RL; rises as mass drops (30 Myr for Sun)
Haish, Lada & Lada 2001

6. Main Sequence
Strong mass-luminosity relationship predicted by theory and observed
Hydrostatic balance => Pc
High Pc => high T for ionized ideal gas
High Tc => nuclear energy generation
Thermal balance => loss at surface balances generation

7. Why the young Sun was faint
Hydrostatic balance sets Pc
Pc set by Ideal Gas Law
N drops as you convert 4 p+4 e- => He4+2 e-
=> Tc + r rise;
=> L rises
Proof? Helioseismology!

8. Frank et al. 1999
Habitable Zone
Range of locations where liquid water can exist; depends on stellar mass and age
Guided by basic stellar properties
Sensitive to treatment of the atmosphere! (Kasting et al. 1993; Frank et al. 1999)

9. The Special Case of M Stars
Low mass stars (0.08 – 0.5 Msun) are common, faint, and long-lived
Minimal nuclear evolution
HZ is very close; tidal synchronization likely
Recent interest in M stars as hosts (Tarter et al. 2007)
Rotation and magnetism are crucial for these stars….

10. Eclipsing binary stars are too large!
Differences are at the 10% level
Mass dependence
Difficult to explain within standard models
Ribas 2006
Morales et al. 2008
Verdict: Activity!
Eclipsing Binaries are close, tidally synchronized, and heavily spotted
Spots puff up stars (Andronov & Pinsonneault 2004)

11. Supporting Evidence… Lithium is destroyed during the pre-MS
Large dispersion in the Pleiades (Soderblom et al. 1993)
Evidence for a range in Tc (and R) for stars of the same mass during the pre-MS
Soderblom et al. 1993

12. Teff reversal in a pre-MS EB (Stassun et al. 2008)
Gomez Macao
Chew et al. 2009
(astro-ph last
night)

13. Consequences! Impacts estimates of protostellar mass and age
Diversity of timescales and evolutionary properties at fixed mass
Magnetism is not simply a perturbation…
Although sunspots are shallow!

14. Main Sequence Spindown
Stars lose angular momentum to magnetized winds
Loss ~ w3 to threshold, then saturates
Threshold depends on mass
Andronov &
Pinsonneault 2003

15. Rotation matters! High rotation => strong magnetic fields, spots
Magnetism drives chromospheric and coronal heating
Lx/Lbol => 10-3 for active stars (1000x Sun);
Strong correlation with far-UV flux as well
CMEs

16. Cool Stars are Heavily Spotted

17. Irwin & Bouvier 2009

18. Rotation in a star cluster: M35
Rapid rotation survives for longer in lower mass stars (right)
Range of rotation rates even among coeval stars
Hartmann et al. 2009

19. Amplitude of variability can be high
Up to 20% brightness fluctuations from spots
Young system (550 Myr), but these effects persist for a long time in low mass stars
Hartmann et al. 2009

20. SDSS Data Sets Spindown Timescale
West et al. 2008
M dwarf activity
Activity vs. galactic latitude in a huge sample from the Sloan survey mapped onto activity lifetime as a function of age

22. Variability is Robust and Common
Aigrain et al. 2009: Variability in the CoRoT fields

23. Final Thoughts Diversity of Pathways from the Assembly History
Rotation Determined by Disk Interactions and Assembly
Selection Effects in Samples (esp. Doppler)
Activity and Rotation Backgrounds
Rotation as a Clock…