Stellar Evolution and the Habitable Zone Marc Pinsonneault

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

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 ~ 0.1-6 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

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

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

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

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!

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)

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….

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)

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

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

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!

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

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

Cool Stars are Heavily Spotted

Irwin & Bouvier 2009

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

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

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

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

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…