Transcript Assembly Phase

Stellar Evolution and the
Habitable Zone
Marc Pinsonneault
Outline
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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
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Assembly Phase (t ~ 0.1 Myr)
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Disk Accretion Phase (t ~ 0.1-6 Myr)
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Star-Disk interactions set stellar rotation
Planet assembly phase
Pre-Main Sequence (t 30 Myr)
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Star acquires its mass rapidly
L governed by D burning of accreted matter
Star contracts and heats up until nuclear fuel ignited
Main Sequence (t 10 Gyr)
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Core H burning
Gradual brightening with age
Stars spin down from magnetized winds
Assembly Phase
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Spitzer C2D: Stars assemble at a wider range of rates
than previously believed (Evans et al. 2009)
Pre-Main Sequence
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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
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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
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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
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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
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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!
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Ribas 2006
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Differences are at the 10%
level
Mass dependence
Difficult to explain within
standard models
Morales et al. 2008
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Verdict: Activity!
Eclipsing Binaries are close,
tidally synchronized, and heavily
spotted
Spots puff up stars (Andronov &
Pinsonneault 2004)
Supporting Evidence…
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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!
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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
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Stars lose angular
momentum to
magnetized winds
Loss ~ w3 to
threshold, then
saturates
Threshold depends
on mass
Andronov &
Pinsonneault 2003
Rotation matters!
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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
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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
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Hartmann et al. 2009
Up to 20%
brightness
fluctuations
from spots
Young system
(550 Myr), but
these effects
persist for a
long time in
low mass stars
SDSS Data Sets Spindown Timescale
West et al. 2008
M dwarf activity
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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
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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…