Prelim Review

1
Prelim Review
2
lt1.2 M?
3
lt1.2 M?

• Hayashi track - fully convective
• cooler surface temp. requires super-
• sonic convection. Ends with burning
• of 2H
• Star becomes radiative from core outward,
  becoming more compact hotter. Occurs on thermal
  timescale
• Burn Li,Be,B at 1-2x106K - Li depletion happens
  when you ? M (? Teff) because convective envelope
  is deeper, carries material to Li burning T.
  Higher M stars with shallower convective
  envelopes have more surface Li
• Late F stars also show Li depletion due to
  coupled rotational/wave mixing at base of shallow
  envelope

4
lt1.2 M?

• 3. Bump from CN part of CNO cycle
• Convective core develops while
• C/N goes to equilibrium -
• short timescale after which L drops until PP
  takes over on main sequence
• 4. Main Sequence - PP chain dominates up to 1.15
  M?
• no convective core since T dependence of PP
  (relatively) small, ? T7
• efficiency of H burning 0.7mc2, star burns
  10 of its mass
• Stars w/ radiative cores go up in L at const
  Teff

5
lt1.2 M?

• Leaving main sequence - transition
• to shell H burning smooth because H
• still present at small ?r from center -
• inert core becomes degenerate
• As shell burns it gets thinner as ?T,??
• get steeper - narrow shell has to support star
  against gravity of inert core - high L in shell
  which goes into mechanical work expanding star -
  as core contracts, envelope expands, moving star
  to red at const L
• Shell burning lasts 4 Gyr for sun before RGB

6
lt1.2 M?

• Red giant branch - star has moved
• as red as it can go - L of star now increases
• as convective envelope moves inward all
• the way to shell - R also increases
• He core is degenerate, L? Mcore. First dredge-up
  mixes processed material to surface
• 7. Tip of the RGB - Core reaches 0.45 M? and T
  reaches He ignition (T2e8) - Pdegeneracy not
  dependent on T, so no feedback like HSE -
  explosive burning He flash. Star moves back
  down RGB as L goes into expanding core. Since L
  depends on core mass, and all stars must reach
  0.45 M?, tip of RGB at fixed luminosity (for
  given z) so acts as standard-ish candle.

7
lt1.2 M?

• 8. Core He burning - 2?(?,?)12C until YHe
• low, then 12C(?,?)16O takes over
• red clump coincides approximately
• with transition to 12C(?,?)16O. C/O
• decreases with stellar mass.
• 9. Asymptotic Giant Branch - He shell burning
  drives star back up parallel to but bluer than
  RGB. Star goes to much higher L. High mass loss
  rates from winds driven by line opacity,
  pulsations, dust. Second dredge-up of nuclear
  processed material as convective envelope expands

8
lt1.2 M?

• 10. Envelope lost through winds, late ejection
  possibly by flashes in degenerate shells.
  Evolution of PN central star more rapid for
  higher mass. End up w/ CO white dwarf with thin
  layers of He, H. Star very compact, high Teff,
  evolves on lines of constant radius until
  crystallization. Stars low mass enough to make He
  white dwarfs havent evolved off MS. Only
  binaries w/ mass ejection make He WDs now. Solar
  mass star should make 0.5 M? WD distribution
  peaks at 0.6.

9
1.2-8 M?

1-3. Similar to low mass stars. 4. Main Sequence
Stars above 1.15 M? dominated by CNO cycle H
burning. ??T17 so all energy deposited in very
small radius - convection necessary to transport
energy. Convective core retreats as He increases
(e-/nucleon ?), core also become more compact -
star moves to red. Note Mixing length gives
wrong answers - based on thermodynamic instead of
hydrodynamic stability. Waves and rotation also
relevant to evolution
10
1.2-8 M?

5. H exhaustion H depleted out to extent of
convective core Star has to contract before T
high enough where H remains for shell to
ignite - star moves to blue briefly 6. H shell
burning - no degeneracy in core over 2.2 M? so
star crosses in Kelvin-Helmholtz time -
Hertzsprung Gap 7a. For stars lt 2.2 M? rest of
evolution as for low mass 7b. As M ?, S?, so ? is
lower for given T - no degeneracy before
He ignition 8. As M ? blue loops get bluer, so
red clump turns into horizontal branch. Same
for ?z
11
1.2-8 M?

• 9. Thermal Pulse AGB - He burns faster than H
  because of lower Q, catches up to quenches H
  shell by ??. He shell runs out of fuel, H
  reignites burns out until enough fuel for He -
  repeat from a few times for low masses to a few
  dozen times for high masses.
• Convective envelope gets deep during He phases -
  third dredge-up (actually many). Protons mixing
  with C-rich material generate neutrons. N capture
  on heavy seeds makes S process 1/2 of material
  above Fe peak. Dredge-up gets s-process and
  C-rich material to surface - C/O gt 1 at low
  metallicity
• 10. Intermediate mass stars produce CO white
  dwarfs with C/O ltlt1. Most massive may become
  ONeMg WD.

12
gt8 M?

13
gt8 M?

1-8. Very much the same as intermediate mass
stars for masses lt 30 M? 9. Core evolution
proceeds too quickly for TPAGB to develop. C
ignition at T6e8 K. Off-center degenerate C
flash for lowest masses. Neutrino cooling
dominates over photon cooling for T gt 5e8 K.
Burning must proceed at much larger rate so small
fraction of energy in photons can provide
pressure support. Evolution proceeds more
rapidly than thermal adjustment timescale of star
- not seen at surface.
14
gt8 M?

9a. C burning T6e8, 12C ? 20Ne, some
23,24Mg,23Na Ne burning T1e9, 20Ne ?
16O,some Mg,Si. Weak s-process in these stages
O burning T2e9, 16O ? 32S at low T, 28Si at
high T, some Mg,P, neutron fraction starts to
increase - only shell O burning material get out
Si burning Tgt3e9, 28Si ? Ca,Ti,Cr ? Fe peak
by ?-glomming. Neutron excess gets large. 1.5 M?
of material processed in a few days - QSE and NSE
dominate
15
gt8 M?

9a. Shell burning is highly dynamic process with
significant asphericity, thermodynamic
perturbations, mixing. Shells are likely a
single connected region at late stages with
plumes burning in flashes determined by
composition T(r). Presupernova state will
imprint substantially on explosion
16
Divergences at large M

He burning begins earlier for higher M, lower z.
Core He burning may begin early on Hertzsprung gap

17
Divergences at large M

Above 30-35 M? at solar z LBV eruptions mass
loss, mixing of He to surface force evolution to
blue, eliminating red supergiant phases