Transcript Review of Evolutionary Tracks

Prelim Review
<1.2 M
9
7
8
6
5
10
1
4 3
2
9
<1.2 M
8
1.
2.
6
Hayashi track - fully convective
cooler surface temp. requires supersonic convection. Ends with burning
of 2H
7
10
5
43
1
2
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
<1.2 M
9
8
7
6
3. Bump from CN part of CNO cycle
5
10
1
43
Convective core develops while
2
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 ~ 0.7%mc2, star burns ~10% of
its mass
`
Stars w/ radiative cores go up in L at ~ const Teff
9
<1.2 M
8
5.
7
6
Leaving main sequence - transition
10
5
43
1
to shell H burning smooth because H
2
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
9
<1.2 M
8
6.
7
6
Red giant branch - star has moved
10
5
4 3
1
as red as it can go - L of star now increases
2
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 (T~2e8) - 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.
<1.2 M
9
8
2(,)12C
7
6
8. Core He burning until YHe
5
10
1
4
3
low, then 12C(,)16O takes over
2
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
9
<1.2 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 haven’t 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.
8
7
6
10
5
4 3
1
2
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
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 < ~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
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 > 1
at low metallicity
10. Intermediate mass stars produce CO white dwarfs with C/O <<1.
Most massive may become ONeMg WD.
>8 M
>8 M
1-8. Very much the same as intermediate mass stars for
masses < 30 M
9. Core evolution proceeds too quickly for TPAGB to
develop. C ignition at T~6e8 K. Off-center degenerate C
flash for lowest masses. Neutrino cooling dominates over
photon cooling for T > 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.
>8 M
9a. C burning: T~6e8, 12C  20Ne, some 23,24Mg,23Na
Ne burning: T~1e9, 20Ne  16O,some Mg,Si. Weak sprocess in these stages
O burning: T~2e9, 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: T>3e9, 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
>8 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
Divergences at large M
He burning begins earlier for higher
M, lower z. Core He burning may
begin early on Hertzsprung gap
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