Stellar Evolution in general and in Special Effects: Core Collapse, C- Deflagration, Dredge-up Episodes Cesare Chiosi Department of Astronomy University of Padova, Italy
Dec 28, 2015
Stellar Evolution in general and in Special Effects:
Core Collapse, C-Deflagration, Dredge-up Episodes
Cesare Chiosi
Department of Astronomy
University of Padova, Italy
Part C: Low and intermediate mass
starsThe kingdom of Type IA SN
The formation of CO White Dwarfs
Low and intermediate mass stars (to about 6 Mo) at the end of their evolution generate “White Dwarfs”made of a mixture of C & O
Burnings in the Tc vs c plane
Generalities
• Low mass stars are those igniting core He-burning in degenerate conditions (M < 2.2 Mo)
• Intermediate mass stars are those igniting He-burning in non degenerate conditions, but develop
strongly degenerate CO core (2.2 Mo < M < 8 Mo).
• In these stars C-ignition fails unless the CO core mass can grow to Mch = 1.4 Mo
• All present the AGB phase.
The beginning of the AGB phase
Mr/M
Typical structure of an AGB
star
• Following central He-exhaustion, the star is structured as follows:
• A contracting, degenerate CO core;
• Two burning shells;• An expanding convective
envelope;• The star climbs the Hayashi line
and loses mass by stellar wind.
Two important facts
• Because of the expansion the H-burning shell temporary extinguishes. The external convection penetrates very deeply almost reaching the He-burning shell. Variations in the surface chemical abundances (II dredge-up).• Extiction of the H-shell causes contraction of the envelope followed by reignition of the H-shell very close to the He-shell. A very thin layer of matter separates the two shells.• Because of this, two important facts occur: (1) the two shells get thermally coupled;
(2) the He-shell becomes thermally unstable.
Thermal coupling• Two burning shells close each other do thermally interact as both require a
certain temperature to exixt. For instance if a He-shell gets
very close to a H-shell we expect an enormous increase of
H-burning.
• Each shell has its own speed in processing matter unless the corresponding
luminosities are in a suitable ratio.
• Let Xi be the abundance of a fuel and qi the energy liberated per gram
• Thermal coupling requires
K) 102(T 8
K) 103(T 7
ii
ii
Xq
L
dt
dM
HHH
HeHeHe
He
H
XqL
XqL
dM
dt
dt
dM
Stationarity conditions
Given that
qH/qHe = 10, XH=0.7 and XHe =1 the situation is stationary only if
LH = 7 LHe
THIS REQUIRES THAT TIME TO TIMETHE He-SHELL UNDERGOES STRONGACTIVITY
He-shell thermal instability
• In general the He-shell is quiet and scarcely efficient, whereas the H-shell dominates the energy production.• However, with regular periodicity, the He-shell dramatically increases the energy production, burns out the available fuel, induces a tiny convective region just above it, and triggers the expansion of the overlaying envelope causing the temporary extinction of the H-shell.• The He-shell gets quiet again thus allowing the envelope to recontract and the H-shell to reignite.• The cycle repeats itself from several to many times (depending on the star mass and efficiency of mass loss).
WHY?
Positive gravothermal specific heat
D
ro
Suppose we have a shell of thickness D confined
between ro and ro + dr (D<< ro)
The mass in the shell is
Suppose that following a perturbation in sh it
expands dr=D and suppose that the mass variation
is negligile
P A
dρ dD r dr dP dr=- =- and =-4
ρ D D r P r
the gravotermal specifi c heat is
4δc* =c (1- )
r4 -
Df or D very small c* can be > 0 even f or a normal gas.
Thermal Runaway of the He- shell.
DrM osh2
The third dredge-up
Complicate interplay between the extinction of the H-burning shell, the penetration of the external convection and possible overlap with the intermediate thin convective shell
Mr/M
Some general considerations
• While the thermal cycles work, and the nuclear shell increases the mass of the CO core (which becomes more and more electron degenerate), mass loss by stellar wind continuously decreases the mass of the external envelope until it is completely expelled. a CO White Dwarf is left or the CO core can grow to the Chandrasekhar limit and C-deflgration may occur (depends on the total mass of the star).
• The number of cycles depends on the envelope mass with respect to the core
and total mass: low mass AGBs have a small envelope and hence a few cycles,
stars of intermediate mass have bigger envelopes and hence a large number of cycles.
• Every cycle may bring some C to the surface. When the abundance of C exceeds that of O, a M-star is turned into C-star.
• Finally, as a result of the game between the H- and He-shells and internal +
external convection, the intershell region can become a good site for s-process nucleosynthesis.
Number of pulses and Mc vs L
Interplay between internal andexternal convection from pulseto pulse
Number of pulses
Mc(luminosity)
S-process nucleosynthesis in AGB
• S-process nucleosythesis is the capture of neutrons by heavy elements on
time scale slow with respect to beta-decay.
• There are at least two sources of neutrons
• During the thermal pulses, external convection extends to layers in which
H-burning was active in the previous pulse. H-rich material is brought to
regions in which He-burning occurs and protons are used in the reactions
MgnNeOnC 25221612 ),( and ),(
.and ith interact wcan then neutrons The
),(
),(),(),(
),(
),(),(),(
2013
20
14171716
16
14131312
NeC
Nn
NpOFpO
On
NpCNpC
In addition……
• Alternatively the H-shell converts C and O (via the CNO cycle) into N which is mixed into the He-shell thus activating the series of reactions
MgnNeOFN 2522181814 ),(),(),(),(
• Another source of neutrons.• The efficiency of the various reactions, processes
depends on many parameters. They are responsible of the synthesis
of elements heavier than Fe via a complex story of n-
captures, followed by and -decays
The path to WD-deflagration & detonation
Gravity in close binaries: 1
Gravity in close binaries: 2
Gravity in close binaries: 3
Gravity in close binaries: 4
Mass transfer & accretion disk
Different types of close binaries
Useful definitions for abundances
Origin of SN IA
SN IA in a snapshot
Type Ia SN: Nuclear Deflagration
How does explosionproceed?
The case of a WD + MS companion
Most popular model for Type Ia SN consists of a WD growing to MCh, presumably by accretion from a companion, and being disrupted by thermonuclear explosion. No remnant is left.
C-ignition: in brief………….
• C-burning first ignites quietly in the WD core but convectively unstable DT cause local run-away.
• Explosive burning starts near center or off-center; flame front propagates subsonically (deflagration) until it may or may not change into a detonation (supersonic) at lower densities, and eventually disrupts the star.
• Turbulence may develop turbulent flame Rayleigh-Taylor instabilities by buoyancy of hot ashes with respect to dense unburned material.
• The consequences of all this are……………
Turbulent Combustion
Merging Flame Fronts
EASY TO CHECK THAT ON A SHORT
TIME SCALE En >> |g|
Chemical structure
Before…. After….
Chemical Structure
Chemical abundances
Elements production & energetics
Wide and Close Binary Systems: CO+CO
Secret everybody favoured scheme !!
Wide Close
But others are possible: CO+He & He+He
CO+He He+He
Formation Frequency of SNI Precursors
Connection between SN Types and Progenitors
Single
Binaries
To conclude: Two Nasty Questions
• Why do Type II SN not explode ?
• Where are the progenitors of Type Ia ?
What are Type II SN……..
……….and Modelers doing?
Virginia Trimble 2004
Is CO+CO in troubles ?
Virginia Trimble 2004