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