Transcript their evolution, nucleosynthesis and dusty end

LMS & IMS:
their evolution, nucleosynthesis and dusty end
S. Cristallo
in collaboration with
Oscar Straniero and Luciano Piersanti
Osservatorio Astronomico di Teramo - INAF
AGBs: a theoretician perspective
Very luminous
(103-104 our SUN)
Very cold
(2000-3000 K)
AGB structure
CO Core
He-shell
H-shell
Earth-Sun
(~200 RSUN)
Earth radius
(~10-2 RSUN)
Practically, a nut
in a 300 mts hot air balloon
The s-process in AGB stars
13C(α,n)16O reaction
22Ne(α,n)25Mg reaction
TDU
TDU
HOT BOTTOM BURNING (Boothroyd & Sackmann 1991)
Busso et al. 1999
The FRANEC Code
(Frascati RAppson-Newton Evolutionary Code)
(Chieffi & Straniero 1989; Straniero et al. 1997; Chieffi et al. 2001;
Straniero et al. 2006; Cristallo et al. 2007; Cristallo et al. 2009)
Four
first-order non-linear
constant coefficients
differential equations
Three
characteristic relations
HYDROSTATIC, NO ROTATION, NO MAGNETIC FIELDS
Major uncertainty sources in stellar
evolutionary codes and their link with grains
1.
2.
3.
4.
5.
Opacities;
Mass-loss law;
Equation of State (IMS);
Convection treatment;
Non convective mixing mechanisms (LMS).
Opacities
Atomic
opacities
T
Molecular
opacities
4000-5000 K
Grains
2000 K
C/O<1
TiO – H2O - CO
C/O>1
CO – C2 – CN - C2H2 – C3
Marigo 2002; Cristallo et al. 2007
C and N enhancements
Metallicity
12C
& 14N enh. factors
2x10-2
Solar ≡ 1.4x10-2
1x10-2 & 8x10-3
3x10-3 & 6x10-3
1x10-3
1, 1.5, 1.8, 2.2, 5
1, 1.5, 1.8, 2.2, 4
1, 1.8, 2.2, 5, 10
1, 2, 5, 10, 50
1, 5, 10, 50, 200
1x10-4
1, 10, 100, 500, 2000
See also:
Lederer & Aringer 2009; Weiss & Ferguson 2009
Ventura & Marigo 2009; Marigo & Aringer 2009
Karakas et al. 2010
Results
The C-enhanced low
temperature opacities
make the stars redder in the
AGB phase
Effects on surface temperatures and, therefore, on
mass-loss and nucleosynthetic yields
Mass loss law
AGB PHASE
1. BCK - temperature
(Fluks et al. 1994)
2. Luminosity - MBOL
3. MK=MBOL-BCK
4. Period-MK
(Whitelock et al. 2003)
5. Period – Mass-loss
GRAINS
DRIVE THE
MASS-LOSS
Vassiliadis&Wood 1993
Straniero et al. 2006
Grains: opacities and mass-loss
Winds of carbon stars are considered to be dust-driven winds. Photons lead to
a radiative acceleration of grains away from the star.
Subsequently, momentum is transferred to the surrounding gas by gas-grains
collisions.
UNKNOWNS
1.
2.
3.
4.
grains opacity (how they interact with radiation);
grains growth process;
grains nucleation phase (in particular for C/O>>1);
stellar pulsation physics.
It is commonly assumed that grain sizes are small compared to the relative
wavelenght: that’s not always true (see e.g. Mattsson et al. 2011)
The Luminosity function of Galactic C-stars
Guandalini et al. 2006 (A&A, 445, 1069)
Cristallo et al. 2011 (ApJS, 197, 2)
The Luminosity function of Galactic C-stars
Guandalini & Cristallo, in preparation
Distances from van Leeuwen 2007
P-L from Whitelock et al. 2006
First attempt (to my knowledge) to evaluate the amount and type of dust
production in AGB stars with a stellar evolutionary model
1.
2.
3.
4.
Amount of silicates scales with Z
Silicates are produced in IMS (strongly
dependence on HBB)
Mass-loss rate dtermines the dust condensation
degree
For C-stars, the main source of uncertainty is
the amount of dredged up carbon
Total mass of dust as a
function of the stellar mass
Ventura et al. 2012
(MNRAS 424, 2345)
Ventura et al. 2012
(MNRAS 420, 1442)
Mass of silicates
Mass of carbon dust
EOS for IMS
For Intermediate Mass Stars, the temperature at the bottom of the convective
envelope is high enough (T>4e7 K) to allow proton captures:
HOT BOTTOM BURNING (Boothroyd & Sackmann 1991)
Convection treatment
•
•
•
•
Schwarzschild criterion: to determine convective borders
Mixing length theory: to calculate velocities inside the convective zones
Mixing efficiency: proportional to the ratio between the convective
time scale and the time step of the calculation (Spark & Endal 1980);
ΔX depends linearly on Δr (NOT diffusive approach).
At the inner border of the convective
envelope, we assume that the velocity
profile drops following an exponentially
decaying law
REF: Freytag (1996), Herwig (1997),
Chieffi (2001), Straniero (2006),
Cristallo (2001,2004,2006,2009)
v = vbce · exp (-d/β Hp)
•
•
•
•
Vbce is the convective velocity at the
inner border of the convective
envelope (CE)
d is the distance from the CE
Hp is the scale pressure height
β = 0.1
WARNING: vbce=0 except during Dredge Up episodes
Gradients
Gradients
profiles
profiles
WITHOUT
WITH exponentially
exponentially
decaying
decaying
velocity
velocity
profile
profile
RADIATIVE
He-INTERSHELL
CONVECTIVE
ENVELOPE
During
a TDU
episode
An interesting by-product: the formation of the 13C pocket
13C-pocket
14N-pocket
23Na-pocket
Variation of the 13C-pocket pulse by pulse
X(13Ceff)=X(13C)-X(14N)*13/14
14N
strong neutron
poison via
14N(n,p)14C reaction
1st
11th
Cristallo et al. 2009
13C
pocket and dredge up as a function of b
Third TP of 2 Mʘ at Z=Zʘ and Z=10-4
Convective
13C
burning
He-intershell elements enrichments
J=Iω=mr2ω
Cristallo et al. 2009
F.R.U.I.T.Y.
(Franec Repository of Updated Isotopic Tables & Yields)
August the 9th 2012: added 1.3 MSUN models at all metallicities
Z=10-4 models (within the end of November)
Dedicated mailing list with upgrades
On line at www.oa-teramo.inaf.it/fruity
(1.5,2.0,2.5,3.0) MSUN with Z=(1x10-3,3x10-3,6x10-3,8x10-3,1x10e-2,sun,2x10e-2)
M=2Mʘ
A key quantity:
the neutron/seed ratio, that is
n(13Ceff) /n(56Fe)
13C
Final AGB
composition for
0.0001<Z<Z
is primary like
56Fe is secondary like
s-process indexes (I)
[ls/Fe]=([Sr/Fe]+[Y/Fe]+[Zr/Fe])/3
[hs/Fe]=([Ba/Fe]+[La/Fe]+[Nd/Fe] +[Sm/Fe])/4
[ls/Fe]
[hs/Fe]
[Pb/Fe]
Observations vs theory (II): [hs/ls] distributions
Ba & CH stars
Post-AGB
Intrinsic C-rich
Intrinsic O-rich
Cristallo et al. 2011
[ls/Fe]=([Sr/Fe]+[Y/Fe]+[Zr/Fe])/3
[hs/Fe]=([Ba/Fe]+[La/Fe]+[Nd/Fe] +[Sm/Fe])/4
FRUITY Models vs Grains (measurements from Barzyk et al. 2007)
FRUITY Models vs Grains (measurements from Barzyk et al. 2007)
FRUITY and MONASH models vs Grains (measurements from Avila et al. 2012)
The most interesting data are those that do not agree
with theoretical models.
Ernst Zinner (this morning)
A new set of FRANEC rotating AGB models
1. Centrifugal forces lead to deviations from spherical symmetry;
2. Differential rotation is considered and, following Endal & Sofia (1976,1978), the evolution of
angular momentum (J) through the star is followed via a nonlinear diffusion equation (except at
the inner border of the convective envelope, where we apply the same formalism of the chemical
transport), by enforcing rigid rotation in convective regions (constant angular velocity);
3. Efficiency of both dynamical (Solberg-Hoiland, dynamical shear) and secular (Eddington-Sweet
circulation, Goldreich-Shubert-Fricke, secular shear) instabilities are evaluated by computing the
corresponding diffusion coefficients as described in Heger et al. (2000), but without their
proposed fμ and fc;
4. Angular momentum transport equation is solved contemporary to the chemical evolution
equations to take into account the feedback of chemical mixing on molecular weight profile,
which could inhibit secular instabilities (μ-current);
5. In solving the angular momentum transport and chemical mixing equations, we computed the
effective diffusion coefficient as the sum of the convective one and those related to secular and
dynamical rotationally instabilities;
6. No magnetic braking is considered.
THANKS!