Evolution & Nucleosynthesis in 1 to 12Msun Stars

PhD Dissertation Defense

by

Ghina Mahmoud Halabi

Committee: Dr. Mounib El Eid (Advisor) Dr. Leonid Klushin (Chair) Dr. Brad Meyer Dr. Jihad Touma Dr. Jordi Jose

American University of Beirut Beirut, Lebanon Dec. 9. 2014

Motivation

• Stars in this mass range are very numerous in the Galaxy (about 23% of

the stellar population).

• They have substantial contribution to the chemical composition of our Milky Way galaxy and other galaxies. They are the site of nucleosynthesis of about half of the elements beyond iron in the universe.

• Several observations of the oldest clusters and stars put a lower limit on the age of the universe and thus provide valuable insight into the accuracy of our cosmological models.

• They are evolved stars and thus are important for the theory of stellar evolution.

Evolution & Nucleosynthesis in 1 to 12Msun Stars

Observations of the Evolutionary Phases

Estimated age: 11.39 Gyr

M3 globular cluster

Theory of Stellar Evolution: General Overview Hertzsprung-Russell Diagram

5Mʘ Main Sequence: (pt. 1 to 3) • Core hydrogen burning • Convective core • 90% of a star’s lifetime

1

RGB: • Core helium burning • hydrogen shell burning • blue loop (pt. 8 to 10)

2 AGB: • CO core • hydrogen & helium burning shells

3 SAGB: Stars of mass above ≈ 8Mʘ undergo C-burning and develop ONe cores.

4

Back regions convective

Outline

1. How stellar models are constructed and the main uncertainties in stellar evolution calculations.

2. Evolution towards the AGB phase. a. Blue loops in intermediate mass stars b. 16O/17O surface abundances of Red Giant stars. 3. Evolution during the AGB phase

4. Evolution during the SAGB phase

5. Conclusions & Future work

Evolutionary Code • Models are constructed using a Lagrangian 1D hydrodynamic evolutionary

code that solves the partial differential equations describing stellar structure & evolution on an adaptive grid.

• Includes updated input physics (equation of state, opacities, nuclear reaction rates..)

• Convection is described within the framework of the local 1D MLT. So the treatment of convective boundaries includes free parameters which need to be constrained by observations.

• Existing uncertainties in the evaluation based on experimental grounds of certain key nuclear reaction rates at the low energies encountered in stars!. These uncertainties propagate into the stellar models and introduce uncertainties in the evolution and nucleosynthesis yields.

Uncertainties

Treatment of convective boundaries: Overshooting (Extra Mixing)

2 2(4 ) i i

r r

dX Xr D

dt M M

D = 0 in radiative zones D ≠ 0 in convective zones calculated according to the MLT D at convective boundaries given by:

2

0

0.25

( ) p

z

fH

os c

p

f

H

D z D e z r r

= pressure scale height (distance over which the pressure drops an e-fold) Based on multi-D simulations (Freytag et al. 1996)

ov p z H

Models consistent with observations

Parameterized approach

Evolution towards the AGB

a. Blue loops in intermediate mass stars

b. Nucleosynthesis (16O/17O surface abundances)

Blue Loops (I): The effect of 14N(p,γ)15O reaction rate

Obtained with the same physic except the rate of the 14N(p,γ)15O reaction which controls H-burning by the CNO cycle:

Old rate (Angulo et al. 1999) overestimated

New rate: based on experimental data by Marta et al. (2011) Lower than the old rate by a factor of two at stellar temperatures.

Halabi, G. M., El Eid, M. & Champagne, A. 2012, ApJ, 761, 10

nuc g

r

L

M

Hydrogen-shell burning is affected

Blue Loops (II): Effect of core & envelope overshooting

Standard calculation

With overshooting, γov=0.1

6Mʘ

• Core overshooting pushes the H-shell out in mass and establishes higher temperatures in the H-shell region. • Envelope overshooting causes a deeper penetration of the H-profile (closer to the H-shell source).

• The H-shell burning is more efficient and the extension of the loop is restored.

Blue Loops (III): Restored Loops Comparison with observations

Halabi, G. M. 2014, AIP 1595, 264

Including observed Cepheids by Fernie et al. (1995) and Schmidt (1984).

Interpretation of observed Cepheids by Criscienzo et al. (2012).

Masses & Surface Abundances (I): Observational data

Halabi, G. M. & El Eid, M. 2014, MNRAS, submitted and under revision

Observational data by Tsuji (2008)

16O/17O

Critical indicator of the depth of convective mixing.

Masses & Surface Abundances (III): Abundance Profiles

Standard Calculation with envelope overshooting

Halabi, G. M. & El Eid, M. 2014, MNRAS, submitted and under revision

Masses & Surface Abundances (IV): Surface Abundance of 16O/17O

Evolution & Nucleosynthesis during the AGB phase

Evolution during the AGB phase (I): Structure of the Star & Thermal Pulsations

LHe

LH

Lstar

41

3 8T

Evolution during the AGB phase (II): Details of One Thermal Pulse

A. I, Boothroyd, Science Vol 314 nb 5806, 2006

Third (of 12C & s-process elements)

13C pocket

12 13 13( , ) ( , )C p N e C

1 2

3

4

2 3

3

16

rrad

rad ad

κ L P

π ac r T

Evolution during the AGB phase (III): The Third Dredge Up

12 13 13 16( , ) ( , ) ( , )C p N e C n O

1. TDUP is very challenging to achieve! 2. The right amount & depth! 3. Physical process (??) 4. Not easy: numerical adjustments timing of TDUP operation..

Evolution during the AGB phase (IV): Production of 13C

13 16( , )C n O

410 M

Evolution & Nucleosynthesis during the SAGB phase

Evolution during the SAGB phase (I): C-burning rate

Recommended rate (CR)

Upper limit (CU)

Lower limit (CL)

Caughlan & Fowler (1988): CF88 rate

Pignatari et al. (2013):

CL

CU

CR

Evolution during the SAGB phase (II): Dependence on the C-burning rate

Halabi, G. M. & El Eid, M. 2014, AIP, submitted

with the CR rate with the CF88 rate

• Carbon ignites at T~7x108 K very close to the center, at mass shell 0.07Mʘ

• Burning proceeds in a convective core

• Carbon ignites remarkably off-center, at mass shell 0.23Mʘ

• The temperature gradient drives the energy ux to propagate inwards, fueled by convection

9Mʘ

Evolution during the SAGB phase (III): C-burning mechanism with the CR rate

nuc

r

L

M

Evolution during the SAGB phase (III): C-burning mechanism with the CR rate

Evolution during the SAGB phase (III): C-burning mechanism with the CR rate

Halabi, G. M., El Eid & J. Jose, in preparation

New novae models

Evolution during the SAGB phase (III): C-burning mechanism with the CR rate

Rate Mup

(Standard mixing)

Mup

(Core overshooting)

CF88 8.5 Mʘ 8 Mʘ

CR 8.3 Mʘ 7.5 Mʘ

CU 7 Mʘ --

CL 9 Mʘ --

Evolution during the SAGB phase (IV): Effect of C-rate and overshooting on Mup

Halabi, G. M. & El Eid, M. 2014, AIP, submitted

Core overshooting constrained from study on blue loops (γov=0.1)

Conclusion 1

nuclear physics

stellar evolution

treatment of mixing

Understanding this connection based on the underlying

physics.

Conclusion 2

Complicated models

input parameters (treatment of instabilities like convection)

important consequences on evolution & nucleosynthesis

Verified by observations!

Conclusion 3

Mechanism is used (overshooting)

Dramatic consequences on details of evolution nucleosynthesis

Blue loops, 16O/17O powerful constraints on our parameter choices

Direct link to observations!

Conclusion 4

State-of-the-art nuclear reaction rates: C-burning, CNO, (s-process network)!

I investigated their uncertainties on details of evolution & nucleosynthesis

Conclusions could be drawn based on how well models match

observations

Mass-loss carefully considered and taken into account

Future Work

• s-process nucleosynthesis.

1. Factors affecting the amount of 13C

2. Efficiency of the diffusion process

3. Constraining amount of 13C based on observations.

(CNRS project in intensive collaboration with Dr. Bradley Meyer).

• The new C-burning rate will be further investigated in novae simulations. Consequences on novae outbursts in binary systems, SN Ia rates.. (Halabi, El-Eid, Jose (2014), in preparation).

Acknowledgments

• Dr. Mounib El Eid (advisor)

• Committee members:

Dr. Leonid Klushin (AUB)

Dr. Brad Meyer (Clemson University, USA, SC)

Dr. Jihad Touma (AUB)

Dr. Jordi Jose (Universitat Politècnica de Catalunya, Spain)

• Dean’s Office (Dr. Patrick Mcgreevy &

Mrs. Abeer Khoury)

• Physics Department (Dr. Samih Isber,

Dr. Ghassan Antar & Mrs. Joumana Abi Falah)

• IT support (Saad Itani)

• Family, friends & colleagues!

Evolution during the pre-AGB phase: Core He-flash in low mass stars

Blue Loops (II):

Blue Loops (III): Why Core Overshooting Solves the Problem

Blue Loops (IV): How Much Core Overshooting?

Nucleosynthesis during the pre-AGB phase (III): Can we trust the mass-loss treatment?

Halabi, G. M. & El Eid, M. 2014, MNRAS, submitted and under revision

• Mass-loss taken into account • Lower error bars than Tusji (2008)

Masses & Surface Abundances (II): Mass Determination

Nucleosynthesis during the pre-AGB phase (IV): Abundance Profiles

Nucleosynthesis during the pre-AGB phase (VII): Surface Abundances: 12C/13C and Nitrogen

Evolution during the SAGB phase (VI): C-burning mechanism with the CF88 rate

Evolution during the SAGB phase (VI): C-burning mechanism with the CF88 rate

Evolution during the SAGB phase (VI): C-burning mechanism with the CF88 rate

Evolution during the SAGB phase (VI): C-burning mechanism with the CF88 rate

Thick lines show the main path of the s-process, while the thinner lines show the less important branchings (Nicolussi et al. 1998).

Spectra of stars in the Large Magellanic Cloud with some of the strongest molecular bands identified (Lattanzio & Wood 2003).

• Momentum equation:

• Mass conservation: • Definition of velocity: • Energy equation: • Equation of energy transfer: • Equation of state:

Stellar Modeling: Basic Equations

22

2 24 r

r

GMr Pr

t M r

2

2

1 ; 4

4r

r

rM r dr

M r

ut

r

2( )nuc

r

L P E

M t t

4

2 3

ln

4 ln

ln 3 radiative regions

ln 16

ln ln, | convective regions

ln ln

r

r

r

P

p

GM TT T

M π r p P

κ L PT

P π ac r T

T P

P T C T

41

3rad gasP P P aT T

Shell convection entrainment

four snapshots of the the logarithm of the fractional volume of the ’H+He’ fluid is shown

as it is entrained into the convection zone. (Woodward et al. 2014)

f=0.01, gamma=0.1

f=0.01, gamma=0.1

f=0.01, gamma=0.1

f=0.20, gamma=0.1

f=inf , gamma=0.1

Excerpt from From Herwig et al. A&A, 324, L81–L84 (1997)