EVOLUTION MODELS OF LOW METALLICITY STARS

Georges Meynet, André Maeder, Sylvia Ekström

Geneva Observatory

Reionization at high redshift

Spite et al. 2005Cayrel et al. 2004

Early chemical evolution of galaxies

Stellar population in metal poor and/or high redshifted galaxies

Pelló et al. 2004

and Raphael HirschiBasel University

What is different at very low Z ?

• The initial masses of the stars (?)

• The ignition of H-burning in massive stars (no CNO element catalysts at the beginning)

• The opacities are lower

Stars more compact: R(popIII) = R(Zsol)/4

Stellar winds are weaker

El Eid et al 1983; Ober et al 1983; Bond et al 1984; Klapp 1984; Arnett 1996; Limongi et al. 2000; Chieffi et al. 2000; Chieffi and Limongi 2002;Siess et al. 2002; Heger and Woosley 2002; Umeda and Nomoto 2003; Nomoto et al. 2003; Picardi et al. 2004; Gil-Pons et al. 2005

GRATTON-

ÖPIK CELL

Cells of meridional circulation

Very importantprocess for thetransport of theangular momentum

Inner cell inwards transportof angular momentumOuter cell outwards transportof angular momentum

20 Msol on the ZAMS

Timescale a few times the Kelvin-Helmholtz timescale

y000 14034000

2

4

KHKEP

THE SHEAR INSTABILITY

Where does the energy come from ?

When does it occur ?

The timescale

From the excess energy in the shear

When the excess energy in the shear canovercome the stable pressure gradients

Secular shear much longer than MS lifetimeDynamical shear dynamical timescale

Brueggen & Hillebrandt 2001

ROTATION AND MASS LOSS

WHEN Z

SURFACE VELOCITY DURING THE MAIN-SEQUENCE PHASE

WHAT CHANGES AT VERY LOW Z FOR ROTATING MODELS ?

Less angular momentum removed by stellar winds

Steeper gradients of the angular velocity in the interiors

MORE EFFICIENT MIXING

BREAK-UP LIMIT

Meridional velocities smaller

MORE ANGULAR MOMENTUM IN THE CORE

Meynet and Maeder 2003

New grids of stellar models

Also Z=0.040; 0.008, 0.004, 0.00001,10-8

+Pop IIISee talk by S. Ekström

60 Msol, vini= 0 km s-1 vini= 300 km s-1

WR:post H-b.Mass loss

WR:in H-b.Rot. mixing

<V>O=189km/sMtot

Mcc

H-b. He-b.

Enter WR phase during the MS phase

Rotating model

With a much higheractual mass: 45 Msol instead of 27 Msol

WR phase longerMass loss and Mixing

Both important

cf also Maeder 1987Fliegner and Langer 1995

For a given metallicity, the minimum initial mass of single stars which become Wolf-Rayet star is decreased for higher rotation velocities

37Msol

22Msol

WR lifetimes alsoincreased for a given initial mass

Meynet and Maeder 2005

Observed points from Prantzos and Boissier (2003)

Meynet and Maeder 2005

Israelian et al. 2004Centurion et al 2003 (DLA)

Spite et al 2005

IMPORTANT AMOUNTS OF PRIMARY NITROGEN NEED TO BE PRODUCED AT LOW Z

60 Msol, Z=10-5, ini/= 0.85

ROTATION INDUCES NUMEROUS PROCESSES WHICH ENHANCES MASS LOSS

1) Reaching of the break-up limit during the Main Sequence phase

Cf also Sackman & Anand 1979; Langer 1998

Vcrit

Vcrit

Vini=500 km/s

Vini=300 km/sVini=300 km/s

Vini=800 km/s

60 Msol, Z = 0.020 60 Msol, Z = 0.00001

2) Redwardsevolutionfavoured

3) -limit at the tip of the blue loop

4) Surface enrichments

During the MS phaseN increased C and O decreasedbut CNO/Z remainsconstant equal to theinitial value

At the end of the coreHe-burning phase, apparitionat the surface of bothH and He-burning productsPrimary N Primary C Primary OCNO/Z increases

Vini/Vcrit~0.6 - 0.7Z=10-5

60 Mo

7 Mo

7 Mo stellar model more mixed than the 60 Mo

Gradient of steeperRadius smaller (~factor 5)Lifetime longer (~factor 9)

7 Mo

7 Msol, Z=10-5

E-AGB phase

60 Msol, Z=10-5,

C-burningphase

NUMEROUS INTERESTING CONSEQUENCES

• Higher surface enrichments at low Z Maeder &Meynet 2001; Venn & Przybilla 2003

• Change with Z of populations of Be stars Maeder et al. 1999

of blue to red supergiant ratio Langer & Maeder 1995; Maeder &Meynet 2001

of LBV and WR stars Fliegner & Langer 1995; Meynet & Maeder 2005

of type Ibc to II SN ratio Prantzos & Boissier 2003; Meynet & Maeder 2005

of collapsar progenitors MacFadyen & Woosley 1999; Hirschi et al 2005

• Change with Z of the stellar yields Meynet & Maeder 2002; Ekström et al. in prep.

A LOT OF INTERESTING PROBLEMS TO STUDY…

CONCLUSION

The effects of rotation are amplified at low metallicity mixing enhanced induce mass loss

A FEW EXAMPLES

Observations

• How fast are rotating low metallicity massive stars ?• Are their surface enrichment on average higher than at solar metallicity ?

Theory

• How stellar winds behave when CNO is increased at the surface during a red supergiant phase ?• How the evolution into the Pair Instability regime is changed by rotation ?• Might the first stellar generations enrich the ISM in new synthesized Helium ?• How the core collapse supernova explosions are affected by rotation ?• How the formation process of massive star is affected by rotation ?

ENERGY OF ROTATION

15 Msol

crit (ini)=~0.6

Gravitational energy

1050.3 ergs

Veq [km s-1]

Energy of reference gravitational energy

Thermal energy ~50%

Energy of radiation ~10%

Rotational energy ~0.3%

Excess of energy in the shear ~0.003%

Thermal energy

Energy of radiation

Energy of rotation

Excess energy of shear

Mass fraction of hydrogen at the center

GLOBAL MASS LOSS RATES

11

1,2

2

11

.

.

94

1

1

)0(

)(

critvvM

M

Maeder and Meynet 2000

Enhancement at break-up velocity

Log Teff 4.35 4.30 4.00 3.90

!

• The - Limit: classical Eddington limit. Non rotating LBV

• The - Limit: rotational effect dominates. Be – stars.

• The - Limit: both rotation and radiation. Most LBV.

Humphreys, 2002

0.90

0.53

0.36

What physicsmakes the Humphreys-Davidson Limit ?

LBV

WN

LBV earlierthan WN starsin general, butalso simultaneous

TRANSITION

Same mass lossfor LBV and WN9-11Crowther 1997

EVOLUTIONARYSTATUS OF LBV

A STRIKING OBSERVATIONAL FACTS

Very Helium-rich stars in Centuri ?

Y/Z

remaining mass in solar masses

60 Msol

Z = 10-8

r

XDDr

rrt

X ischeareff

i )(1 2

2

rDr

rrUr

rrt

rschear

42

42

2 1

5

1)(

Transport of the chemical speciesTransport of the angular momentum

Meridional circulation Shear turbulence

Zahn 1992

• Different evolution of the surface velocity

•When stellar winds are weak, stars can reach more easily the critical velocity

•The account for anisotropic massloss favours break-up Maeder, 2002

ROTATION AND MASS LOSS

40 Msol, Vini=400 km/s