Diapositive 1

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EVOLUTION MODELS OF LOW METALLICITY STARS
Georges Meynet, André Maeder, Sylvia Ekström
Geneva Observatory
and Raphael Hirschi Basel University
Spite et al. 2005 Cayrel et al. 2004
Early chemical evolution of galaxies
Reionization at high redshift
Pelló et al. 2004
Stellar population in metal poor and/or high
redshifted galaxies
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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
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Cells of meridional circulation
Very important process for the transport of
the angular momentum
20 Msol on the ZAMS
Inner cell ? inwards transport of angular
momentum Outer cell ? outwards transport of
angular momentum
GRATTON- ÖPIK CELL
Timescale? a few times the
Kelvin-Helmholtz timescale
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THE SHEAR INSTABILITY
Where does the energy come from ?
From the excess energy in the shear
When does it occur ?
When the excess energy in the shear can overcome
the stable pressure gradients
The timescale
Secular shear ? much longer than MS
lifetime Dynamical shear ? dynamical timescale
Brueggen Hillebrandt 2001
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ROTATION AND MASS LOSS
WHEN Z
SURFACE VELOCITY DURING THE MAIN-SEQUENCE PHASE
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WHAT CHANGES AT VERY LOW Z FOR ROTATING MODELS ?
Meridional velocities smaller
MORE ANGULAR MOMENTUM IN THE CORE
Steeper gradients of the angular velocity in the
interiors
MORE EFFICIENT MIXING
Less angular momentum removed by stellar winds
BREAK-UP LIMIT
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Wolf-Rayet Stars
These stars are seldom (227 WR known in our
Galaxy, a few thousands estimated) however
Contribute through their winds to the enrichment
in new synthesized elements of the interstellar
medium
4He, 12C, 19F, 26Al, s-process elements
Progenitors of type Ib, Ic supernovae, of at
least some of the gamma ray bursts
May reveal their presence in remote galaxies
through their very broad emission lines ? study
of star formation and evolution in different
environments
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Remaining problems with WR stars
Possible to reproduce the WR/O number ratios
observed at different Z only using models with
enhanced mass loss rates
Maeder and Meynet 94
Not satisfactory ! Clumping in the winds of hot
stars tends to reduce the observed mass loss
rates by a factor 2 to 3
Nugis et al 98 Hamann and Koesterke 98
Other difficulties ? Observation ?smooth
transition from high surface
abundances to H-free atmospheres
? Observed number of stars
in the transition WN/WC
phase (Conti Massey 89 Langer 91
Crowther 95,02) ? In
Magellanic Clouds, WR binarity similar to
that in solar vicinity
(Foelmi et al. 2003 ab)
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What are the effects of rotation on the
Wolf-Rayet star formation process ?
Hot stars ? Log Teff gt 4.0
Mass fraction of hydrogen At the surface below 0.4
In non-rotating models Mass loss the key
parameter
In rotating models Rotational diffusion and mass
loss
Maeder 1987 Fliegner and Langer 1995 Meynet and
Maeder 2003
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New grids of stellar models
Also Z0.040 0.008, 0.004, 0.00001,10-8
Pop III See talk by S. Ekström
Meynet and Maeder 2003
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Rotating model

• Enter WR phase
• during the MS
• phase

WRpost H-b. Mass loss
WRin H-b. Rot. mixing
?
With a much higher actual mass 45 Msol instead
of 27 Msol
60 Msol, vini 0 km s-1
vini 300 km s-1
ltVgtO189km/s
Mtot
Mcc
WR phase longer Mass loss and Mixing Both
important
H-b.
He-b.
cf also Maeder 1987 Fliegner and Langer 1995
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For a given metallicity, the minimum initial mass
of single stars which become Wolf-Rayet star is
decreased for higher rotation velocities
WR lifetimes also increased for a given initial
mass
22Msol
37Msol
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Mass loss rates depend on Z
Cf Kudritzki 2002
Vini300 km/s
NO MASS LOSS
Mass loss rates from Vink et al. 20002001
FINAL MASS
Meynet and Maeder 2004
INITIAL MASS
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Meynet and Maeder 2005
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Meynet and Maeder 2005
Observed points from Prantzos and Boissier (2003)
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IMPORTANT AMOUNTS OF PRIMARY NITROGEN NEED TO BE
PRODUCED AT LOW Z
Israelian et al. 2004 Centurion et al 2003 (DLA)
Spite et al 2005
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60 Msol, Z10-5, Wini/W 0.85
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Vini800km/s
Vini300km/s
Vini0km/s
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Meynet Maeder 2002
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ROTATION INDUCES NUMEROUS PROCESSES WHICH
ENHANCES MASS LOSS
1) Reaching of the break-up limit during the Main
Sequence phase
60 Msol, Z 0.020
60 Msol, Z 0.00001
Vcrit
Vcrit
Vini800 km/s
Vini500 km/s
Vini300 km/s
Vini300 km/s
Cf also Sackman Anand 1979 Langer 1998
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3) WG-limit at the tip of the blue loop
2) Redwards evolution favoured
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4) Surface enrichments

• During the MS phase
• N increased
• C and O decreased
• but CNO/Z remains
• constant equal to the
• initial value

• At the end of the core
• He-burning phase, apparition
• at the surface of both
• H and He-burning products
• Primary N
• Primary C
• Primary O
• CNO/Z increases

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7 Mo stellar model more mixed than the 60 Mo
7 Mo
W
60 Mo
Vini/Vcrit0.6 - 0.7 Z10-5
Gradient of W steeper Radius smaller (factor
5) Lifetime longer (factor 9)
7 Mo ?
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7 Msol, Z10-5 E-AGB phase
60 Msol, Z10-5, C-burning phase
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CONCLUSION
The effects of rotation are amplified at low
metallicity
? mixing enhanced
? induce mass loss
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
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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 ?

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ENERGY OF ROTATION
Veq km s-1
15 Msol
W/W crit (ini)0.6
Gravitational energy
1050.3 ergs
Energy of reference gravitational energy
Thermal energy
Thermal energy 50
Energy of radiation 10
Energy of radiation
Rotational energy 0.3
Energy of rotation
Excess of energy in the shear 0.003
Excess energy of shear
Mass fraction of hydrogen at the center
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Relation SN - GRB
COLLAPSAR
Woosley 1993
Hjorth et al. 2003
Precursor Rotating WR star ? Is there enough
rotation ? 1 of all WR
would be enough.
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WO STARS PREFERENTIALLY OBSERVED IN METAL POOR
REGIONS
WHY ? cf Smith and Maeder 1991
Z 0.004 Z 0.008 Z 0.020 Z 0.040
A few percents of WR stars
Galaxy 3 over 227 LMC 1 over 134 SMC 1
over 12
Models with Vini300 km s-1 without magnetic
fields would retain enough angular momentum in
their core to be progenitors of a GRB Woosley
and Heger 2003 Hirschi et al. 2005

• Apparition of GRB favoured
• for a given range of metallicities

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GLOBAL MASS LOSS RATES
Maeder and Meynet 2000
Enhancement at break-up velocity
Log Teff 4.35 4.30 4.00
3.90
!
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Humphreys, 2002
0.90 0.53 0.36
What physics makes the Humphreys- Davidson
Limit ?

• The G - Limit classical Eddington limit.
  Non rotating LBV
• The W - Limit rotational effect dominates.
  Be stars.
• The WG - Limit both rotation and radiation.
  Most LBV.

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EVOLUTIONARY STATUS OF LBV

• LBV earlier
• than WN stars
• in general, but
• also simultaneous
• TRANSITION
• ??
• Same mass loss
• for LBV and
• WN9-11
• Crowther 1997

LBV
WN
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Humphreys-Davidson limit
When the mass loss rates decrease and/or the
metallicity decreases ? more stars can encounter
the Humphreys-Davidson limit
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A STRIKING OBSERVATIONAL FACTS

• Very Helium-rich stars in ? Centuri ?

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DY/DZ
60 Msol
Z 10-8
remaining mass in solar masses
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Zahn 1992
Meridional circulation
Shear turbulence
Transport of the angular momentum
Transport of the chemical species
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ROTATION AND MASS LOSS

• Different evolution of the surface velocity
• When stellar winds are weak, stars can reach more
  easily the critical velocity
• The account for anisotropic mass
• loss favours break-up

40 Msol, Vini400 km/s
Maeder, 2002