Formation of the first and second generation stars

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Formation of the first and second generation
stars
Aug. 17 _at_ Tartu Workshop
Kazu Omukai (NAOJ)
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Outline

• Formation of the First Stars
• Why they are supposed to be very massive
  (100-1000Msun)?
• Formation of the 2nd-generation stars
• When and how did the transition to low-mass stars
  occur ?
• Second-generation includes
• 2nd-gen. zero-metal stars
• Extremely metal-poor stars

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Formation of the First Stars

• First Stars (definition)
• made of primordial pristine gas (H, He, small
  Li)
• formed from the cosmological initial condition
• (no astrophysical feedback)

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Scenario of the First Star Formation
2. Fragmentation of the First Objects
1. Formation of the First Object
3. Collapse of Dense Cores Formation of
Protostar
4. Accretion of ambient gas and Relaxation to
Main Sequence Star
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Fragmentation of First Objects Formation
of Dense Cores

• 3D similation
• (Bromm et al. 2001 Abel et al. 2002)
• filamentary clouds
• (Nakamura Umemura 2001)

Typical mass scale of fragmentation Dense cores
a few x 102-103Msun
Bromm et al.. 2001
These massive cores will collapse and form
protostars.
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First Star Accretion

• High accretion rate
• Mdot cs3/G T3/2
• 0.001-0.01Msun/yr
• for metal-free clouds (T 300 K)
• ? short formation time
• (c.f.10-6-10-5Msun/yr
• for the present-day case)
• (2) low opacity in accreting matter
• because of no dust
• ? lower radiation pressure
• (smaller stellar feedback)

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Protostellar Evolution with Accretion
Protostellar Radius
3b?expansion
1?adiabatic phase tKH gttacc
2, KH contr.
3a, ZAMS
(K.O. Palla 2003)

• Owing to fast accretion, the star becomes massive
  before H burning. (H burning via CN cycle starts
  at 40-100Msun)
• Accretion continues if accretion rate
  ltMdotcrit4x10-3Msun/yr
• (if gtMdotcrit , no
  stationary solution for gt100Msun)

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Protostellar Evolution with ABN(2002) Accretion
Rate
Evolution of radius under the ABN accretion rate

• Accretion continues.
• the final stellar mass will be 600Msun
• Or accretion may stop owing to photoevaporation
  of the disk at 200Msun (Tan McKee 2004)

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Mass of First Stars
Mstarmin( Mfrag, Mdot tOB, Mfeedback)
Mfragfragmentation mass
1000Msun Mdotaccretion rate 10-3Msun tOB
massive star lifetime 106yr Mdot tOB
1000Msun Mfeedback mass of star when the
accretion is halted by stellar feedback gt
100Msun
Mstar100-1000Msun
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2nd Generation Star Formation

• Different Initial Condition
• Ionization by the first stars
• Density fluctuation by SN blast wave, or HII
  region
• Different Environment
• External Radiation (UV, Cosmic Ray)
• Different Composition
• Metal Enrichment
• Dust formation

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Star formationin fossil HII regions
(Oh Haiman 2004 Nagakura K.O. 2005)

• After the death of the exciting star, star
  formation restarts inside the fossil HII region.

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Star formation in fossil HII regions
(Nagakura K.O. 2005)
Chemical evolution

• High ionization degree facilitates the formation
  of H2 and HD.
• HD cooling T30K
• ?low-mass star formation (ltMsun e.g. Uehara
  Inutsuka 1999).

Temperature evolution
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Metallicity Effects
Omukai, Tsuribe, Schneider Ferrara (2005)
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Metals and Fragmentation scales
Schneider, Ferrara, Natarajan, K.O. (2002)

• Formation of massive fragments by H2 cooling
  continues until some metallicity, say Z10-5Zsun
• For higher metallicity, sub-solar mass
  fragmentation is possible by dust cooling.

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Conclusion

• Metal-free stars
• consist of
• first-generation stars
• (cosmological initial condition, H2cooling)
  
• typically very massive 102-103Msun
• second-generation stars
• (e.g., HD cooling)
• can be less massive
• Metal enrichment
• Slight amount of metals (10-5Zsun)can
  induce the transition from massive to low-mass
  star formation mode.

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END
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Metallicity Effect Radiation Pressure on to Dust
Grains

• if kdgtkes, radiation pressure onto dust shell
  is more important.
• gt massive SF
• This occurs 0.01Zsun
• For Zlt0.01Zsun
• Accretion process is not changed from Z0

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Effects of UV Radiation Field
Star Formation in Small Objects (Tvir lt 104K)
(K.O. Nishi 1999)

• Only one or a few massive stars can
  photodissociate entire parental objects.
• Without H2 cooling, following star formation is
  inhibited.

Only One star is formed at a time.
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Effects of external FUV radiation
Star formation in large objects (Tvirgt104K)
K.O. Yoshii 2003
Evolution of T in the prestellar collapse
radiation JnW Bn(105K) from massive PopIII
stars

• log(W)-15
• critical value
• WltWcrit H2 formation, and cooling
• WgtWcrit no H2
• (Lya H- f-b cooling)

• Fragmentation scale
• H2 cooling clumps
• (logW lt -15)
• Mfrag2000-40Msun
• Atomic cooling clumps (logW gt -15)
• Mfrag0.3Msun

In starburst of large objects, subsolar mass Pop
III Stars can be formed.
Fragmentaion scale decreases for stronger
radiation
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Metals and Mass of Stars
10-2Zsun
0
Zsun
10-5Zsun
Massive frag.
Low-mass frag. possible
Accretion halted by dust rad force
Accretion not halted
Massive stars
Low-mass massive stars
Low-mass stars
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Critical accretion rate

• Total Luminosity (if ZAMS)

Exceeds Eddington limit if the accretion rate is
larger than
In the case that Mdot gt Mdot_crit, the stars
cannot reach the ZAMS structure with continuing
accretion.
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3D simulations for prestellar collapse

• The 3D calculations have reached ngt1012cm-3
• (radiative transfer needed for higher density
  cf. n1022cm-3 for protostars)
• Overall evolution is similar to the 1D
  calculation.

Abel, Bryan Norman 2002
Bromm Loeb 2004
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Pop I vs Pop III Star Formation
Pop I core Mstar 10-3Msun Mfrag
gt0.1Msun Mdot 10-5Msun With dust grains
Pop III core Mstar 10-3Msun Mfrag
gt103Msun Mdot 10-2Msun No dust grain
Massive stars (gt10Msun) are difficult to form.
Accretion continues. Very massive star formation
(100-1000Msun)
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Metals from the First SNe
Heger, Baraffe, Woosley 2001
PISN
SN II
Two windows

• Type II SN 8-25Msun
• Pair-instability SN 150-250Msun

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Metals and Dusts from the First Stars
Schneider, Inoue, K.O., Ferrara in prep.
Progenitor SN II (22Msun)
Progenitor PISN (195Msun)

• Dust from SNe (c.f. present-day dust from AGB
  stars)
• larger area per unit dust mass (smaller radius)
• more refractory composition (silicates,
  amorphous carbon)

Becomes important even with smaller amount of dust
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Scenario of Present-day Star Formation
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How much is the accretion rate onto the first
protostars?

• Several groups found
• similar accretion rates.
• The rate is very high 0.01Msun/yr
• because of high prestellar temperature 300
  K
• (c.f.10-6-10-5Msun/yr
• for the present-day case)
• The rate decreases with time.

Glover (2004)
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Key Observations

• Early reionization of IGM
• te0.17 zreion17 (WMAP)
• caused by first stars?
• Number and abundance pattern of metal poor stars
  ( Fe/H -5 -2 )

So far still very limited !!!
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Before the First Stars
SIMPLE

• Cosmological initial condition (well-defined)
• Pristine H, He gas, no dusts, no radiation field
  (except CMB), CR
• simple chemistry and thermal process
• No magnetic field (simple dynamics)

After the First Stars
COMPLICATED

• Feedback (SN, stellar wind) turbulent ISM
• metal /dust enriched gas
• radiation field (except CMB), CR
• complicated microphysics
• magnetic field MHD

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Pop III Dense Cores to Protostars Dynamical
Evolution

(K.O. Nishi 1998)

• self-similar collapse up to n1020cm-3
• protostar formation
• state 6 n1022cm-3, T30000K,
  Mstar10-3Msun
• (very similar to Pop I
  protostars )