Formation of the first and second generation stars
description
Transcript of Formation of the first and second generation stars
Kazu Omukai (NAOJ)
Formation of the first and second generation stars
Aug. 17 @ Tartu Workshop
Outline
Formation of the First Stars Why they are supposed to be very massive (100-1000Msun)?
Formation of the 2nd-generation starsWhen and how did the transition to low-mass stars occur ?
“Second-generation” includes2nd-gen. zero-metal starsExtremely metal-poor stars
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)
2. Fragmentationof the First Objects
Scenario of the First Star Formation
1. Formationof the First Object
3. Collapse of Dense Cores: Formation of Protostar
4. Accretion of ambient gas andRelaxation to Main Sequence Star
3D similation (Bromm et al. 2001; Abel et al. 2002)filamentary clouds (Nakamura & Umemura 2001)
Bromm et al.. 2001
Typical mass scale of fragmentation;
Dense cores
~a few x 102-103Msun
Fragmentation of First Objects: Formation of Dense Cores
These massive cores will collapse and form protostars.
First Star Accretion(1) 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)
Protostellar Evolution with Accretion
Protostellar Radius yrMM /101.1 ,2.2 ,4.4 ,8.8 3
3 a, ZAMS
3b、 expansion
2, KH
contr.
1、 adiabatic phase
tKH >tacc
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 <Mdotcrit=4x10-3Msun/yr
(if >Mdotcrit , no stationary solution for >~100Msun)
(K.O. & Palla 2003)
Protostellar Evolution with ABN(2002) 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)
Evolution of radius under the ABN accretion rate
Mass of First Stars
Mstar=min( Mfrag, Mdot tOB, Mfeedback)
Mfrag:fragmentation mass ~1000Msun
Mdot:accretion rate ~10-3Msun
tOB: massive star lifetime ~106yr Mdot tOB ~1000Msun
Mfeedback: mass of star when the accretion is halted by stellar feedback > 100Msun
Mstar=100-1000Msun
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
ionized
Star formation
in fossil HII regions
After the death of the exciting star, star formation restarts inside the fossil HII region.
(Oh & Haiman 2004;Nagakura & K.O. 2005)
ionizedneutral
Star formation in fossil HII regions
High ionization degree facilitates the formation of H2 and HD.
HD cooling T~30K
low-mass star formation (<Msun; e.g. Uehara & Inutsuka 1999).
(Nagakura & K.O. 2005)
Temperature evolution
Chemical evolution
Metallicity Effects
Omukai, Tsuribe, Schneider & Ferrara (2005)
1
1) Dust cooling: [Z/H]~-5
2
2) H2 formation on dust: [Z/H]~-4
3
3) metal-line cooling: [Z/H]~-3
Metals and Fragmentation scales
Formation of massive fragments by H2 cooling continues until some metallicity, say Z~10-5Zsun
For higher metallicity, sub-solar mass fragmentation is possible by dust cooling.
Schneider, Ferrara, Natarajan, & K.O. (2002)
ConclusionMetal-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 tr
ansition from massive to low-mass star formation mode.
END
Metallicity Effect: Radiation Pressure on to Dust Grains
ifd>es, radiation pressure onto dust shell is more important.
=> massive SF This occurs ~0.01Zsun
For Z<0.01Zsun
Accretion process is not changed from Z=0
Effects of UV Radiation Field
Only one or a few massive stars can photodissociate entire parental objects.
Without H2 cooling, following star formation is inhibited.
(K.O. & Nishi 1999)
Star Formation in Small Objects (Tvir < 104K)
Photodissociation
Only One star is formed at a time.
Effects of external FUV radiation
log(W)=-15 ; critical value
W < Wcrit H2 formation, and cooling
W>Wcrit no H2
( Lyα –– H- f-b cooling)
Evolution of T in the prestellar collapse
radiation : J=W B(105K) from massive PopIII stars
Star formation in large objects (Tvir>104K)
Fragmentation scale H2 cooling clumps ( logW < -15 )
Mfrag~2000-40Msun
Atomic cooling clumps (logW > -15)
Mfrag~0.3Msun
Fragmentaion scale decreases for stronger radiation
Fragmentaion scale vs UV intensity
In starburst of large objects, subsolar mass Pop III Stars can be formed.
K.O. & Yoshii 2003
Metals and Mass of Stars
0 Zsun10-5Zsun 10-2Zsun
Massive frag. Low-mass frag. possible
Accretion not haltedAccretion halted by dust rad force
Massive stars Low-mass & massivestars
Low-massstars
Critical accretion rate
Total Luminosity (if ZAMS)
ZAMSZAMStot RMGMLL /
Exceeds Eddington limit if the accretion rate is larger than
yrM
RLLcM esZAMSEddZAMScrit
/104
/)/1(43-
In the case that Mdot > Mdot_crit, the stars cannot reach the ZAMS structure with continuing accretion.
3D simulations for prestellar collapse
The 3D calculations have reached n>1012
cm-3 (radiative transfer needed for
higher density; cf. n~1022cm-3 for protostars)
Overall evolution is similar to the 1D calculation.
Abel, Bryan & Norman 2002
Bromm & Loeb 2004
Pop I vs Pop III Star Formation
Pop I coreMstar : 10-3Msun
Mfrag: >0.1Msun
Mdot: 10-5Msun
With dust grains
Pop III coreMstar : 10-3Msun
Mfrag : >103Msun
Mdot : 10-2Msun
No dust grain Massive stars (>10Msun)are difficult to form.
Accretion continues.Very massive star formation (100-1000Msun)
Metals from the First SNe
Type II SN 8-25Msun
Pair-instability SN 150-250Msun
Heger, Baraffe, Woosley 2001
PISNSN II
Two windows
Metals and Dusts from the First Stars
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
Schneider, Inoue, K.O., Ferrara in prep.
Scenario of Present-day Star Formation My talk covers these phases.
How much is the accretion rate onto the first protostars?
Several groups found
similar accretion rates.
The rate is very high ~0.01Ms
un/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)
Key Observations
Early reionization of IGM
ezreion=17 (WMAP)
caused by first stars?
Number and abundance pattern of metal poor stars ( [Fe/H] = -5 – -2 )
So far still very limited !!!
Before the First StarsCosmological 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 StarsFeedback (SN, stellar wind) turbulent ISMmetal /dust enriched gas radiation field (except CMB), CR
complicated microphysics magnetic field MHD
SIMPLE
COMPLICATED
( K.O. & Nishi 1998) self-similar collapseup to n~1020cm-3
protostar formation
state 6; n~1022cm-3, T~30000K, Mstar~10-3Msun
( very similar to Pop I protostars )
Pop III Dense Cores to Protostars: Dynamical Evolution