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ASTROPHYSICS: From Black Holesto the First Stars
(Lecture 21: Introduction to the First Stars)
Instructor: Volker BrommTA: Jarrett Johnson
The University of Texas at Austin
Astronomy 353
(Spring 2007)
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From the Dark Ages to the Cosmic Renaissance
• First Stars Transition from Simplicity to Complexity
(Larson & Bromm, Scientific American, Dec. 2001)
First Stars (Pop III)
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Initial Simplicity: Chemical Elements
Chemical Abundances (in Solar System)
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Synthesis of Chemical Elements
Big Bang: Massive Stars:
à H, He (D, Li, Be)
à C, N, O, …, U
• Before First Stars: primordial gas was pure H/He!
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Initial Simplicity: The Primordial Fireball
Cosmic Microwave Background (CMB):à Temperature map
Remove “dipole”(due to Solar System’sMotion w.r.t. CMB): à
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Initial Simplicity: Tiny Irregularities
Cosmic Microwave Background (CMB):à Temperature map
~10-3
~10-5
~10-5 Disk of Milky Way
Seeds for galaxyformation
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Initial Fluctuations: Quantum noise + Inflation
“Planck time”
(Quantum gravity???)
Photons of CMBemitted
13.7
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Dark Matter Models
Two basic models: Fluctuation Spectrum:
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The Neutrino Universe
- Q: How much mass is needed to confine (coral in)neutrinos?
- Early on (first 10,000 years), neutrinos move (almost)with speed of light (thus: `Hot Dark Matter’)
Small mass
neutrino
Normal particles
Large mass~ 1015 solar massesà mass of a cluster
of galaxies (e.g., Coma)
à Small structuresare `erased’ byneutrino free-streaming!
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Dark Matter Models
Hot Dark Matter: Cold Dark Matter:
à now known to be correct!
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The Cold Dark Matter Model:
Hierarchical Merger Tree:
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Cold Dark Matter Model: The Cosmic Web
• Big Q: What happens to cosmic gas?
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Cosmic gas (baryons): Dissipative Collapse
CDM
gas
``Dark Matter Halo’’
• Dissipation = energy loss by emitting radiation
photons
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Atomic cooling
H_2 cooling
Cooling of Primordial Gas
• Simplified physicsNo magnetic fields yet (?)No metals no dustInitial conditions given by CDM
Well-posed problem
• Problem:How to cool primordial gas?
No metals different coolingBelow 104 K, main coolant is H2
• H2 chemistryCooling sensitive to H2 abundanceH2 formed in non-equilibrium
Have to solve coupled set of rate equations
Metals
Tvir for Pop III
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Cooling via Molecular Hydrogen (H2)
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Q: Where do First Stars Form?A: In ``minihalos’’!
• Gravitational Evolution of DM
• Gas Microphysic:
- Can gas sufficiently cool?- tcool < tff (Rees-Ostriker)
• Collapse of First Luminous Objects expected:
- at: zcoll = 20 – 30
- with total mass: M ~ 106 Mo
Mass vs. redshift
Gas can cool
Gas cannotcool
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What happens inside primordial minihalos?
• Most important question: How massive werethe first stars?
M ~ 106 Mo
Massive Black Hole Stars (single or multiple)
normal IMF Top-heavy IMF
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Simulating the Formation of the First Stars:
• use state-of-the-art supercomputers
• multi-processor (parallel)
“Beowulf” machines
• “Lonestar” atTexas Advanced Computing Center
• UT Austin, J.J. Pickle Research Campus
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The Universe at the End of the Dark Ages
~ 20,000 light-years
~300 light-years
200 million years after Big Bang
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Formation of the First Star: Zooming-in
~ 75 LY3,000 LY
(Bromm, Coppi, & Larson 1999, 2002; Bromm & Loeb 2004)
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Zooming in FurtherBromm & Loeb 2004, New Astronomy, 9, 353
75 Light-years 1 Light-years
• Computer simulation with very high resolution
• Result: The First Stars were very massive! . (100 times the solar mass)
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The Crucial Role of Accretion
• Final mass depends on accretion from dust-freeEnvelope
Clump:
M~MJ
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Accretion onto a Primordial Protostar
dM/dt vs. time M vs. time
MtM 500)yr103( 6* ≈×=
6.0−∝t
4.0t∝
75.0t∝
Upper limit:
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First Stars were Massive
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First Stars: High-energy Radiation(Alvarez, Bromm, & Shapiro, Astrophysical Journal 2006)
40,000 Light-years
200 million years after Big Bang
à A bubble of ionized gas!
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Reionizing the Universe(Iliev et al. 2006)
~300 million lightyears
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The Death of the First Stars:(Heger et al. 2003)
Initial Stellar Mass
Z
Pop III
Pop I
PISN
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Physics of Pair-instability Supernovae
M ~ 140 - 260 Mo
-T>109K- ph+ph e- e+
- grav. runaway collapse
- large jump in core T- explosive nuclear burning- implosion explosion
- no compact remnant - all heavy elements dispersed- distinct nucleosynthetic pattern
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First Stars: Supernova-Explosion(Bromm, Yoshida, & Hernquist, Astrophysical Journal 2003)
~ 3
,000
Lig
ht-y
ears
• 100 times the explosion energy of normal supernova!
• Complete disruption(no remnant
left behind)!
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The James Webb Space Telescope:(NASA’s successor to the Hubble)
• Launch in ~2013
• Near IR sensitivity of ~ 1 nJy
• ~ 4’ x 4’ FOV
Direct Imaging of the First Stars
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Gamma-Ray Bursts as Probes of the First Stars:
• GRB progenitors massive stars
• GRBs expected to trace star formation
• Swift mission:
- Launched in 2004
- Sensitivity
GRBs from z > 15
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Relic from the Dawn of Time:• HE0107-5240: [Fe/H] = - 5.3 (Christlieb et al. 2002)
• How could such a low-mass star have formed ?
O8.0~ MM
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Probing the First Stars: Hobby-Eberly Telescope
• High-resolution
Spectra of oldest stars in our Milky Way
• pattern of chemical
elements tells us
about first stars and
supernovae
àààà Extending famous Sloan Sky Survey!
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Perspectives:
• Very dynamic, rapidly developingfield
• Closing the final gap in our worldview
• driven by supercomputers and our best telescopes
• Texas will play an important role (HET, theory,…)