Transcript Document

Radiative and Chemical Feedback
by the First Stars
Daniel Whalen
McWilliams Fellow
Carnegie Mellon University
My Collaborators
• Joe Smidt (UC Irvine)
• Thomas McConkie (BYU)
• Brian O’Shea (MSU)
• Mike Norman (UCSD)
• Rob Hueckstaedt (LANL)
Numerical Simulations of Local Radiative
Feedback on Early Star Formation
Uniform Ionizing/LW
Backgrounds
Rad Hydro Models
of Single Halos
• Machacek, Bryan & Abel 2001,
MNRAS, 548, 509
• Susa & Umemura 2006,
• Machacek, Bryan & Abel 2003,
MNRAS, 338, 273
• Ahn & Shapiro 2007, MNRAS,
375, 881
• O’Shea, Abel, Whalen & Norman
2005, ApJL, 628, 5
• Whalen et al 2008, ApJ, 679,
925
• Mesinger, Bryan & Haiman 2006,
ApJ, 648, 835
• Whalen, Hueckstaedt &
McConkie 2010, ApJ 712,101
• Mesinger, Bryan & Haiman 2009,
MNRAS, 399, 1650
The Universe
at Redshift 20
128 kpc comoving
ZEUS-MP Reactive Flow
Radiation Hydrodynamics Code
• massively-parallel (MPI) Eulerian hydrocode with 1-, 2-,
or 3D cartesian, cylindrical, or spherical meshes
• 9-species primordial H/He gas network coupled to photon
conserving multifrequency UV transfer
• Poisson solver for gas self-gravity
• includes the dark matter potential of cosmological halos,
which remains frozen for the duration of our calculations
• 40 energy bins < 13.6 eV, 80 bins from 13.6 eV to 90 eV
• self-shielding functions of DB 96 corrected for thermal
Doppler broadening are used to compute H2 photodissociation
Adaptive Subcycling
• three characteristic timescales emerge when one couples
radiation to chemistry and hydrodynamics:
• the trick is to solve each process on its own timescale without
holding the entire algorithm hostage to the shortest time step
• procedure:
(1) while holding densities and velocities fixed, evolve
reaction network and gas energy on global min of tchem
and th/c until the global min of th/c is crossed
(2) perform density and velocity updates (the hydro) every
th/c
Parameter Space of Surveyed Halos
• We sample consecutive evolutionary stages of a single
1.35 x 105 solar mass halo rather than the entire cluster
at a single redshift
• Since halos in the cluster tend to be coeval, exposing
just one at several central densities spans the range of
feedback better than a few at roughly the same density
• We chose this halo mass because it is the smallest in
which we expect star formation, so feedback would be
less prominent than in a more massive halo
Spherically-Averaged Enzo AMR Code Halo Radial Density and
Velocity Profiles (O’Shea & Norman 2007b)
z = 23.9, 17.7, 15.6 and 15.0
Evolution of Halo Cores in the Absence of Radiation
Halo Photoevaporation Model Grid
023_500pc: complete disruption
I-Front Structure
monoenergetic:
20 - 30 mfp
e , T
105 K blackbody:
e , T
T-Front
quasar:
e , T
secondary ionizations by photoelectrons
25 Msol
40 Msol
60 Msol
Whalen et al 2008,
ApJ, 682, 49
Whalen et al 2010,
ApJ, 712, 101
80 Msol
120 Msol
Local Radiative Feedback
• due to coeval nature of halos within the cluster, feedback
tends to be positive or neutral
• halos with nc > 100 cm-3 will survive photoevaporation
and host star formation (accelerated in many instances)
• feedback sign is better parameterized by central halo
density than halo mass
• radiation drives chemistry that is key to the hydrodynamics
of the halo -- multifrequency transfer is a must
• these results are mostly independent of the spectrum of
the illuminating star--more LW photons don’t make much
difference
Nucleosynthetic Forensics
of the First Stars
Daniel Whalen
Candace Joggerst
2010, ApJ, 709, 11
2011, ApJ, 728, 129
Our Collaborators
• Ann Almgren (LBNL)
• John Bell (LBNL)
• Alexander Heger (University of Minnesota)
• Stan Woosley (UC Santa Cruz)
The Primordial IMF Remains Unconstrained
Numerical simulations, although proceeding from well-posed
initial conditions, lack the physics to model star formation up
to the main sequence (and likely diverge from reality well
before)
Direct observation of primordial supernovae, in concert with
gravitational lensing, may be possible with JWST (Kasen &
Woosley 2010; Whalen et al 2011a,b,c, in prep)
Stellar archaeology, in which we search for the
nucleosynthetic imprint of Pop III stars on low-mass
subsequent generations that survive today, is our best
bet for indirectly constraining the primordial IMF
Final Fates of the First Stars
Heger & Woosley 2002, ApJ 567, 532
Stellar Archaeology: EMP and HMP Stars
• Hyper Metal-Poor (HMP) Stars:
-5 < [Fe/H] < -4  thought to be enriched by one or
a few SNe
• Extremely Metal-Poor (EMP) Stars:
-4 < [Fe/H] <-3  thought to be enriched by an entire
population of SNe because of the
small scatter in their chemical
abundances
No PISN?
• original non-rotating stellar
evolution models predict a
strong ‘odd-even’ nucleosynthetic
signature in PISN element
production
• to date, this effect has not been
found in any of the EMP/HMP
surveys
• intriguing, but not conclusive,
evidence that Pop III stars had
lower initial masses than suggested
by simulation
• this has directed explosion models
towards lower-mass stars
Heger & Woosley 2002
2D Rotating Progenitor Pop III Explosion
Models
• progenitors evolved in the 1D KEPLER
stellar evolution code, exploded, and then
followed to the end of nucleosynthetic burn
(~ 100 sec)
• KEPLER profiles then mapped into the new
CASTRO AMR code and then evolved in 2D
out to shock breakout from the star
• 2 rotation rates, 3 explosion energies, 3
masses, and 2 metallicities, for a total of 36
models
• self-gravity of the gas plus the gravity of the
compact remnant (the latter is crucial for
capturing fallback)
CASTRO Code (Almgren et al 2009)
Mixing & Fallback vs Mass,
Eex, and Metallicity
• mixing falls and fallback rises
with increasing mass
• mixing increases and fallback
decreases with rising Eex
• mixing and fallback rise with
metallicity
Elemental Yield Comparison to HMP Stars
IMF-Averaged Yields and the EMP Stars
Mixing in 150 –
250 Msol Pop III
PI SNe
Joggerst & Whalen 2011,
ApJ, 728, 129
The Nucleosynthetic Imprint of 15 – 40 Msol Pop III
Stars on the First Generations
• 15 – 40 M rotating Pop III progenitors are a good fit to one of the
HMP chemical abundances but not the other two
• an IMF-average of the chemical yields of our zero-metallicity models
provides a good fit to the abundances Cayrel et al 2004 and Lai et al
2008 measured in their EMP star surveys
• since EMP stars are imprinted by well-established populations of
SNe, they are a better metric of primordial SN progenitors
• direct comparison of nucleosynthetic yields from numerical models
to the abundances of MP stars is problematic—intervening
hydrodynamical processes complicate elemental uptake into new
stars
• newly completed surveys (like SEGUE-II) will greatly expand our sample
of metal-poor stars and yield great insights into the primordial IMF
Thanks!