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A Somewhat Philosophical Approach
Unraveling the Local Group Unraveling the Local Group to Find the First Stars to Find the First Stars
Jason TumlinsonYale Center for Astronomy and Astrophysics
The First Stars in the Universe are a major frontier of extragalactic astrophysics and driver for new
facilities such as JWST and ALMA.
Four motivations:
Understand stellar evolution at low Z & Z = 0;
Constrain the IMF in primordial gas;
Reconstruct the early mass assembly of galaxies;
Understand the origin of important and rare elements.
This field is moving from a theoretical pursuit to an empirical science, largely on the basis of data collected
at low redshift and new theoretical frameworks.
Beers & Christlieb (2005) ARA&A
VLT data - Cayrel et al. (2004) and Barklem et al. (2005)
HERES Survey - Barklem et al. (2005)
[Ba/
Fe]
82% at [Fe/H] ≤ -2.5 show r-process enrichment
What are metal-poor stars saying?
Figure from Aoki et al. (2006)
Christlieb et al. (2004)
Frebel et al. (2005)
Whatever they are saying, there is now a lot of it.
253 stars
17 elements
Much more is on the way.
How can we make efficient use of all this
information?HERES Survey - Barklem et al. (2005)
A Fundamental Disconnect
-4 -3 [Fe/H]
[Co/Fe]
[Ni/Fe]
[Zn/Fe]1
0
-1
1
0
-1
1
0
-1
Tominaga et al.(2005)courtesy K. Nomoto
“ab initio yields”
?
Two-track Approach in the Field (and the talk)
Because the observed abundance patterns involve “nucleosynthesis” and “chemical evolution”, the full synthesis
demands a set of coupled equations including:
1. “Source terms” – existence proofs for abundance patterns.
2. “Coupling” terms – construct global pattern by mixing sources within the mass accretion and star formation history, including mass function.
These tracks are mutually interactive, informative and iterative, as we search for and solve the governing equations.
We must be sure to include in these equations the proper physical context of early star formation.
The Cosmological Context for the First StarsThe Cosmological Context for the First Stars
20 15 10 5 0z
0.0
0.2
0.4
0.6
0.8
1.0C
olla
psed
Bar
yon
Fra
ctio
n
20 15 10 5 0z
0.0
0.2
0.4
0.6
0.8
1.0C
olla
psed
Bar
yon
Fra
ctio
nT
vir = 10 3– 10 4K
T vir= 104 – 105 K
Tvir > 105 K
A Compelling Need for New Theory
A model must meet these necessary, but maybe not sufficient, conditions:
STOCHASTIC, because some elements show huge scatter, and
HIERARCHICAL, because that’s the way real galaxies form.
The goal is to unify high and low-z data in a new approach to the first stars.
A New Synthesis of Chemical Evolution & Structure Formation
Tie chemical evolution to the larger picture of galaxy formation:
- The hierarchical theory of galaxy formation provides a natural approach to inhomogeneous chemical evolution, in the framework of halo merger trees.
- Each node in tree is a semi-closed box within which stellar birth, death, and ISM mixing evolve stochastically, “one-star-at-a-time”.
- Best of all, these “nodes” can be modeled as individual galaxies for direct comparisons to data at high redshift – this is also a galaxy formation code.
Tumlinson 2006, ApJ 641, 15x1011/104 K
Key Component: The Log-Normal IMF
1 10 100Mc
10-5
10-4
10-3
10-2
10-1Thu Apr 14 10:41:49 2005
σmc
STRONG
WEAK
α = -2.35
Key Result: The Metallicity Distribution Function (MDF)
-5 -4 -3 -2 -1 0[Fe/H]
0.01
0.10
1.00
10.00
100.00N
([F
e/H
])Ryan & Norris (1991)
(511/13/18)
Tue May 10 09:59:53 2005
Zcrit
Fo ≤ 1/N(<2.5) ≤ 0.0019
Tumlinson 2006
Unraveling Chemical Evolution, “One Star at a Time”
Pure Z = 0 progenitors!
PISN
Tumlinson, Venkatesan, & Shull (2004) Yields: Heger+Woosley - Data: McWilliam95, Carretta02, Cayrel04
HN
Yields: Umeda+Nomoto04 - Data from McWilliam95, Carretta02, Cayrel04
Constraints on Primordial IMF
C
B
A
Tumlinson 2006, ApJ 641, 1
The First Stars IMF(s)?
α = -2.35
Tumlinson 2006, ApJ 641, 1
Simulations
The First Stars IMF(s)?
α = -2.35
Tumlinson 2006, ApJ 641, 1
?
High-z to Low-z Connection
Three Emergent Themes
– “Ordinary massive Pop III stars – is there anything they can’t do?”
– “Physics is much too hard for physicists.”
– “What data carries the most information?”
Can “Ordinary Mass Pop III Stars” Do It All?
It has now emerged that VMS are unnecessary for– The abundance patterns of Pop II stars (Truran,
Venkatesan, Nomoto, Heger, Tumlinson, etc.) – The infrared background (Ferrara), and – Reionization (Ferrara, Ostriker).
Thus the “common picture” of Pop III star formation -massive stars forming monolithically from primordial dense cores of M > 100 Mּס, has yet to enjoy any positive evidence in the data. HOWEVER, it may be sufficient to include feedback in the formation models, or use rotation to remap the “IMF” to the “NMF” ( = nucleosynthetic mass function).
What is “Pop III”?
• The purist – “Pop III is an observers definition that extends Baade’s original categories, and so can refer strictly only to stars in the nearby Universe, if they exist”.
• The theorist – “Pop III is a theoretical heuristic that describes stars forming in minihalos with primordial composition”.
• The pragmatist – “Pop III is metal-free stars, wherever and whenever they are found”.
Some Progress Noted Here
Simulations with different techniques (AMR – Norman/Abel/O’Shea; SPH – Yoshida/Bromm) have agreed on the basic starting
conditions for primordial star formation from self-gravitating clouds.
The hope is that these converged results can now be used as the initial conditions for simulations of protostar formation, leading up to predictions of final mass, working on smaller scales with more
sophisticated physics for accretion and feedback.
“Our methods and physics will fail us before our resolution fails us.” - Matt Turk, summarizing Tom Abel
“This is not the end. It is not even the beginning of the end. But it is, perhaps, the end of the beginning.”
– Winston Churchill following the Allied victory at Casablanca.
Hard Physics I: Stellar Rotation and Nucleosynthesis
What is the role of rotation in determining the mass loss and end state of evolution for metal-free stars? As demonstrated by Nomoto,
Woosley, Vink, Meynet, rotation may be critical to determining the unique nucleosynthetic patterns that reveal Pop III.
Questions for discussion:
Do yields, and the emergent abundances, vary more with mass, or with initial rotation, or with explosion energy?
What new physics is needed?
Is there any guidance available for angular momentum and/or rotation rates to be gained from simulations and semi-analytic models of
protostar formation?
Can we reasonably hope that simulations, nucleosynthetic yields,and/or abundance studies can be mutually constraining?
Hard Physics II: Forming the Second Star
“We not only don’t know the Pop III IMF, we don’t even know the early Pop II IMF.”
- Simon Glover
The problem here is that all the simplifying assumptions for Pop III stars break down, and stars form in the presence of UV radiation, dust, magnetic fields, cosmic rays, and
metals in relative ratios that are likely far from their “solar” values.
Every single one of these is a complicated physical process with its own parametric uncertainty and varying degree
of observational constraint.
Note to observers: the IMF at [Fe/H] = -3 is critical to our interpretation of F0 from star counts in the halo, so
investigations into nucleosynthesis and binary fraction at the lowest metallicities (Johnson, Lucatello) is a
promising approach.
Hard Physics III: Metal Ejection and Mixing
“Contamination and mixing of metals is a major unsolved problem” – JPO
Following Nobel Laureate (and IAP founder) Jean Perrin’s comment that the spirit of science is to “explain a
complex visible with a simple invisible” –
What is the simple invisible that reduces mixing to its physical essence? Can we derive a simple prescription that is statistically faithful to the distribution of f(M, Z),
even if it is not comprehensively physical?
This is the core problem facing both theoretical formulations of the “second stars” and the proper
interpretation of their living descendants.
The Observers Have Homework Too
• The data we have seen presented here, for the MW and dSphs, from Li to n-capture, is beautiful and unprecedented in scope and quality.
• However, we must ask – what is the proper balance between survey size and data quality, given limited telescope time? Rare cases are valuable! (see Tim’s comments on absolute frequencies)
• As [Fe/H] -> -4, we may be seeing decoupling between different elements (Cohen, Primas) –but how can we quantify the degree of coupling, to compare to expectations (and what are the expectations?).
• The corresponding theoretical challenge is to learn how to make optimal use of all this extraordinary new information.