Unraveling the Local Group to Find the First Stars€¦ · Two-track Approach in the Field (and the...

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.