Chemical compositions of extragalactic systems: beyond α ...Methods used in C & N abundance...

Chemical compositions of extragalactic systems: beyond α and

Fe peak elements

Ricardo SchiavonLiverpool John Moores University

IAU Symp 311, Oxford, 22 July, 2014

Wednesday, July 23, 14

Focus of this Talk

• Motivation

• Methodology

• Consistency checks

• Directions for improvement


Motivation

Disk stars Halo starsDamped Ly-α systems

Pettini et al. (2008)

Elements formed in different nucleosynthetic may probe time scales other than those characteristic of SN II / SN Ia


Motivation

Disk stars Halo starsDamped Ly-α systems

Pettini et al. (2008)

If timescales are set by stellar evolution, constraints can be placed on models of galaxy formation


State of the art

• Elements that have been studied so far: C, N, Na, Ba, Sr (in galaxies) and r-process elements in extragalactic GCs

• Focus of this talk on C and N


SP Synthesis in a nutshell

Stellar observables:•Spectra- Empirical- Synthetic

•Magnitudes•Line indices

Stellar evolution theory:•Isochrones•Lifetimes

IMF

IntegratedStellar Population

Properties

Stellar Physics


SP Synthesis in a nutshell

Stellar observables:•Spectra- Empirical- Synthetic

•Magnitudes•Line indices

Stellar evolution theory:•Isochrones•Lifetimes

IMF

IntegratedStellar Population

Properties


Stellar Evolution

• Small impact from C, mostly at “young” ages

• Negligible impact from N, zero from other non-α• O, Si, and Mg are important, though!

Dotter et al. (2008)

See also Vandenberg et al. (2012)

C enrichedSolar Solar

N enriched


Stellar atmospheres & spectra: BLUE

C and N are among the most spectroscopically active elements. Strong impact on SP spectra!

Conroy et al. (2014)

% c

hang

e of

BLU

E sp

ectr

um o

f a 1

3 G

yr s

olar

met

allic

ity

stel

lar

popu

latio

n fo

r +

0.3

dex

incr

emen

t in

ele

men

tal

abun

danc

e (+

0.15

in t

he c

ase

of C

)


C and N are among the most spectroscopically active elements. Strong impact on SP spectra!


% c

hang

e of

RED

spe

ctru

m o

f a 1

3 G

yr s

olar

met

allic

ity

stel

lar

popu

latio

n fo

r +

0.3

dex

incr

emen

t in

ele

men

tal

abun

danc

e (+

0.15

in t

he c

ase

of C

)

Stellar atmospheres & spectra: RED


Coelho et al. (2005)

Uncertainties in log gfs (mostly) and model atmospheres prevent adoption of purely synthetic models to be used in accurate abundances

Uncertainties in synthetic spectra

SynthesisObservation


In view of these inaccuracies, one resorts to a hybrid approach, by which:

-- Stellar population spectra or index strengths are based on an empirical library

-- Spectrum synthesis is used differentially to correct spectra for variable abundance ratios

-- The hope is that spectrum synthesis inaccuracies are less important in a differential sense

Hybrid Approach


Methods used in C & N abundance measurements

• Graves & Schiavon 2008 (EZ_Ages): line indices, empirical stellar library, spectrum synthesis corrections, isochrones (e.g., Schiavon 2007, Graves et al. 2007, Smith et al., Price et al., etc)

• Thomas et al. 2011 (T10,J12) : line indices, empirical stellar library, spectrum synthesis corrections, fuel consumption theorem

• Worthey et al. 2011 (W13): line indices, empirical stellar library, detailed spectrum synthesis corrections, chemistry considered in isochrones

• Conroy et al. 2012: spectral fits, empirical stellar library, detailed spectrum synthesis corrections, limited isochrones


Other Promising Methods

• Colucci, Greenstein & McWilliam

• Soren Larsen

Analysis of high resolution spectra (R ∼ a few 10,000s), using spectrum synthesis and curve of growth methods

Applied to extragalactic clusters only

Abundances of multiple elements, including Fe peak, α, neutron capture


Comparison across groups

Comparison between Thomas et al. and Schiavon for Galactic GCs.

In general, good agreement



Comparison between Thomas et al. and Schiavon for Galactic GCs.

In general, good agreement, except for C/Fe

Needs to be understood



Comparison of abundances of M31 globular clusters from Schiavon et al. (Lick indices) and Colucci et al. (based on high resolution HIRES spectra). Excellent agreement!



Agreement within 0.1-0.2 dex for abundances and ratios




But important differences remain!



State of the art SAMs + GCE

State of the art models cannot match the N/O vs mass relation. Is that a problem with N yields, or some more fundamental issue with

models?

Yates et al. (2014)


N/O vs σ

The slope of the correlation between N/O and σ (and O/H) can help decide between primary and secondary N production, which

may help constrain star formation timescales.But there is disagreement between different groups

Conroy et al. (2014)Johansson et al. (2012)


N/O vs σ

Are these disagreements due to sample differences?Needs checking


“Morphological” sample“Passive” sample


N/O vs σ

Are these disagreements due to the fact that “O” means different things for these different models?

Needs checking


“Morphological” sample“Passive” sample


Summary• Need to address fundamental uncertainties from stellar

evolution theory (e.g., TP-AGB and HB phase)

• Need to improve theoretical stellar spectra: more accurate line opacities, and also better model atmospheres

• Improved and extended empirical libraries

These efforts will have a broad impact on our understanding of stars, clusters and galaxies and are

within reach


Summary

An EXCELLENT starting point:

• A consistency check via application of various methods/models to a common data set, representative of all systems of scientific interest (e.g., globular clusters, massive galaxies)

• Construction of a data set to enable good quality reality checks (using open and globular clusters, mostly)


A Provocative Result• It’s been known that

GCs have multiple stellar populations for about a decade now

• N abundances in M31 GCs correlate with mass

• N-rich stars found in the field. formed in GCs

• Are GC-like systems (or their precursors) the sites of N formation in galaxies?


Chemical compositions of extragalactic systems: beyond α ...Methods used in C & N abundance...

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