Globular Clusters: testing stellar evolution and nucleosynthesis theories

Peter CottrellBeatrice Tinsley Institute, Dept of Physics & AstronomyUniversity of Canterbury, Christchurch, New Zealand

&Max Planck Institute for Astrophysics, Garching,

Germany (mid-May to mid-August)

Science issues:

• globular clusters (GC) : large samples of stars for testing stellar evolution and nucleosynthesis

• what are the variations in some key nuclear products : C, N, Na, s- & r-process elements?

• what stars are the origin of these elements?

Current and recent collaborators:

Clare Worley, Obs Côte d’Azur, France; Jeffrey Simpson, Canterbury, NZ;

Iain McDonald, Manchester, UK; Jacco van Loon, Keele, UK;

Ken Freeman, Liz Wylie de Boer, ANU, Australia

Colour Magnitude Diagram (CMD) :Stellar Evolution

Main Sequence: H-core burning

First Giant Branch: H-shell burning

Asymptotic Giant Branch: H- and He-shell burning

Lum

inos

ity

Temperature

Nuclear burning in stars

• H- (p-p and CNO-cycle) and He-burning (3

• n-capture reactions onto Fe “seeds” during late stages of evolution

• rapid, slow compared to -decay

• n sources [ 13C(,n)16O and 22Ne(,n)25Mg ]

• key products around magic peaks • s- : light (Y, Sr, Zr); heavy (La, Ba, Nd); Pb• r- : (Eu)• plus effects on other elements (C, N, Na)

Abundance distribution of the nuclides

light s- Pbheavy s-& Eu (r-)

Outline

• large scale surveys of globular clusters– statistically significant samples

• C, N, Na, s-process abundance results– Medium and high spectral resolution– AAT (Australia), SALT (South Africa), VLT (Chile)

• SALT High Resolution Spectrograph (from 2012)

Large scale surveys of globular clusters

VLT

AAT

SALT

AAT

High resolution

single object

Medium resolution

multi-object (MOS)

Fabry – Pérotspectrophotometry

Wavelength

Y, Zr & Eu abundances : fitting synthetic spectra (coloured lines) to the data ( ••• )

CN-weak/strong pairs with Na correlation

La II Nd II

47 Tuc

AAOmega data from AAT

Na, CN correlation

CMD : surface gravity (log g) vs surface temperature (Teff) increasing Na

abundance

increasing CN band strength

Worley & Cottrell, 2011

Ba abundance variation in Cen (part of Jeffrey Simpson’s PhD research)

Fig 4 of Villanova et al 2010

Fig 7 of Stanford et al 2010

Fig 2 of Marino et al 2011

increasing Ba abundance

increasing Fe abundance

R (=) 2,000 of 1500 Cen stars

van Loon et al 2007

R (=) 2,000 of 1500 Cen stars

Spectrum synthesis grid + real spectrum

Statistical test for match

van Loon et al 2007

Spectrum synthesis grid + real spectrum

[Ba/Fe] variations

C,N abundances C normal, N enhanced by ~10

observations (•••) fitted to synthetic spectrum (line)

CH molecular linesCN lines

All CN, CH molecular lines removed from synthetic spectrum (line)

CH molecular linesCN lines

Southern African Large Telescope (SALT)

is an international consortium* that has built and is operating

an 11m (91 hexagonal segment) telescope in South Africa

* South African, Polish, German, USA, UK, Indian institutions and the University of Canterbury in New Zealand

SALT image from 2010 following recalibration of post-primary optics

8 arc minute field of view

What can SALT instruments do?

SALT Robert Stobie Spectrograph (RSS)– R (= ) up to 10,000

SALT High Resolution Spectrograph (HRS)– R from 16,000 to 65,000

SALT RSS

SALT HRS

47 Tuc

CMD

Teff = 4500 K log g = 4.5

Summary

• can use globular clusters to test stellar evolution theory & stellar nucleosynthesis yields (of C, N, Na & s- and r-process elements)

• used high- & medium-resolution studies to infer abundances- 47 Tuc, Cen, but also NGC 6388, NGC 362, NGC 6752

• plans to extend to other techniques on SALT (multi-object spectroscopy, Fabry-Pérot interferometry, high-resolution spectroscopy) & to other galactic & extragalactic globular clusters