Among 'empirical' methods of estimating oxygen abundances in extragalactic H II regions, the use of the ratio of nebular lines of [O III] and [N II], first introduced by Alloin et al., is reappraised with modern calibration data and shown to have certain advantages over Ρ 23 ≡ ([On] + [O III])/Hβ and N2 ≡ [N II] λ6583/Hα, particularly when applied to star-forming galaxies at high redshifts.

[O III] / [N II] as an abundance indicator at high redshift

▪ Abstract We discuss the importance of very metal-poor stars to develop an understanding of the nature of the first stars that formed in the Universe and the nucleosynthesis events associated with them, as well as to refine models of galaxy formation, in particular for large spiral galaxies such as the Milky Way. After briefly reviewing the history of the search for very metal-deficient stars in the Galaxy, we summarize ongoing efforts, concentrating on the two large objective-prism surveys that have led to the discovery of the majority of stars with [Fe/H] < −2.0 known at present: the HK survey of Beers and collaborators and the Hamburg/ESO survey of Christlieb and collaborators. We then consider the wealth of information that can be gleaned from high-resolution spectroscopic study of very metal-poor stars. We close with a list of open questions and a discussion of new survey techniques that will expand the sample of recognized very metal-deficient stars in the Galaxy by several orders of magnitude.

/pdf/the-discovery-and-analysis-of-very-metal-poor-stars-in-the-2ic6fxtoys.pdf

The discovery and analysis of very metal-poor stars in the galaxy

▪ Abstract The formation and evolution of galaxies is one of the great outstanding problems of astrophysics. Within the broad context of hierachical structure formation, we have only a crude picture of how galaxies like our own came into existence. A detailed physical picture where individual stellar populations can be associated with (tagged to) elements of the protocloud is far beyond our current understanding. Important clues have begun to emerge from both the Galaxy (near-field cosmology) and the high redshift universe (far-field cosmology). Here we focus on the fossil evidence provided by the Galaxy. Detailed studies of the Galaxy lie at the core of understanding the complex processes involved in baryon dissipation. This is a necessary first step toward achieving a successful theory of galaxy formation.

/pdf/the-new-galaxy-signatures-of-its-formation-1a420kr2z3.pdf

The New Galaxy: Signatures of Its Formation

▪ Abstract We present a review of nucleosynthesis in AGB stars outlining the development of theoretical models and their relationship to observations. We focus on the new high resolution codes with...

/pdf/nucleosynthesis-in-asymptotic-giant-branch-stars-relevance-1594it3sxd.pdf

Nucleosynthesis in asymptotic giant branch stars: Relevance for galactic enrichment and solar system formation

The content of neutron-capture (trans-iron-peak) elements in the lowmetallicity Galactic halo varies widely from star to star. The differences are both in bulk amount of the neutron-capture elements with respect to lighter ones and in element-to-element ratios among themselves. Several well-defined abundance distributions have emerged that reveal characteristic rapid and slow neutron-capture nucleosynthesis patterns. In this review we summarize these observed metal-poor star’s abundances, contrasting them with the Solar-system values, comparing them to theoretical predictions, using them to assess the types of stars responsible for their specific anomalies, and speculating on the timing and nature of early Galactic nucleosynthesis.

/pdf/neutron-capture-elements-in-the-early-galaxy-12cijxx72u.pdf

Neutron-Capture Elements in the Early Galaxy

Evidence for composition gradients across the disks of spiral galaxies.

The Horizontal-Branch Stars of the Globular Cluster NGC 6397

I-band CCD images of a large area of the nearby globular cluster NGC 6397 have been used to construct a surface density profile and two luminosity and mass functions. The surface density profile extends out to 14' from the cluster center and shows no sign of a tidal cutoff. The inner profile is a power law with slope −0.8 steepening to −1.7 outside of 1'. The mass functions are for fields at 4' and 11' from the cluster center and confirm the upturn in the mass function for stars less massive than about 0.4 M ○. . There appears to be an excess of low-mass stars over higher-mass stars in the outer field with respect to the inner, in qualitative agreement with expectations for mass segregation

Star counts in NGC 6397

The Recombination Spectrum of Nebular Hydrogen.

The Spectrum of the Polarized White Dwarf GD 229

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