THE MOST METAL-POOR STARS. IV. THE TWO POPULATIONS WITH [Fe/H] ≲ −3.0

By J. Norris, David Yong, M. Bessell, N. Christlieb, Martin Asplund, G. Gilmore, R. Wyse, Timothy C. Beers, Paul Barklem, A. Frebel and 1 more. 2012

140 citations recorded by the source corpus.

We discuss the carbon-normal and carbon-rich populations of Galactic halo stars having [Fe/H] ≲ −3.0, utilizing chemical abundances from high-resolution, high signal-to-noise model-atmosphere analyses. The C-rich population represents ∼28% of stars below [Fe/H] = −3.1, with the present C-rich sample comprising 16 CEMP-no stars, and two others with [Fe/H] ∼ −5.5 and uncertain classification. The population is O-rich ([O/Fe] ≳ +1.5); the light elements Na, Mg, and Al are enhanced relative to Fe in half the sample; and for Z > 20 (Ca) there is little evidence for enhancements relative to solar values. These results are best explained in terms of the admixing and processing of material from H-burning and He-burning regions as achieved by nucleosynthesis in zero-heavy-element models in the literature of “mixing and fallback” supernovae (SNe); of rotating, massive, and intermediate-mass stars; and of Type II SNe with relativistic jets. The available (limited) radial velocities offer little support for the C-rich stars with [Fe/H] < −3.1 being binary. More data are required before one could conclude that binarity is key to an understanding of this population. We suggest that the C-rich and C-normal populations result from two different gas cooling channels in the very early universe of material that formed the progenitors of the two populations. The first was cooling by fine-structure line transitions of C ii and O i (to form the C-rich population); the second, while not well defined (perhaps dust-induced cooling?), led to the C-normal group. In this scenario, the C-rich population contains the oldest stars currently observed.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/f933fa60a565cae5e185d71991629c9fd3a8a6ab

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In Astronomy and Astrophysics

1. A. Helmi
2. M. Walker
3. Timothy C. Beers
4. J. Norris
5. James S. Bullock
6. A. Frebel
7. R. Hirschi
8. Hans-Walter Rix

Citations in the corpus

• CARBON-ENHANCED METAL-POOR STARS IN SDSS/SEGUE. I. CARBON ABUNDANCE ESTIMATION AND FREQUENCY OF CEMP STARS (2013, background): “…with peculiar chemical patterns, such as strong enrichments or deficiencies of light elements such as C, N, O, Na, Mg, Si, etc. (e.g., McWilliam et al. 1995; Ryan et al. 1996; Norris et al. 2001, 2013; Johnson 2002; Cayrel et al. 2004; Aoki et al. 2008, 2013; Cohen et al. 2008; Lai et al. 2008).”
• Near-Field Cosmology with Extremely Metal-Poor Stars (2015, background): “Taken together, these data led Norris et al. (2013) to suggest that there are two distinct stellar populations below [Fe/H] = −3.0: one C-rich, the other C-normal.”
• The Pristine survey - I. Mining the Galaxy for the most metal-poor stars (2017, background): “…power of new observations in this field, it seemed that, based on the handful of stars found in this regime, all ultra iron-poor stars showed a very high carbon abundance, typically [C/Fe]>1.0, up to [C/Fe]>4.0 (see for instance the compilations in Spite et al. 2013; Norris et al. 2013).”
• DETAILED CHEMICAL ABUNDANCES IN THE r-PROCESS-RICH ULTRA-FAINT DWARF GALAXY RETICULUM 2 (2016, background): “CEMP stars with no enhancement of slow n-capture process (s-process) material, like those in Ret 2, likely reflect their natal composition (e.g., Ryan et al. 2005; Norris et al. 2013; Starkenburg et al. 2014; Hansen et al. 2015a,c).”
• s-process production in rotating massive stars at solar and low metallicities (2015, background): “Two kinds of models are presently emerging (Norris et al. 2013): the mixing and fallback models of faint supernovae (Nomoto, Kobayashi & Tominaga 2013; Tominaga, Iwamoto & Nomoto 2014) and the models of spinstars, i.e. of massive stars with fast rotation and mass-loss (Meynet et al. 2006), a…”
• CARBON-ENHANCED METAL-POOR STAR FREQUENCIES IN THE GALAXY: CORRECTIONS FOR THE EFFECT OF EVOLUTIONARY STATUS ON CARBON ABUNDANCES (2014, background): “Carbon, on the other hand, has several different formation channels (e.g., Norris et al. 2013b) in the early universe, making it unsuitable 5 [A/B] = log(NA/NB ) − log(NA/NB ) , where N is the number density of atoms of a given element in the star ( ) and the Sun ( ), respectively. for tracing…”
• Binarity in carbon-enhanced metal-poor stars (2014, methodology): “…of their abundance patterns, focussing onthe abundances of light elements as well as their neutron-capture elements (see for instance the work of Masseron et al. 2012; Norris et al. 2013b; Ito et al. 2013; Hansen et al. 2014; Keller et al. 2014,emphasizing the importance of the lighter elements).”