[Paper Review] First Stars II. Elemental abundances in the extremely metal-poor star CS 22949--037: A diagnostic of early massive supernovae

É. Depagne, V. Hill+11 more|ArXiv.org|May 15, 2002

Gamma-ray bursts and supernovaePhysics and Astronomy35 references122 citations

TL;DR

This study presents high-resolution VLT-UVES spectroscopy of the extremely metal-poor giant CS 22949–037 ([Fe/H] ≈ −4.0), revealing extreme enhancements in oxygen ([O/Fe] = 1.97 ± 0.1) and nitrogen ([N/Fe] = 2.56 ± 0.2), along with a low 12C/13C ratio (4 ± 2), indicating significant CNO processing. The data favor a 30–40 M☉ core-collapse supernova with fallback as the progenitor, rather than pair-instability supernovae, due to inconsistent odd-even and Zn abundance predictions in the latter model.

ABSTRACT

CS 22949--037 is one of the most metal-poor giants known ([Fe/H]$ \approx-4.0$), and it exhibits large overabundances of carbon and nitrogen (Norris et al.). Using VLT-UVES spectra of unprecedented quality, regarding resolution and S/N ratio, covering a wide wavelength range (from $λ= 350$ to 900 nm), we have determined abundances for 21 elements in this star over a wide range of atomic mass. The major new discovery is an exceptionally large oxygen enhancement, [O/Fe] $= 1.97\pm0.1$, as measured from the [OI] line at 630.0 nm. We find an enhancement of [N/Fe] of $2.56\pm 0.2$, and a milder one of [C/Fe] $= 1.17\pm$0.1, similar to those already reported in the literature. This implies $Z_{\star}=0.01 Z_{\odot}$. We also find carbon isotopic ratios $^{12}$C/$^{13}$C$ =4\pm2.0$ and $^{13}$C/$^{14}$N$ =0.03 ^{+0.035}_{-0.015}$, close to the equilibrium value of the CN cycle. Lithium is not detected. Na is strongly enhanced ([Na/Fe] $= +2.1 \pm 0.2$), while S and K are not detected. The silicon-burning elements Cr and Mn are underabundant, while Co and Zn are overabundant ([Zn/Fe]$ = +0.7$). Zn is measured for the first time in such an extremely metal-poor star. The abundances of the neutron-capture elements Sr, Y, and Ba are strongly decreasing with the atomic number of the element: [Sr/Fe] $\approx +0.3$, [Y/Fe] $\approx -0.1$, and [Ba/Fe] $\approx -0.6$. Among possible progenitors of CS 22949--037, we discuss the pair-instability supernovae. Such very massive objects indeed produce large amounts of oxygen, and have been found to be possible sources of primary nitrogen. Other scenarios are also discussed. A 30-40$M_{\odot}$ supernova, with fallback, seems the most likely progenitor for CS 22949--037.

Motivation & Objective

• To determine elemental abundances in the extremely metal-poor star CS 22949–037 using high-resolution, high signal-to-noise VLT-UVES spectra.
• To investigate the nucleosynthetic origin of the star's unusual abundance pattern, particularly the extreme overabundance of oxygen and nitrogen.
• To test whether the observed abundances can be explained by pair-instability supernovae or alternative core-collapse supernova models with zero or low metallicity.
• To assess the role of internal mixing processes, such as CN cycle processing, in altering surface abundances in the star or its progenitor.
• To evaluate the implications of the observed neutron-capture element trend (Sr, Y, Ba) for the neutron exposure history in the progenitor.

Proposed method

• Acquired high-resolution (R ≈ 45,000) and high signal-to-noise (S/N ≈ 100–200) UVES spectra covering 350–900 nm from the VLT at Paranal Observatory.
• Performed detailed spectral synthesis using model atmospheres and line list data to derive elemental abundances in local thermodynamic equilibrium (LTE), with corrections for non-LTE effects on Na and Al.
• Applied corrections for internal mixing processes in the star, particularly for C and N, assuming transformation via the CN cycle.
• Compared observed abundance ratios with theoretical yields from zero-metallicity supernova models, especially the Z35Z model by Heger and Woosley.
• Used the forbidden [O I] line at 630.0 nm for the first time in such a metal-poor star to derive oxygen abundance with high precision.
• Analyzed isotopic ratios (12C/13C and 13C/14N) to infer the extent of CNO processing and compare with equilibrium values.

Experimental results

Research questions

• RQ1 What is the origin of the extreme oxygen enhancement ([O/Fe] = 1.97 ± 0.1) in CS 22949–037, and can it be explained by pair-instability supernovae?
• RQ2 Why is nitrogen overabundant ([N/Fe] = 2.56 ± 0.2) in a star with such low metallicity, and what role does internal CNO processing play?
• RQ3 How do the observed trends in neutron-capture elements (Sr, Y, Ba) with atomic number compare to predictions from standard supernova yields?
• RQ4 Can the observed abundance pattern be reproduced by a single core-collapse supernova model, such as the Z35Z model, or is a multi-progenitor scenario required?
• RQ5 To what extent have surface abundances been altered by post-main-sequence mixing, particularly via the CN cycle?

Key findings

• The [O I] line at 630.0 nm was measured for the first time in an extremely metal-poor star, yielding [O/Fe] = 1.97 ± 0.1, indicating a strong oxygen overabundance.
• Nitrogen is highly enhanced with [N/Fe] = 2.56 ± 0.2, and the 12C/13C ratio is measured at 4 ± 2, consistent with near-equilibrium conditions of the CN cycle.
• Carbon is mildly enhanced with [C/Fe] = 1.17 ± 0.1, and sodium is strongly enhanced ([Na/Fe] = +2.1 ± 0.2), while sulfur and potassium remain undetected.
• The neutron-capture elements show a rapid decline with atomic number: [Sr/Fe] ≈ +0.3, [Y/Fe] ≈ −0.1, and [Ba/Fe] ≈ −0.6, suggesting a truncated neutron exposure.
• Zinc is detected for the first time in such a star, with [Zn/Fe] = +0.7, which is inconsistent with predictions from pair-instability supernova models.
• The observed abundance pattern shows fair agreement with the Z35Z zero-metallicity core-collapse supernova model of Heger and Woosley, supporting a 30–40 M☉ progenitor with fallback.