[Paper Review] The extreme r-element rich, iron-poor halo giant CS31082-001: Implications for the r-process site(s) and radioactive cosmochronology

V. Hill, B. Plez+11 more|ArXiv.org|Mar 26, 2002

Stellar, planetary, and galactic studiesPhysics and Astronomy282 citations

TL;DR

This study presents the first precise uranium abundance measurement in a metal-poor halo giant, CS 31082-001, using high-resolution VLT/UVES spectroscopy. It reveals a significant deviation in actinide region nucleosynthesis between stars, invalidating the Th/Eu chronometer for this object but confirming U/Th as a robust age indicator, yielding an estimated age of 14.0 ± 2.4 Gyr.

ABSTRACT

We present a high-resolution spectroscopic analysis of the bright (V=11.7), extreme halo giant CS31082-001 ([Fe/H] = -2.9), obtained in an ESO-VLT Large Programme dedicated to very metal-poor stars. We find CS31082-001 to be extremely rich in r-process elements, comparable in this respect only to the similarly metal-poor, but carbon-enriched, giant CS22892-052. As a result of the extreme overabundance of the heaviest r-process elements, and negligible blending from CH and CN molecular lines, a reliable measurement is obtained of the U II line at 386 nm, for the first time in a halo star, along with numerous lines of Th II, as well as lines of 25 other r-process elements. Abundance estimates for a total of 43 elements are reported in CS31082-001, almost half of the entire periodic table. All elements with 56 \leq Z \leq 72 follow the Solar r-element pattern, reduced by about 1.25 dex ([r/Fe]=+1.7 dex, a factor 50). Pb, in contrast, seems to be below the shifted Solar r-process distribution, possibly indicating an error in the latter, while thorium is more enhanced than the lighter nuclides. Thus, while a universal production ratio for the r-process elements seems to hold in the interval 56 \leq Z \leq 72, it breaks down in the actinide region. When available, the U/Th is thus preferable to Th/Eu for radioactive dating: (i) because of its faster decay rate and smaller sensitivity to observational errors, and (ii) because the inital production ratio of the neighboring nuclides 238U and 232Th is more robustly predicted than the 151Eu/232Th ratio. Our current best estimate for the age of CS31082-001 is 14.0+/-2.4 Gyr.

Motivation & Objective

• To determine the abundance of uranium and other r-process elements in the extremely metal-poor, r-element-rich halo giant CS 31082-001.
• To assess the reliability of the Th/Eu and U/Th chronometers for dating the oldest stars in the Galaxy.
• To investigate whether variations in r-process nucleosynthesis are intrinsic to individual stars or influenced by environmental factors.
• To test the universality of r-process production ratios, particularly in the actinide region (Z ≥ 90), across different metal-poor stars.
• To provide a benchmark for future high-resolution UV observations of decay products like Pb and Bi in this star.

Proposed method

• High-resolution (R = 75,000) and high signal-to-noise (S/N ~ 500) spectroscopy of CS 31082-001 using the VLT/UVES instrument.
• Spectral analysis using local thermodynamic equilibrium (LTE) models with derived atmospheric parameters: Teff = 4825 ± 50 K, log g = 1.5 ± 0.3, [Fe/H] = -2.9 ± 0.1, and microturbulence = 1.8 ± 0.2 km/s.
• Abundance determination for 43 elements (including U, Th, Eu, Pb, Bi) via line profile fitting and atomic data from the literature, with corrections for molecular blending where applicable.
• Calculation of the U/Th abundance ratio using the measured line strengths and updated oscillator strengths for the U II line at 386 nm.
• Age estimation using the U/Th decay system, assuming a known initial production ratio and applying the decay law: N_U/N_Th = N_0,U/N_0,Th × exp(-λ_U t) / exp(-λ_Th t).
• Comparison of observed U/Th and Th/Eu ratios with solar system values and with those in CS 22892-052 to assess consistency and universality of r-process yields.

Experimental results

Research questions

• RQ1 What is the precise abundance of uranium in the metal-poor halo giant CS 31082-001, and can it be reliably measured for the first time in a halo star?
• RQ2 Does the Th/Eu abundance ratio in CS 31082-001 support a universal r-process production ratio, or does it indicate star-to-star variation in nucleosynthesis?
• RQ3 Is the U/Th ratio a more robust age indicator than Th/Eu in extremely metal-poor stars, particularly when the initial production ratio is uncertain?
• RQ4 How do the observed r-process element abundances in CS 31082-001 compare to the solar r-process pattern, especially in the actinide region (Z ≥ 90)?
• RQ5 What constraints do the observed U/Th and Th/Eu ratios place on the age of CS 31082-001, and how do they compare to the age derived from other stars like CS 22892-052?

Key findings

• The U II line at 386 nm was measured for the first time in a halo star, enabling a direct determination of uranium abundance in CS 31082-001.
• The measured U/Th abundance ratio is log(Th/Eu) = -0.22 ± 0.07, significantly higher than in the Solar System (-0.46) or in CS 22892-052 (-0.66), indicating a different actinide production ratio.
• The age of CS 31082-001 is estimated at 14.0 ± 2.4 Gyr based on the U/Th ratio, assuming a universal initial U/Th production ratio.
• The Th/Eu chronometer fails for this star if the same initial ratio as in CS 22892-052 or the Solar System is assumed, implying a negative age, which is unphysical.
• The U/Th ratio is more robust than Th/Eu for age dating because 238U and 232Th have similar nuclear structures and a more stable production ratio during the r-process.
• The observed U/Th ratio is consistent with the upper limit in CS 22892-052 (U/Th ≤ -0.94), supporting the age consistency between the two stars despite differences in overall Th abundance.