Recent radiative lifetime measurements accurate to +/- 5% using laser-induced fluorescence (LIF) on 43 even-parity and 15 odd-parity levels of Ce II have been combined with new branching fractions measured using a Fourier transform spectrometer (FTS) to determine transition probabilities for 921 lines of Ce II. This improved laboratory data set has been used to determine a new solar photospheric Ce abundance, log epsilon = 1.61 +/- 0.01 (sigma = 0.06 from 45 lines), a value in excellent agreement with the recommended meteoritic abundance, log epsilon = 1.61 +/- 0.02. Revised Ce abundances have also been derived for the r-process-rich metal-poor giant stars BD+17 3248, CS 22892-052, CS 31082-001, HD 115444 and HD 221170. Between 26 and 40 lines were used for determining the Ce abundance in these five stars, yielding a small statistical uncertainty of 0.01 dex similar to the Solar result. The relative abundances in the metal-poor stars of Ce and Eu, a nearly pure r-process element in the Sun, matches r-process only model predictions for Solar System material. This consistent match with small scatter over a wide range of stellar metallicities lends support to these predictions of elemental fractions. A companion paper includes an interpretation of these new precision abundance results for Ce as well as new abundance results and interpretations for Pr, Dy and Tm.
Deep Dive into Improved Laboratory Transition Probabilities for Ce II, Application to the Cerium Abundances of the Sun and Five r-process Rich, Metal-Poor Stars, and Rare Earth Lab Data.
Recent radiative lifetime measurements accurate to +/- 5% using laser-induced fluorescence (LIF) on 43 even-parity and 15 odd-parity levels of Ce II have been combined with new branching fractions measured using a Fourier transform spectrometer (FTS) to determine transition probabilities for 921 lines of Ce II. This improved laboratory data set has been used to determine a new solar photospheric Ce abundance, log epsilon = 1.61 +/- 0.01 (sigma = 0.06 from 45 lines), a value in excellent agreement with the recommended meteoritic abundance, log epsilon = 1.61 +/- 0.02. Revised Ce abundances have also been derived for the r-process-rich metal-poor giant stars BD+17 3248, CS 22892-052, CS 31082-001, HD 115444 and HD 221170. Between 26 and 40 lines were used for determining the Ce abundance in these five stars, yielding a small statistical uncertainty of 0.01 dex similar to the Solar result. The relative abundances in the metal-poor stars of Ce and Eu, a nearly pure r-process element in the
The study of elemental abundances in stellar photospheres continues to be a rich area of investigation. Halo stars are among the oldest objects in the Galaxy and these stars provide a "fossil" record of the Galactic chemical evolution. The discovery and detailed study of a class of metal-poor Galactic halo stars with variable n(eutron)-capture elemental abundances (e.g. Sneden et al. 1995, Smith et al. 1995, Cowan et al. 1996, Woolf et al. 1995, Sneden et al. 1996, Burris et al. 2000, Hill et al. 2002, Sneden et al. 2003a) is particularly significant.
Rare-Earth (RE) elements are among the most spectroscopically accessible of the n(uetron)-capture elements. The open f-shell of the RE neutral atoms and ions yields many strong lines in the visible and near-IR where spectral line blending is less of a problem than in the UV, and where ground-based observations are possible. Observations with a high Signal-to-Noise ratio (S/N) and a high spectral resolving power on an increasing number of stars are now available from large ground-based telescopes. The quality of these new astronomical data can be fully exploited only if similar quality basic spectroscopic data, especially data on transition probabilities, are available. The combination of improved astronomical data and improved laboratory data has reduced lineto-line and star-to-star scatter in abundance values for many RE elements.
Cerium is the last of the RE elements in need of additional work and it has the richest line spectra of the RE elements. Experimental work since the mid-1990s has emphasized LIF lifetime measurements (e.g. Langhans et al. 1995, Li et al. 2000, Zhang et al. 2001, Xu et al. 2003, Den Hartog & Lawler 2008). These LIF lifetimes have typically been combined with theoretical branching fractions to determine transition probabilities for individual Ce II lines (e.g. Zhang et al. 2001, Palmeri et al. 2000, Biémont & Quinet 2005).
In spite of the somewhat daunting line density, a large set of Ce II branching fractions measurements based on FTS data is now complete. The branching fraction measurements are combined with the most recent and extensive LIF radiative lifetimes from Den Hartog & Lawler (2008) to determine transition probabilities for 921 lines of Ce II reported herein.
Although progress has been made in the theoretical determination of atomic transition probabilities, a recent comparison strongly suggests that modern experimental methods yield more reliable atomic transition probabilities (Lawler et al. 2008a). The method used in this study on Ce is to combine radiative lifetimes from time-resolved laserinduced-fluorescence (TR-LIF) with emission branching fraction measurements from high resolution data recorded using a Fourier transform spectrometer (FTS). This method for measuring transition probabilities is accurate, flexible, and efficient. The systematic determination of experimental transition probabilities by combining radiative lifetimes from TR-LIF with branching fractions from emission data recorded with a FTS has played a central role in providing the basic atomic data needed for RE abundance determinations. This method yields absolute transition probabilities which are accurate to ±5% (~0.02 dex) for strong lines.
These laboratory data are applied to re-determine the Solar abundance of Ce and to refine the Ce abundance in five r-process rich, metal poor Galactic halo stars. The Appendix to this paper includes a summary to all of our RE lab data in machine readable form. This work on Ce II completes a multi-year effort to improve laboratory spectroscopic data for Rare Earth (RE) ions and to apply these data in abundance studies (Lawler et al. 2008b and references therein).
The emergence of a tightly defined r-process only abundance pattern in many very metal-poor Galactic halo stars, at least for the RE elements, has been an exciting development (e.g. Sneden et al. 2003b, Ivans et al. 2006, Lawler et al. 2006, Den Hartog et al. 2006). As this abundance pattern becomes even more tightly defined, it will: i) provide a powerful constraint on future modeling of the r-process nucleosynthesis; ii) help determine a definitive r-process site; and iii) unlock other details of the r-process and of the Galactic chemical evolution.
As discussed above, radiative lifetimes from Den Hartog & Lawler (2008) provide a foundation for this study of branching fractions and the transition probabilities of Ce II.
Branching fraction measurements were attempted on lines from all 74 levels of the lifetime experiment, and were completed for lines from 43 even-parity and 15 odd-parity upper levels. As in earlier work on other RE spectra, we used the 1.0 meter FTS at the National Solar Observatory (NSO) on Kitt Peak for Ce II branching fraction measurements. The Kitt Peak FTS has the large etendue of all interferometric spectrometers, a limit of resolution as small as 0.01 cm -1 , wavenumber accuracy to 1 part in 10 8 , broad spectral coverag
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