Detection of Pb II in the Ultraviolet Spectra of Three Metal-poor Stars
Abstract
We report the first detection of the Pb II line at 2203.534 Šin three metal-poor stars, using ultraviolet spectra obtained with the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope. We perform a standard abundance analysis assuming local thermodynamic equilibrium (LTE) to derive lead (Pb, Z = 82) abundances. The Pb II line yields a higher abundance than Pb I lines by +0.36 ± 0.34 dex and +0.49 ± 0.28 dex in the stars HD 94028 and HD 196944, where Pb I lines had been detected previously. The Pb II line is likely formed in LTE, and these offsets affirm previous calculations showing that Pb I lines commonly used as abundance indicators underestimate the Pb abundance in LTE. Pb is enhanced in the s-process-enriched stars HD 94028 ([Pb/Fe] = +0.95 ± 0.14) and HD 196944 ([Pb/Fe] = +2.28 ± 0.23), and we show that 208Pb is the dominant Pb isotope in these two stars. The $\mathrm{log}\varepsilon $ (Pb/Eu) ratio in the r-process-enhanced star HD 222925 is 0.76 ± 0.14, which matches the solar system r-process ratio and indicates that the solar system r-process residuals for Pb are, in aggregate, correct. The Th/Pb chronometer in HD 222925 yields an age of 8.2 ± 5.8 Gyr, and we highlight the potential of the Th/Pb chronometer as a relatively model-insensitive age indicator in r-process-enhanced stars. * Based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute (STScI), which is operated by the Association of Universities for Research in Astronomy, Inc. (AURA) under NASA contract NAS 5-26555. These observations are associated with programs GO-14161, GO-14765, and GO-15657. This Letter includes data taken at The McDonald Observatory of The University of Texas at Austin.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- October 2020
- DOI:
- arXiv:
- arXiv:2009.14251
- Bibcode:
- 2020ApJ...902L..24R
- Keywords:
-
- Nucleosynthesis;
- R-process;
- S-process;
- Stellar abundances;
- Ultraviolet astronomy;
- 1131;
- 1324;
- 1419;
- 1577;
- 1736;
- Astrophysics - Solar and Stellar Astrophysics;
- Astrophysics - Astrophysics of Galaxies
- E-Print:
- Accepted for publication in the Astrophysical Journal Letters (7 pages, 2 figures)
