Elizabeth Ann den Hartog

NEW RARE EARTH ELEMENT ABUNDANCE DISTRIBUTIONS FOR THE SUN AND FIVE r-PROCESS-RICH VERY METAL-POOR STARS

We have derived new abundances of the rare earth elements Pr, Dy, Tm, Yb, and Lu for the solar photosphere and for five very metal-poor, neutron-capture r-process-rich giant stars. The photospheric values for all five elements are in good agreement with meteoritic abundances. For the low-metallicity sample, these abundances have been combined with new Ce abundances from a companion paper, and reconsideration of a few other elements in individual stars, to produce internally consistent Ba, rare earth, and Hf (56 ≤ Z ≤ 72) element distributions. These have been used in a critical comparison between stellar and solar r-process abundance mixes.

IRON-GROUP ABUNDANCES IN THE METAL-POOR MAIN-SEQUENCE TURNOFF STAR HD 84937

We have derived new very accurate abundances of the Fe-group elements Sc through Zn (Z = 21-30) in the bright main-sequence turnoff star HD 84937, based on high-resolution spectra covering the visible and ultraviolet spectral regions. New or recent laboratory transition data for 14 species of seven elements have been used. Abundances from more than 600 lines of non-Fe species have been combined with about 550 Fe lines in HD 84937 to yield abundance ratios of high precision. The abundances have been determined from both neutral and ionized transitions, which generally are in agreement with each other. We find no substantial departures from standard LTE Saha ionization balance in this [Fe/H] = -2.32 star. Noteworthy among the abundances are: [Co/Fe] = 0.14 and [Cu/Fe] = -0.83, in agreement with past studies abundance trends in this and other low metallicity stars; and = 0.31, which has not been noted previously. A detailed examination of scandium, titanium, and vanadium abundances in large-sample spectroscopic surveys reveals that they are positively correlated in stars with [Fe/H] < -2; HD 84937 lies at the high end of this correlation. These trends constrain the synthesis mechanisms of Fe-group elements. We also examine the GCE abundance trends of the Fe-group elements, including a new nucleosynthesis model with jet-like explosion effects.

Atomic data for stellar spectroscopy: recent successes and remaining needs

Stellar chemical composition analyses provide vital insights into galactic nucleosynthesis. Atomic line data are critical inputs to stellar abundance computations. Recent lab studies have made significant progress in refining and extending knowledge of transition probabilities, isotopic wavelength shifts, and hyperfine substructure patterns for the absorption lines that are of most interest to stellar spectroscopists. The observable neutron-capture (n-capture) element species (Z 30) have been scrutinized in lab studies by several groups. For many species the uncertainties in experimental oscillator strengths are 10%, which permits detailed assessment of rapid and slow n-capture nucleosynthesis contributions. In this review, extreme examples of r-process-enriched stars in the galactic halo will be shown, which suggest that the description of observable n-capture abundances in these stars is nearly complete. Unfortunately, there are serious remaining concerns about the reliability of observed abundances of lighter elements. In particular, it is not clear that line formation in real stellar atmospheres is being modeled correctly. But for many elements with Z 30 the atomic transition data are not yet settled. Highlights will be given of some recent large improvements, with suggestions for the most important needs for the near future.

Atomic Data for Stellar Nucleosynthesis

Stellar chemical composition analyses can only yield reliable abundances if the atomic transition parameters are accurately determined. During the last couple of decades a renewed emphasis on laboratory spectroscopy has produced large sets of useful atomic transition probabilities for species of interest to stellar spectroscopists. In many cases the transition data are of such high quality that they play little part in the abundance uncertainties. We summarize the current state of atomic parameters, highlighting the areas of satisfactory progress and noting places, where further laboratory progress will be welcome.

Abundance Signatures in Halo Stars: Clues to Nucleosynthesis in the First Stars

We are using both space‐based (Hubble Space Telescope, HST) and ground‐based telescopes to make extensive studies of Galactic halo stars. These stars contain the nucleosynthesis products (from the rapid neutron capture process, i.e., the r‐process) from the earliest generations of stars ‐ the progenitors of the halo stars. The observed stellar abundance distributions—from the lightest neutron‐capture elements, such as Ge, along with some of the heaviest, including Pt—are providing new clues about the earliest Galactic r‐process nucleosynthesis. These in turn will help to identify the characteristics and nature of the first stars in the Galaxy.

FTS Branching Fraction Comparison in Hf II

New branching fraction measurements in HfII from FTS data are described and compared to independent measurements. Strong lines of HfII have been detected in low metallicity stars. Hf is a potential reference element in nucleocosmochronometry.