Improved Laboratory Transition Probabilities for Neutral Chromium and Re-determination of the Chromium Abundance for the Sun and Three Stars

TL;DR

This paper presents new laboratory transition probabilities for neutral chromium, leading to revised solar and stellar chromium abundances, and discusses ionization imbalance issues in stellar spectra.

Contribution

It provides improved transition probabilities for neutral chromium lines and applies them to determine updated chromium abundances in the Sun and three stars.

Findings

New transition probabilities for 263 Cr I lines.

Revised solar chromium abundance: log ε(Cr I) = 5.64.

Detected ionization imbalance between Cr I and Cr II in stars.

Abstract

Branching fraction measurements from Fourier transform spectra in conjunction with published radiative lifetimes are used to determine transition probabilities for 263 lines of neutral chromium. These laboratory values are employed to derive a new photospheric abundance for the Sun: log $\epsilon$(Cr I)$_{\odot}$ = 5.64$\pm$0.01 ($\sigma = 0.07$). These Cr I solar abundances do not exhibit any trends with line strength nor with excitation energy and there were no obvious indications of departures from LTE. In addition, oscillator strengths for singly-ionized chromium recently reported by the FERRUM Project are used to determine: log $\epsilon$(Cr II)$_{\odot}$ = 5.77$\pm$0.03 ($\sigma = 0.13$). Transition probability data are also applied to the spectra of three stars: HD 75732 (metal-rich dwarf), HD 140283 (metal-poor subgiant), and CS 22892-052 (metal-poor giant). In all of the selected stars, Cr I is found to be underabundant with respect to Cr II. The possible causes for this abundance discrepancy and apparent ionization imbalance are discussed.