Validating the C I 5052.17 Å/Mg II 4481 Å Equivalent Width Ratio as a Diagnostic for F-type Lambda Boo Stars

The Lambda Boo-type stars are chemically peculiar stars with deficiencies of iron-peak elements but near-solar C, N, O, and S abundances. Since the prototype Lambda Boötis was first reported as peculiar, this group has been expanded from a small group of early A-type stars to a larger group of late B to early F-type dwarfs. Although a detailed abundance analysis that supports the Lambda Boo-like abundance pattern is the definitive confirmation of this Lambda Boo characteristic, the rapid rotation of many Lambda Boo stars generally limits how much detail can be derived from an abundance analysis. Traditionally, Lambda Boo candidates have been classified by visually examining the difference between their spectra and spectra of standard stars. Therefore, some ambiguity remains especially for mild or borderline Lambda Boo stars. This is the third paper in a series that establishes a straightforward yet reliable way to identify Lambda Boo-type stars. In previous papers, we identified line equivalent width (EW) ratios in the ultraviolet and visible regions that can distinguish Lambda Boo stars from other metal-weak stars. In this paper, we apply the visible line EW ratio diagnostic to 25 Lambda Boo candidates and carry out a detailed abundance analysis of HD 81290, an F2 star with a C I/Mg II EW ratio in the range expected for Lambda Boo-type stars. Our elemental abundance analysis results confirm HD 81290's Lambda Boo membership and demonstrate the utility of our EW ratio as a diagnostic for cooler F-type Lambda Boo stars.