A re-examination of the β versus (R-I) diagram for normal and metallic-lined A stars

• Published in: The Astronomical journal 1993-07, Vol.106 (1), p.383-389
• Main Authors: HAKKILA, J, MAYER, J. G
• Format: Article
• Language: English
• Subjects: Astronomy; Earth, ocean, space; Exact sciences and technology; Normal stars (by class): general or individual; Stars
• ISSN: 0004-6256; 1538-3881
• DOI: 10.1086/116647

In a photometric plot of the Stromgren beta index versus the (R-I) broadband color, Weiss (1979) has identified a 'gap' corresponding to an absence of late-type normal A main-sequence stars that is occupied by the metallic-lined A (Am) stars. This gap apparently corresponds to the (B - V) depletion observed by Bohm-Vitense and Canterna (1974), which has been explained by the onset of convection at around 7700 K. Since the beta vs (R-I) gap is limited to bright A stars with nonvariable nonpeculiar spectra, the most important observational selection effects are thought to have been removed. Reexamination of this plot is necessitated by (1) updated spectral classifications and modern photometric observations, and (2) an alternative theory involving main-sequence mass loss. The revised beta vs (R-I) plot indicates that (1) a depletion (as opposed to a 'gap') exists at about the position of the red edge of the instability strip for normal main-sequence stars, (2) normal stars in the depleted region are rapid rotators, (3) pulsating variable stars might be depleted here and elsewhere in the instability strip, and (4) Am stars are found in the depleted region and 'strong' Am stars appear to be slightly redder and more luminous than 'marginal' or 'weak' ones. The results are discussed in terms of both the onset of eonvection and the main-sequence mass-loss hypothesis. (Author (revised))