Convection, Thermal Bifurcation, and the Colors of A Stars

Broad-band ultraviolet photometry from the TD-1 satellite and low dispersionspectra from the short wavelength camera of IUE have been used to investigate along-standing proposal of Bohm-Vitense that the normal main sequence A- andearly-F stars may divide into two different temperature sequences: (1) a hightemperature branch (and plateau) comprised of slowly rotating convective stars,and (2) a low temperature branch populated by rapidly rotating radiative stars.We find no evidence from either dataset to support such a claim, or to confirmthe existence of an "A-star gap" in the B-V color range 0.22 <= B-V <= 0.28 dueto the sudden onset of convection. We do observe, nonetheless, a large scatterin the 1800--2000 A colors of the A-F stars, which amounts to ~0.65 mags at agiven B-V color index. The scatter is not caused by interstellar orcircumstellar reddening. A convincing case can also be made against binarityand intrinsic variability due to pulsations of delta Sct origin. We find nocorrelation with established chromospheric and coronal proxies of convection,and thus no demonstrable link to the possible onset of convection among the A-Fstars. The scatter is not instrumental. Approximately 0.4 mags of the scatteris shown to arise from individual differences in surface gravity as well as amoderate spread (factor of ~3) in heavy metal abundance and UV line blanketing.A dispersion of ~0.25 mags remains, which has no clear and obvious explanation.The most likely cause, we believe, is a residual imprecision in our correctionfor the spread in metal abundances. However, the existing data do not rule outpossible contributions from intrinsic stellar variability or from differentialUV line blanketing effects owing to a dispersion in microturbulent velocity.Comment: 40 pages, 14 figures, 1 table, AAS LaTex, to appear in The Astrophysical Journa