Rotation and binary rate among giant F stars

If this hypothesis is right, Am and metallic A-FIII stars need to have the same rate of binaries and a similar distribution of vsini. From our new spectroscopic data and from vsini and radial velocities in the literature, we show that it is not the case.

The metallic giant stars are often fast rotators with vsini larger than 100 km/s, while the maximum rotational velocity for Am stars is about 100 km/s. The rate of tight binaries with periods less than 1000 days is less than 30 % among metallic giants, which is incompatible with the value of 75 % for Am stars (Abt & Levy, 1985ApJS...59..229A). Therefore, the simplest way to explain the existence of giant metallic F stars is to suggest that all normal A and early F stars might go through a short "metallic" phase when they are finishing their life on the main sequence.