Rotational velocities of stars in IC 4756 and the calibration of intermediate band photometry for A stars

In a previous study of IC 4756 with intermediate band photometry, it was found that the A stars above the turn-off point did not seem to obey the absolute magnitude calibration derived by a previous author. Since such an effect in other clusters has been attributed to rotation, rotational velocities have been measured for 26 stars in IC 4756 and photometric observations of stars which are fainter and nearer to zero-age main sequence have been obtained. The rotational velocities show no relation to departures from the calibration. Thus, it is concluded that the calibration does not apply to this cluster but stellar rotation is not the cause. A new true distance modulus of Vsub(c)-Msub(v) = 6.8 is derived by extrapolating the observed photometric indices to the zero-age sequence. (author)