Population II Cepheids in globular clusters

Abstract

B and V data of Population II Cepheids in M2, M5, M10, M13, M80 and ω Cen are combined in order to obtain period-luminosity and period-colour relations. The mean absolute magnitudes are determined adopting as distance modulus the quantily [V(RR)-M _v(RR)], whereV(RR) is the apparent mean magnitude of RR Lyrae andM _v(RR) is the absolute magnitude derived from the transistion periodP _tr. It is shown that the scatter of the points around the (P-L) and (P-C) relations depends also from the width of the instability strip. In fact the position of the variables in the instability strip may be correlated with their amplitude in the sense that the short period variables (logP<1) which have largest amplitudes are brightest and bluest while the long period variables (logP>1) show a reversed relation.

Finally, after correcting the observed colours for the amplitude effect, period-colour-amplitude relations are derived for the both short and long period variables.