A New SX Phoenicis Star in the Globular Cluster M15

Abstract

A new SX Phoenicis star (labeled SXP 1) found from BV CCD photometry is the first to be discovered in the globular cluster M15. It is a blue straggler and is located 1028 north and 2856 west of the center of M15. The mean magnitudes of SXP 1 are B = 18.671 and V = 18.445. The amplitude of variability of SXP 1 is measured to be ΔV ≈ 0.15. From multiple-frequency analysis based on the Fourier decomposition method, we detect two very closely separated pulsation frequencies: the primary frequency at f1 = 24.630 cycles day-1 for both B and V bands, and the secondary frequency at f2 = 24.338 cycles day-1 for the B band and 24.343 cycles day-1 for the V band. This star is the second among known SX Phoenicis stars found to pulsate with very closely separated frequencies (f2/f1 ≥ 0.95). These frequencies may be explained by excitation of nonradial modes; however, we have an incomplete understanding of this phenomenon in the case of SX Phoenicis stars with relatively high amplitudes. The relations between metallicity and period and between the variability amplitude and period for SXP 1 are found to be consistent with those for SX Phoenicis stars in other globular clusters.