Oscillating Blue Stragglers in the Core of 47 Tucanae

Abstract

Six SX Phe variables detected in a 39 hr time series of Hubble Space Telescope Planetary Camera exposures in the F336W (U) filter are discussed. Two of these stars show both the fundamental and first-overtone modes and are among the first double-mode SX Phe stars reported in globular clusters. One of the double-mode stars shows evidence for a third mode near the expected period for the second overtone. Two of the other SX Phe variables in 47 Tuc are shown to oscillate simultaneously in the fourth and fifth radial overtones. The remaining two variables both have multiple oscillations excited which include nonradial modes. To support interpretation of these variables we provide evolutionary computations using current opacities and physics allowing comparison of the blue straggler stars (BSSs) and several cluster-magnitude diagram features for 47 Tuc. Linear nonadiabatic oscillation analysis is performed for a significant range of envelope and full evolutionary models in order to (1) establish the theoretical characteristics of SX Phe pulsations (e.g., location of primary driving and adiabatic nature), (2) establish the theoretical red and blue edges of the instability strip, and (3) derive the theoretical relations necessary for obtaining evolutionary and pulsational masses. Masses inferred from fundamental period-period ratio relations for the double-mode oscillators are well above the turnoff mass in 47 Tucanae and are consistent with expected masses for these blue stragglers based on position in the color-magnitude diagram (CMD) and comparison with theoretical evolutionary tracks. Combining the evolution and pulsation constraints results in mass estimates for the four double-mode BSSs in 47 Tuc of M = 1.35 ± 0.1 to 1.6 ± 0.2 M☉.