STScI Preprint #1274

Oscillating Blue Stragglers in the Core of 47 Tucanae

Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS 5-26555

Six SX Phe variables detected in a 39-hour 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 Stragglers 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: (a) establish the theoretical characteristics of SX Phe pulsations (e.g., location of primary driving and adiabatic nature), (b) establish the theoretical red and blue edges of the instability strip, (c) 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 CMD and comparison with theoretical evolutionary tracks. Combining the evolution and pulsation constraints results in mass estimates for the four double-mode BSS in 47 Tuc of M = 1.35 ± 0.1 to 1.6± 0.2 M.

Status:

Appeared in: The Astrophysical Journal, 507:818-845, 1998

Affiliations:

¹) Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
²) Osservatorio Astronomico di Trieste, via G.B. Tiepolo 11, 34131 Trieste, Italy
³) Present address: Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138;
⁴) Osservatorio Astronomico di Capodimonte, Via Moiariello 16, 80131 Napoli, Italy
⁵) Osservatorio Astronomico di Teramo, Via M. Maggini, 64100 Teramo, Italy;
⁶) Present address: National Optical Astronomy Observatories, P.O. Box 26732, 950 N. Cherry, Tucson, AZ, 85726;