SX-PHOENECIS STARS IN THE EXTREMELY METAL-POOR GLOBULAR-CLUSTER NGC-5053

The results of a major search for photometrically variable blue straggler stars (BSs) in the extremely metal-poor globular cluster NGC 5053 are presented. The survey is based on photometry of over 200 CCD frames (BVI passbands) taken on 18 nights between 1985 and 1994. Five of the 16 BSs monitored for variability are identified as SX Phe stars and their photometric characteristics derived. These five stars are among the shortest-period (49<P<57 min), lowest luminosity (3.0 less than or equal to M(V) less than or equal to 3.3 mag), smallest amplitude (0.08 less than or equal to A(V) less than or equal to 0.25 mag) SX Phe stars presently known. Their colors and locations in both B and V period-luminosity (P-L) diagrams suggest that four of the stars (blue stragglers 11, 13, 14, and 15 in the Nemec & Cohen 1989 catalog) are fundamental-mode pulsators, while the fifth and bluest star, No. 7, appears to be a first-overtone pulsator. One of the stars, No. 11, exhibits cycle-to-cycle light variations that are consistent with multimode pulsations; however, not enough photometric information is available to derive its secondary pulsation period. Masses for the stars, estimated directly from the general equation of stellar pulsation and relative to the RR Lyrae stars, range from 0.53 M(circle dot) (NC7) to 1.8 M(circle dot) (NC13). The SX Phe stars are used to derive improved P-L-[Fe/H] relations for SX Phe stars, from which a revised distance modulus of (m-M)(0)=16.06 is obtained for NGC 5053. (C) 1995 American Astronomical Society.