Image-Subtraction Photometry of Variable Stars in the Field of the Globular Cluster NGC 6934

Abstract

(Abridged) We present CCD BVI photometry of 85 variable stars from the field of the globular cluster NGC 6934. The photometry was obtained with the image subtraction package ISIS. 35 variables are new identifications: 24 RRab stars, 5 RRc stars, 2 eclipsing binaries of W UMa-type, one SX Phe star, and 3 variables of other types. Both detected contact binaries are foreground stars. The SX Phe variable belongs most likely to the group of cluster blue stragglers. Large number of newly found RR Lyr variables in this cluster, as well as in other clusters recently observed by us, indicates that total RR Lyr population identified up to date in nearby galactic globular clusters is significantly (>30%) incomplete. Fourier decomposition of the light curves of RRc variables was used to estimate the basic properties of these stars: M=0.63 M_odot, log(L/L_odot)=1.72, T_eff=7300 and Y=0.27. Using RRab variables we obtain: M_V=0.81, [Fe/H]=-1.53 and T_eff=6450. From the B-V color at minimum light of the RRab variables we obtained E(B-V)=0.09+-0.01. Different calibrations of absolute magnitudes of RRab and RRc available in literature were used to estimate apparent distance modulus of the cluster: (m-M)_V= 16.09+-0.06. We note a likely error in the zero point of the HST-based V-band photometry of NGC 6934 recently presented by Piotto et al. Among analyzed sample of RR Lyr stars we have detected a short period and low amplitude variable which possibly belongs to the group of second overtone pulsators (RRe subtype variables). The BVI photometry of all variables is available electronically via anonymous ftp. The complete set of the CCD frames is available upon request.