Galactic clusters with associated Cepheid variables. III. NGC 1647 and SZ Tauri

Abstract

New photoelectric UBV photometry of 45 stars and spectroscopic observations of 24 stars are presented for NGC 1647, the cluster associated with the 3[d with dot below]15 Cepheid SZ Tauri. The resulting photometry, spectral classifications, and radial velocities are used with previously published photoelectric, photographic, spectroscopic, and proper motion studies of NGC 1647 for a detailed cluster analysis. The newly-obtained cluster distance modulus is V[subscript 0]-M[subscript V]=8.67 [plus or minus] 0.02 s.e. (d=542 [plus or minus] 4 pc), and a value of R=A[subscript V]/E[subscript B-V]=3.09 [plus or minus] 0.16 s.e. is found to describe the dust extinction in the field. A space reddening of E[subscript B-V]=0.29 [plus or minus] 0.01 is derived for SZ Tau from two B-type companions which are located east-west of the Cepheid, and its resulting luminosity as an outlying cluster member is [angled bracket]M[subscript V][angled bracket] = -3.09 [plus or minus] 0.04. A cluster radial velocity of -2.3 [plus or minus] 1.6 km s[superscript -1] and cluster age of ~1.9 x 10[superscript 8] yr are both consistent with the likely membership of SZ Tau. It is argued from its luminosity that this Cepheid is pulsating in the first overtone mode, as is also implied by its light curve and Wesselink radius. In particular, a newly derived period-luminosity relation for cluster Cepheids of the form [angled bracket]M[subscript V][angled bracket] = 1.15-2.94 log P[subscript 0] is consistent with the results for SZ Tau only if its observed variability is due to overtone pulsation.