A photometric investigation of cluster membership for the Cepheid BB Sagittarii

Abstract

Photoelectric photometry (UBV and some VRI) is presented for the close pair of open clusters Cr 394 and NGC 6716 and for the 6[d over dot]64 classical Cepheid BB Sgr. Based upon the photometric data and star counts, the following parameters are derived for these clusters: Cr 394, [angled bracket]E[subscript B - V][angled bracket]= 0.25 [plus or minus] 0.01, V[subscript 0] — M[subscript V] = 9.04 [plus or minus] 0.08, d = 643 [plus or minus] 25 pc, age [similar or equal to]6 x 10[superscript 7] yr, nuclear radius r[subscript n] = 9 arcmin, tidal radius R[subscript c] = 27 arcmin (5 pc); NGC 6716, [angled bracket]E[subscript B - V][angled bracket] = 0.11 [plus or minus] 0.01, V[subscript 0 ] - M[subscript V] = 9.07 [plus or minus] 0.17, d = 650 [plus or minus] 52 pc, age[similar or equal to]6 x 10[superscript 7] yr, r[subscript n] = 7 arcmin. The spatial location and inferred age for BB Sgr are consistent with coronal membership in Cr 394, whose center is located only 19.6 arcmin (3.6 pc) from the Cepheid. Its intrinsic luminosity and mean light color index as a cluster member are [angled bracket]M[subscript V][angled bracket] = — 3.08 [plus or minus] 0.12 and ([angled bracket]B[angled bracket] - [angled bracket]V[angled bracket])[subscript 0] = 0.75 [plus or minus] 0.02, respectively, values which are consistent with a location near the red edge of the Cepheid instability strip. Space-velocity data for Cr 394 members are needed to strengthen the case for cluster membership of this Cepheid.