A CCD Search for Variable Stars of Spectral Type B in the Northern Hemisphere Open Clusters. IX. NGC 457

摘要

We present results of a BVI variability survey in the young open cluster NGC457 based on observations obtained during three separate runs spanning almost20 years. In total, we found 79 variable stars, of which 66 are new. The BVIphotometry was transformed to the standard system and used to derive clusterparameters by means of isochrone fitting. Using the complementary H-alphaphotometry carried out in two seasons separated by over 10 years, we find thatthe cluster is very rich in Be stars. In total, 15 stars in the observed fieldof which 14 are cluster members showed H-alpha in emission either during ourobservations or in the past. Most of the Be stars vary in brightness ondifferent time scales including short-period variability related most likely tog-mode pulsations. A single-epoch spectrum of NGC457-6 shows that this Be staris presently in the shell phase. The inventory of variable stars in theobserved field consists of a single BCep-type star, NGC457-8, 13 Be stars, 21slowly pulsating B stars, seven DSct stars, one GDor star, 16 unclassifiedperiodic stars, 8 eclipsing systems and a dozen of stars with irregularvariability, of which six are also B-type stars. As many as 45 variable starsare of spectral type B which is the largest number in all open clusterspresented in this series of papers. The most interesting is the discovery of alarge group of slowly pulsating B stars which occupy the cluster main sequencein the range between V=11 and 14.5 mag, corresponding to spectral types B3 toB8. They all have very low amplitudes and about half show pulsations withfrequencies higher than 3 c/d. We argue that these are most likelyfast-rotating slowly pulsating B stars, observed also in other open clusters.