1999A&A...341..469C

During many years, several authors discussed whether NGC 2451 was a real open star cluster or not. By using parallaxes and proper motions from Hipparcos satellite, and Geneva multicolour photometric measurements of 64 stars, the existence of two clusters A and B is confirmed. Distances and Geneva colour excesses E[B-V] are respectively 197pc, 0.01 and 358pc, 0.12 for NGC 2451 A and B. The two clusters have the same age (logt=7.7). A unique Be star, HR 2968, which belongs to cluster B, has been monitored from 1978 to 1998 in Geneva photometry and from November 1989 to March 1993 by the Hipparcos satellite. This star shows exceptional luminosity variations: i) The mean luminosity, which has been stable since 1978 (normal B-star phase), increased from 1990 to 1995 (Be phase), and then decreased until 1998; ii) Also, in 1990 started a periodic light variation with a period of 371d. Five periods of this mid-term light variation were observed. Hipparcos and Geneva photometries are in perfect agreement. A model is proposed to explain this periodic variability: the Be star is the main component of a binary system having an eccentric orbit of period 371d; from 1990, the Be star was surrounded by matter expelled in its equatorial plane and, at each periastron passage, the companion star interacts gravitationally and/or radiatively with the disk. The physical parameters of the Be star and its companion (most probably a main sequence star) are determined by using their belonging to NGC 2451 B and by calculating an approximate orbit from published radial velocities.