NGC 6994 - clearly not a physical stellar ensemble

Abstract

The sparse stellar ensemble NGC 6994 = M73 has recently been discussed as a possible remnant of an old open cluster. In order to solve the controversy on the nature of this object we have taken high-resolution spectra of the six brightest stars within 6' angular distance of its nominal position. These stars are the only obvious member candidates for an eventual cluster or cluster remnant since fainter stars do not show any significant concentration in the plane of the sky. The radial velocities, atmospheric parameters, and absolute magnitudes derived from the spectra reveal that the six candidates do not share the same kinematics and lie at different distances from the Sun. The proper motions provided by the Tycho-2 catalogue show that there is also a large spread in the tangential motions of these stars, in agreement with the spectroscopic results. This leads to the conclusion that the few bright stars that constitute NGC 6994 are not a physical system (cluster, cluster remnant, or group). They must instead be understood as a projective chance alignment of physically unrelated field stars.