Not an open cluster after all: the NGC?6863 asterism in Aquila

摘要

Context. Shortly after birth, open clusters startdissolving; gradually losing stars into the surrounding star field. Thetime scale for complete disintegration depends both on their initialmembership and location within the Galaxy. Open clusters undergoing theterminal phase of cluster disruption or open cluster remnants (OCRs)are notoriously difficult to identify. From an observational point, acombination of low number statistics and minimal contrast against thegeneral stellar field conspire to turn them into very challengingobjects. To make the situation even worst, random samples of fieldstars often display features that may induce to classify themerroneously as extremely evolved open clusters. Aims. In this paper, we provide a detailed study of the stellarcontent and kinematics of NGC6863, a compact group of a few starslocated in Aquila and described by the Palomar Observatory Sky Surveyas a non existent cluster. Nonetheless, this object has been recentlyclassified as OCR. The aim of the present work is to either confirm ordisprove its OCR status by a detailed star-by-star analysis. Methods. The analysis is performed using wide-field photometryin the UBVI pass-band, proper motions from the UCAC3 catalogue, andhigh resolution spectroscopy as well as results from extensive N-body calculations. Results. The spectra of the four brightest stars in this fieldclearly indicate that they are part of different populations. Theirradial velocities are statistically very different and theirspectroscopic parallaxes are inconsistent with them being part of asingle, bound stellar system. Out of the four stars, only two of themhave similar metallicity. The color magnitude diagram for the field ofNGC6863 does not show any clear signature typical of actual openclusters. Consistently, spatial scan statistics confirms the absence ofany statistically significant, kinematically supported over-density atthe purported location of NGC6863. Conclusions. Our results show that the four brightest starscommonly associated to NGC6863 form an asterism, a group ofnon-physically associated stars projected together, leading to theconclusion that NGC 6863 is not a real open cluster. Key words: open clusters and associations: individual: NGC 6863- open clusters and associations: individual: NGC 1901 - open clustersand associations: general - Galaxy: evolution