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

C. Moni Bidin¹, R. de la Fuente Marcos², C. de la Fuente Marcos² and G. Carraro^3,4

¹ Departamento de Astronomia, Universidad de Concepcion, Casilla 160-C, Concepcion, Chile
² Suffolk University Madrid Campus, C/ Viña 3, 28003 Madrid, Spain
³ European Southern Observatory, Alonso de Cordova 3107, Vitacura, Santiago, Chile e-mail: gcarraro@eso.org
⁴ Dipartimento di Astronomia, Universitá di Padova, vicolo Osservatorio 3, 35122, Padova, Italy

Received: 25 June 2009
Accepted: 2 December 2009

Abstract

Context. Shortly after birth, open clusters start dissolving; gradually losing stars into the surrounding star field. The time scale for complete disintegration depends both on their initial membership and location within the Galaxy. Open clusters undergoing the terminal phase of cluster disruption or open cluster remnants (OCRs) are notoriously difficult to identify. From an observational point, a combination of low number statistics and minimal contrast against the general stellar field conspire to turn them into very challenging objects. To make the situation even worst, random samples of field stars often display features that may induce to classify them erroneously as extremely evolved open clusters.

Aims. In this paper, we provide a detailed study of the stellar content and kinematics of NGC 6863, a compact group of a few stars located in Aquila and described by the Palomar Observatory Sky Survey as a non existent cluster. Nonetheless, this object has been recently classified as OCR. The aim of the present work is to either confirm or disprove its OCR status by a detailed star-by-star analysis.

Methods. The analysis is performed using wide-field photometry in the UBVI pass-band, proper motions from the UCAC3 catalogue, and high resolution spectroscopy as well as results from extensive N-body calculations.

Results. The spectra of the four brightest stars in this field clearly indicate that they are part of different populations. Their radial velocities are statistically very different and their spectroscopic parallaxes are inconsistent with them being part of a single, bound stellar system. Out of the four stars, only two of them have similar metallicity. The color magnitude diagram for the field of NGC 6863 does not show any clear signature typical of actual open clusters. Consistently, spatial scan statistics confirms the absence of any statistically significant, kinematically supported over-density at the purported location of NGC 6863.

Conclusions. Our results show that the four brightest stars commonly associated to NGC 6863 form an asterism, a group of non-physically associated stars projected together, leading to the conclusion that NGC 6863 is not a real open cluster.