X-ray Source Populations in the Region of the Open Clusters NGC 6633 and IC 4756

Abstract

Using deep exposures (~10^5 s) with the ROSAT HRI, we have performed flux-limited surveys for X-ray sources in the vicinity of the Hyades-age open stellar clusters NGC 6633 and IC 4756, detecting 31 and 13 sources respectively. Our primary aim is to search for so-far unrecognised cluster members. We propose identifications or classifications (cluster member, field star, extragalactic field object) for the X-ray sources, based on published membership lists, and on X-ray:optical flux ratios and optical colour--magnitude diagrams. Results of simulating the expected X-ray-emitting source populations are compared with the ROSAT measurements and with the expected capabilities of the XMM mission. The simulations provide a novel method of comparing the activity levels of NGC 6633 and IC 4756 with that of the Hyades. The measurements and simulations confirm that cluster members are the major class of X-ray emitter in these fields at flux levels f_x > 10^{-14}/erg/cm^2/s (0.1--2.4 keV), contributing ~40 per cent of the total X-ray sources. We find 6 possible new members in NGC 6633, and 4 candidates in IC 4756; all require further observation to establish membership probability.