Chandra/ACIS-I study of the X-ray properties of the NGC 6611 and M16 stellar population

TL;DR

This study analyzes Chandra X-ray observations of the NGC 6611 cluster in M16 to understand X-ray emission mechanisms in young stellar objects and compare their properties with other clusters like Orion.

Contribution

It provides a detailed analysis of X-ray properties of young stars in M16, including OB stars, and compares these with other clusters, highlighting the universality of the X-ray luminosity function.

Findings

Disk-bearing stars show lower X-ray activity than disk-less stars.

X-ray luminosity function of M16 is similar to Orion's, indicating universality.

Most OB stars exhibit soft X-ray spectra with no hard component.

Abstract

Mechanisms regulating the origin of X-rays in YSOs and the correlation with their evolutionary stage are under debate. Studies of the X-ray properties in young clusters allow to understand these mechanisms. One ideal target for this analysis is the Eagle Nebula (M16), with its central cluster NGC6611. At 1750 pc from the Sun, it harbors 93 OB stars, together with a population of low-mass stars from embedded protostars to disk-less Class III objects, with age <= 3Myrs. We study an archival 78 ksec Chandra/ACIS-I observation of NGC6611, and two new 80ksec observations of the outer region of M16, one centered on the Column V, and one on a region of the molecular cloud with ongoing star-formation. We detect 1755 point sources, with 1183 candidate cluster members (219 disk-bearing and 964 disk-less). We study the global X-ray properties of M16 and compare them with those of the Orion Nebula Cluster. We also compare the level of X-ray emission of Class II and Class III stars, and analyze the X-ray spectral properties of OB stars. Our study supports the lower level of X-ray activity for the disk-bearing stars with respect to the disk-less members. The X-ray Luminosity Function (XLF) of M16 is similar to that of Orion, supporting the universality of the XLF in young clusters. 85% of the O stars of NGC6611 have been detected in X-rays. With only one possible exception, they show soft spectra with no hard component, indicating that mechanisms for the production of hard X-ray emission in O stars are not operating in NGC 6611.