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We present a combined X-ray, optical and IR (2MASS) study of NGC 6530, complementing our previous studies of this cluster. We consider di erent indicators of IR excesses, taken as indicator of circumstellar disks and therefore of pre- main-sequence status. We use reddening-free indices to ensure that our results are una ected by highly irregular, di erential reddening. More than the study of the JHK bands alone (33 IR-excess stars found), we find as useful a comparison between various optical and IR colors, finding overall 333 stars with optical-IR excess. In the field of the previously studied Chandra ACIS-I observation, we find 196 stars with optical-IR excess, of which 120 not previously found in X-rays. The total number of estimated cluster members becomes thus 1100. The estimated disk frequency in the ACIS field is 20%. By considering di erent optical-IR excess indices, we find only partially overlapping subsamples, corresponding to di erent characteristics of their spectral energy distributions (longer- or shorter-wavelength IR excesses, or blue-violet excess). In a region displaced towards north-west with respect to the known cluster center we find an unexpected concentration of stars with optical-IR excesses, most of which not detected in X-rays. The spectral energy distribution of some of these objects shows more extreme excesses with respect to most ordinary T Tauri stars, and suggests circumstellar disks with very small inner holes and high accretion rates; some objects are best interpreted as surrounded by reflection nebulosities, as found in some optically detected Class I objects in nearer star-forming regions. These reflection-nebulae candidates have the lowest X-ray detection rate among all subsamples considered here. Optical-IR excess stars in the north of NGC 6530 are nearly co-spatial with a sub-population of cluster stars older than central cluster stars. This leads to the conclusion that in these northern regions of NGC 6530, far from massive cluster stars, star formation (and disk evolution) has proceeded rather undisturbed over longer time scales than near cluster center, where most massive stars are found, and most stars are lacking substantial disks and strong accretion.

The rich young cluster NGC 6530: a combined X-ray-optical-infrared study