NGC 6611: A cluster caught in the act

Abstract

We have combined optical CCD photometry and spectroscopy with infrared imaging photometry to study the young cluster NGC 6611. We use these data to derive improved values for the reddening law (R = 3.75) and the distance modulus (m - M = 11.5), and to construct a physical Hertzsprung-Russell (HR) diagram from which we can probe the ages, masses, and evolutionary states of this stellar ensemble. The HR diagram shows a strong population of high-mass stars, the most massive of which has a mass of roughly 80 solar mass, similar to what we find in other Galactic and Magellanic Cloud clusters and associations. The age of the massive stellar population in NGC 6611 is approximately 2 million yr, with an age spread of, perhaps, 1 million yr, although the data are also consistent with there being no discernible age spread among the most massive stars. However, the HR diagram does reveal that one star of somewhat lower mass (30 solar mass) must have formed approximately 6 million yr ago. The upper end of the mass function of NGC 6611 is found to have a slope of Gamma = -1.1 +/- 0.3, indistinguishable from a Salpeter slope, and similar to what we have found in other Galactic associations, but shallower than what we have found in the Magellanic Clouds. Our most significant result, however, is that we catch this cluster in the act of forming intermediate-mass (3-8 solar mass) stars. This is the first well-established case where large numbers of intermediate-mass stars have been seen on their way to the Zero Age Main Sequence (ZAMS). That intermediate-mass pre-main-sequence stars are indeed present is evidenced both by their location above the ZAMS in the HR diagram, and in some cases by their spectroscopic and infrared signatures of (possibly remnant protostellar) circumstellar material. The pre-main-sequence population ranges from as young as 0.25 million yr to at least 1 million yr of age. We find an highly unusual number (27) of emission-line stars, which appear quite similar in their optical and infrared continuum and optical spectroscopic properties to 'classical Be/Ae' stars (as opposed to Herbig Be/Ae stars). Our data are inconsistent with the traditional interpretation that these classical Be/Ae stars are slightly evolved stars undergoing mass loss. Instead, we offer the conjecture that these may be young stars whose circumstellar disks have become optically thin, and produce Balmer emission lines. The infrared data do indicate a number of stars, particularly among the embedded sample, whose colors are consistent with those of stars thought to be surrounded by optically thick circumstellar accretion disks. The identification of such disks around young massive stars continues to be rare, and implies that the disk survival times around intermediate- and high-mass stars are much shorter (less than 0.5 Myr) than those of disks surrounding lower-mass stars.