NGC-6611 - A CLUSTER CAUGHT IN THE ACT

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 th e distance modulus (m-M=11.5), and to construct a physical Hertzprung- Russell (HR) diagram from which we can probe the ages, masses, and evo lutionary states of this stellar ensemble. The HR diagram shows a stro ng population of high-mass stars, the most massive of which has a mass of roughly 80M., similar to what we find in other Galactic and Magell anic Cloud clusters and associations. The age of the massive stellar p opulation in NGC 6611 is approximately 2 million yr, with an age sprea d of, perhaps, 1 million yr, although the data are also consistent wit h there being no discernible age spread among the most massive stars. However, the HR diagram does reveal that one star of somewhat lower ma ss (30 M .) 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 GAMM A = - 1.1 +/- 0.3, indistinguishable from a Salpeter slope, and simila r 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 M .) stars. This is the first well-established case where large numbers of intermediate-mass stars have been seen on their way to the 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 infrar ed signatures of (possibly remnant protostellar) circumstellar materia l. The pre-main-sequence population ranges from as young as 0.25 milli on yr to at least 1 million yr of age. We find an highly unusual numbe r (27) of emission-line stars, which appear quite similar in their opt ical and infrared continuum and optical spectroscopic properties to '' classical Be/Ae'' stars (as opposed to Herbig Be/Ae stars). Our data a re inconsistent with the traditional interpretation that these classic al Be/Ae stars are slightly evolved stars undergoing mass loss. Instea d, we offer the conjecture that these may be young stars whose circums tellar disks have become optically thin, and produce Balmer emission l ines. The infrared data do indicate a number of stars, particularly am ong the embedded sample, whose colors are consistent with those of sta rs thought to be surrounded by optically thick circumstellar accretion disks. The identification of such disks around young massive stars co ntinues to be rare, and implies that the disk survival times around in termediate- and high-mass stars are much shorter (< 0.5 Myr) than thos e of disks surrounding lower-mass stars.