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NGC 6611: A cluster caught in the act
We have combined optical CCD photometry and spectroscopy with infraredimaging photometry to study the young cluster NGC 6611. We use thesedata to derive improved values for the reddening law (R = 3.75) and thedistance modulus (m - M = 11.5), and to construct a physicalHertzsprung-Russell (HR) diagram from which we can probe the ages,masses, and evolutionary states of this stellar ensemble. The HR diagramshows a strong population of high-mass stars, the most massive of whichhas a mass of roughly 80 solar mass, similar to what we find in otherGalactic and Magellanic Cloud clusters and associations. The age of themassive stellar population in NGC 6611 is approximately 2 million yr,with an age spread of, perhaps, 1 million yr, although the data are alsoconsistent with there being no discernible age spread among the mostmassive stars. However, the HR diagram does reveal that one star ofsomewhat lower mass (30 solar mass) must have formed approximately 6million yr ago. The upper end of the mass function of NGC 6611 is foundto have a slope of Gamma = -1.1 +/- 0.3, indistinguishable from aSalpeter slope, and similar to what we have found in other Galacticassociations, but shallower than what we have found in the MagellanicClouds. Our most significant result, however, is that we catch thiscluster in the act of forming intermediate-mass (3-8 solar mass) stars.This is the first well-established case where large numbers ofintermediate-mass stars have been seen on their way to the Zero Age MainSequence (ZAMS). That intermediate-mass pre-main-sequence stars areindeed present is evidenced both by their location above the ZAMS in theHR diagram, and in some cases by their spectroscopic and infraredsignatures of (possibly remnant protostellar) circumstellar material.The pre-main-sequence population ranges from as young as 0.25 million yrto 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 andinfrared continuum and optical spectroscopic properties to 'classicalBe/Ae' stars (as opposed to Herbig Be/Ae stars). Our data areinconsistent with the traditional interpretation that these classicalBe/Ae stars are slightly evolved stars undergoing mass loss. Instead, weoffer the conjecture that these may be young stars whose circumstellardisks have become optically thin, and produce Balmer emission lines. Theinfrared data do indicate a number of stars, particularly among theembedded sample, whose colors are consistent with those of stars thoughtto be surrounded by optically thick circumstellar accretion disks. Theidentification of such disks around young massive stars continues to berare, and implies that the disk survival times around intermediate- andhigh-mass stars are much shorter (less than 0.5 Myr) than those of diskssurrounding lower-mass stars.
| Rotational velocities of faint stars of early spectral types.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1963AJ.....68..428S&db_key=AST
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Observation and Astrometry data
| Constellation: | Ophiucus |
| Right ascension: | 18h19m54.21s |
| Declination: | +02°40'54.1" |
| Apparent magnitude: | 9.392 |
| Proper motion RA: | 16.3 |
| Proper motion Dec: | -12.9 |
| B-T magnitude: | 9.982 |
| V-T magnitude: | 9.441 |
Catalogs and designations:
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INHABITED SKY
