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The extremely young open cluster NGC 6611: Compiled catalogue, absorption map and the HR diagram
A compiled catalogue of astrometric and photometric data for about 2200stars down to V=16.8() m is presented. The stars are located in an areawith a radius of 22.8 arcmin around the extremely young open cluster NGC6611 with the cluster center at (alpha ,delta )_{2000.0}=18() h18()m40() s,-13(deg) 47.1'. The catalogue is based on new reductions ofpositions, proper motions, and of photometric data that have beenpublished in this region and includes, among others, Hipparcos, AC/GSCand Tautenburg Schmidt plate observations. For the membershipdetermination, spatial and proper motion distributions of stars in theregion were analysed. An extinction map of the area and the clustercolor-magnitude diagram down to M_V=+1() m were constructed. Thedistance of NGC 6611 was determined as 2.14 +/- 0.10 kpc. The completecatalogue is available at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| 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.
| Some Variable Stars in the SRS Catalogue
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| Proper motions, cluster membership and reddening in NGC 6611.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1974A&AS...16....1K&db_key=AST
| The Distribution of the BD M-Type Stars Along the Galactic Equator.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1958ApJ...128..510N&db_key=AST
| Spectral classes, stellar magnitudes and color indices of 3915 faint stars in a area with the center R.A.=18h 0, Dec=15 degrees.
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