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J/AJ/106/1906      NGC 6611: A Cluster Caught in the Act (Hillenbrand+ 1993)

NGC 6611: A Cluster Caught in the Act Hillenbrand L.A., Massey P., Strom S.E., Merrill K.M. <Astron. J. 106, 1906 (1993)> =1993AJ....106.1906H
ADC_Keywords: Clusters, open ; Photometry, UBVRI; Photometry, infrared; MK spectral classification Description: This catalog is the RA-sorted list of astrometry and optical/infrared photometry for optically visible sources in NGC 6611 (table 1 of the paper), resulting from observations made in October 1990 at Kitt Peak 0.90m telescope; the positions in this list are for the equinox J2000. The spectral classifications (table 3 of the paper) used spectroscopic observations made with the Hydra instrument at the 4m telescope, with a dispersion of 2Å, in the wavelength range 390-490nm. 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 H-R diagram from which we can probe the ages, masses, and evolutionary states of this stellar ensemble. The H-R diagram shows a strong population of high-mass stars, the most massive of which has a mass of roughly 80 solar masses, 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 years, with an age spread of, perhaps, a million years, although the data are also consistent with there being no discernible age spread among the most massive stars. However, the H-R diagram does reveal that one star of somewhat lower mass (30 solar masses) must have formed approximately 6 million years 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 masses) 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. That intermediate mass pre-main sequence stars are indeed present is evidenced both by their location above the zero-age main sequence in the H-R 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 years to at least 1 million years 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 amongst 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 (< 0.5Myr) than those of disks surrounding lower mass stars. File Summary:
FileName Lrecl Records Explanations
ReadMe 80 . This file table1.dat 77 1022 NGC 6611 Optical and Near-IR Photometry Data posmk.dat 80 77 MK classification, with accurate position
Byte-by-byte Description of file: table1.dat
Bytes Format Units Label Explanations
1 A1 --- n_ID [ *]? emission line object flag 2- 4 I3 --- ID ? Previous identification number (G1) 6- 7 I2 h RAh [18/18]! hours of right ascension (J2000) (G2) 8 A1 --- --- [:] field separator 9- 10 I2 min RAm [17/20]! minutes of right ascension (G2) 11 A1 --- --- [:] field separator 12- 16 F5.2 s RAs seconds of right ascension (G2) 18 A1 --- DE- sign of declination (G2) 19- 20 I2 deg DEd [13/14]! degrees of declination (J2000) (G2) 21 A1 --- --- [:] field separator 22- 23 I2 arcmin DEm minutes of declination (G2) 24 A1 --- --- [:] field separator 25- 28 F4.1 arcsec DEs seconds of declination (G2) 30- 34 F5.2 mag Vmag V magnitude 35- 36 A2 --- u_Vmag [ :] uncertainty flag in V 37- 41 F5.2 mag B-V B-V color 42- 43 A2 --- u_B-V [ :] uncertainty flag in B-V 44- 48 F5.2 mag U-B ?U-B color 49- 50 A2 --- u_U-B [ :] uncertainty flag in U-B 52- 56 F5.2 mag Kmag ?K magnitude (2.2um) 57- 58 A2 --- u_Kmag [ :] uncertainty flag in K 59- 63 F5.2 mag H-K ?H-K color 64- 65 A2 --- u_H-K [ :] uncertainty flag in H-K 66- 70 F5.2 mag J-H ?J-H color 71- 72 A2 --- u_J-H [ :] uncertainty flag in J-K 74- 77 F4.2 mag V-K ?V-K color
Byte-by-byte Description of file: posmk.dat
Bytes Format Units Label Explanations
1- 3 I3 --- ID ? Identification number, as in table1 (G1) 16- 17 I2 h RAh Right Ascension J2000 (hours) (G2) 19- 20 I2 min RAm Right Ascension J2000 (minutes) (G2) 22- 26 F5.2 s RAs Right Ascension J2000 (seconds) (G2) 28 A1 --- DE- Declination J2000 (sign) (G2) 29- 30 I2 deg DEd Declination J2000 (degrees) (G2) 32- 33 I2 arcmin DEm Declination J2000 (minutes) (G2) 35- 38 F4.1 arcsec DEs Declination J2000 (seconds) (G2) 40 A1 --- rPos [MTUs] Reference of position (G2) 42- 46 F5.2 mag Vmag V mag, as in table1 47 A1 --- u_Vmag [:?] Uncertainty flag on Vmag 49- 60 A12 --- SpType MK spectral type 64- 80 A17 --- Notes Remarks by B. Skiff (G1)
Global Notes: Note (G1): this identifier contains the number assigned by Walker (1961ApJ...133..438W), Kamp (1974A&AS...16....1K) or Tucholke et al. (1986A&AS...66..311T). The values -1 and -2 were assigned at CDS to designate the two stars in table "posmk.dat" without ID number; other names of these two stars are supplied in the "Notes" column of the "posmk.dat" file. Note (G2): the positions in table1.dat are based on the original GSC1.1 reference frame (Cat. I/220), and though their internal scatter is only ∼0".2 rms (better than estimated in the paper), they inherited a systematic error of about +0".9 in RA relative to the ICRS-based catalogues. The positions in the "posmk.dat" file, added by B. Skiff in April 2004, are taken from the following sources: M = 2MASS (Cat. II/246) T = Tycho-2 (2000A&A...357..367H, Cat. I/259) U = UCAC2 (2000AJ....120.2131Z, Cat. I/289) s = SkyView estimate (accuracy ±1")
History: * 28-Jul-1994: Prepared at ADC by N. Paul M. Kuin, with data of table1 kindly supplied by Lynne Hillenbrand * 28-Apr-2004: table "posmk" prepared by Brian Skiff who kindly supplied these data to CDS. * 06-Oct-2005: One object number corrected in table1.
(End) N. Paul M. Kuin (NSSDC/ADC), Francois Ochsenbein [CDS] 28-Apr-2004
The document above follows the rules of the Standard Description for Astronomical Catalogues.From this documentation it is possible to generate f77 program to load files into arrays or line by line

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