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 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:
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FileName Lrecl Records Explanations
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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
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Byte-by-byte Description of file: table1.dat
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Bytes Format Units Label Explanations
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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
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Byte-by-byte Description of file: posmk.dat
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Bytes Format Units Label Explanations
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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)
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Global Notes:
Note (G1): this identifier contains the number assigned by
Walker (1961ApJ...133..438W 1961ApJ...133..438W), Kamp (1974A&AS...16....1K 1974A&AS...16....1K)
or Tucholke et al. (1986A&AS...66..311T 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 2000A&A...357..367H, Cat. I/259)
U = UCAC2 (2000AJ....120.2131Z 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