A&A 406, 893-898 (2003)
DOI: 10.1051/0004-6361:20030834
C. Boeche1 - R. Barbon2 - A. Henden3 - U. Munari1 - P. Agnolin2
1 - Osservatorio Astronomico di Padova, Sede di Asiago,
36012 Asiago (VI), Italy
2 -
Osservatorio Astrofisico di Asiago,
Università di Padova, 36012 Asiago (VI), Italy
3 -
Universities Space Research Association/US Naval Observatory
Flagstaff Station, PO Box 1149, Flagstaff AZ 86002-1149, USA
Received 28 april 2003 / Accepted 3 June 2003
Abstract
A photometric, astrometric and spectroscopic investigation of the poorly
studied open cluster NGC 6738 has been performed in order to ascertain its
real nature. NGC 6738 is definitely not a physical stellar ensemble:
photometry does not show a defined mean sequence, proper motions and
radial velocities are randomly distributed, spectro-photometric parallaxes
range between 10 and 1600 pc, and the apparent luminosity function is identical to
that of the surrounding field. NGC 6738 therefore appears to be an apparent
concentration of a few bright stars projected on patchy background absorption.
Key words: open clusters and associations: general - open clusters and associations: individual: NGC 6738
NGC 6738 (
,
,
,
)
shows up as a group of bright stars on a fairly crowded background located a
few degrees from the galactic equator in Aquila. It is classified as IV2p
meaning that the object is poorly populated and separated from the
surrounding field and spans a moderate range in brightness (Ruprecht
1966). No modern data exist for this object. Collinder
(1931) found a distance of 1190 pc and Roslund (1960)
reported, by means of objective prism spectral classification, that the
stars down to 12 mag in the region of the cluster are dwarfs but they do not
appear to define a main sequence, thus already challenging the reality of
the object. In another study, Sahade et al. (1963) listed the
eclipsing variable V888 Aql as a possible cluster member. In this paper we
report on UBVRI photometry, radial velocities, spectral classifications and
Tycho-2 proper motions of stars in the field of the cluster, in order to
ascertain its true nature. As a matter of fact, looking at the Palomar Atlas
maps, the cluster region seems to show up more like a window of low
absorption in a larger region of high obscuration than as a physical
grouping of stars.
Table 1: Journal of observations. Seeing is the FWHM of stellar images as measured on the CCD frames. Short exposures are a few seconds; medium exposures are around a minute; and long exposures are several minutes in duration.
Photometric observations were made with the 1.0-m Ritchey-Chrétien
telescope of the US Naval Observatory, Flagstaff Station, in two successive runs on 1999 and 2000. The journal of observations is given in
Table 1 and a finding chart for the brighter stars is presented in
Fig. 1. In the first run the surveyed area was
arcmin2 centered on the cluster position and with a
limiting magnitude of V = 20.0, whereas in the second run a larger area of
arcmin2 was imaged down to V=17.0 in order to have a
larger scale sampling of the field surrouding the cluster.
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Figure 1: Finding chart of the stars brighter than V=13 in the field of NGC 6738. Dots scale with magnitude. Stars spectroscopically observed are numbered. |
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Figure 2:
V, (B-V) diagrams ( top) and
(U-B), (B-V) diagrams ( bottom)
of the 44 ![]() ![]() |
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A Tektronix/SITe
thinned, backside-illuminated CCD was used for
the first epoch and a Tektronix/SITe
CCD for the second epoch,
along with Johnson UBV and Kron-Cousins RI filters. Images were processed
using IRAF, with nightly median sky flats and bias frames. Aperture
photometry was performed with routines similar to those in DAOPHOT (Stetson
1987). Astrometry was performed using SLALIB (Wallace
1994) linear plate transformation routines in conjunction with the
USNO-A2.0 reference catalog.
Errors in coordinates were typically under 0.1 arcsec in both coordinates, referred to the mean coordinate zero point of
the reference stars in each field. The telescope scale is 0.6763 arcsec/pixel.
Typical seeing was
2 arcsec. A 9 arcsec extraction aperture with
concentric sky annulus was commonly used. The reported photometry only uses
data collected under photometric conditions (transformation errors under 0.02 mag). Cluster observations were interspersed with observations of
Landolt (1983, 1992) standard fields, selected for wide
colour and airmass range. The mean transformation coefficients (cf. Henden
& Kaitchuck 1990, Eqs. (2.9ff)) are:
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(1) | ||
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(2) | ||
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(3) | ||
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(4) | ||
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(5) |
Table 2: Spectroscopic data. Column from left to right: star number, Tycho (or Hipparcos) identification, V magnitude, B-V colour, our spectral types and those from literature (if avaiable), E(B-V), (B-V)0 and distance moduli. Colour excesses and distance moduli have been computed using Lang's (1992) tables. Colon indicates uncertainties in the intrinsic colours and/or absolute magnitudes. The "negative'' reddening shown by star #28 could trace an undetected binary.
For classification purposes, spectroscopic observations of 30 stars in a
arcmin2 area centered on the cluster have been obtained
with the B&C spectrograph at the 122 cm telescope of the Asiago Observatory.
A 600 gr/mm grating has been used providing a dispersion of 2 Å/pix
(74 Å/mm) in the 3900-4900 Å interval and the observations were reduced
using IRAF. Table 2 shows our spectral classifications
(obtained against an internal atlas of re-observed MK standards from the
list of Yamashita et al. 1977) compared to the scanty data found in the
literature together with colour excesses and corrected distance moduli
obtained from our photometry. The stars observed appear to be distributed
between 10 to 1600 pc with no clear clustering. This is confirmed by the
intrinsic CMD shown in Fig. 4 where, disregarding the obvious late
type dwarfs belonging to the field, no main sequence is present. We arrive
at the same conclusion observing where the classified stars lie in the
photometric diagrams of Fig. 2. From point (b) in the previous
section, we must infer that all of our program stars are field stars. This
is also supported by the reddenings of the observed stars which span about 1 mag, suggesting once more that they do not lie at the same distance.
Using proper motions from Tycho-2, a vector point diagram (VPD) of the stars
located inside a box of 1
on the center of the
cluster is given in Fig. 5. This diagram shows no concentration of
cluster stars distinct from the distribution of field stars (cf. Lattanzi et al. 1991).
From this characteristic, we can infer the absence along the line of sight
to NGC 6738 of a localized group of stars sharing the same projected space motion.
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Figure 3:
Apparent luminosity function for the 44 ![]() ![]() |
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Figure 4: Intrinsic colour-magnitude diagram of observed stars, each of them labeled with its spectral classification. |
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Figure 5:
Vector point diagram (based on Tycho-2 data) of the stars
located inside a
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Table 3: Radial velocities from Echelle spectra. The listed errors are errors of the mean.
Some of the brightest stars in the field of the cluster have been observed
with the Echelle spectrograph mounted at the 182 cm telescope operated by Osservatorio Astronomico di Padova
at Mount Ekar (Asiago, Italy).
The set up used provided a 20 000 resolving power over the 4550-8750 Å interval. Data reduction and analysis has been performed with IRAF.
Table 3 lists the heliocentric radial velocities of program
stars together with their errors as estimated from the comparison of the
results obtained from the various echelle orders. The errors cluster around 1 km s-1. Stars #2 and #8 show large radial velocity variations that
suggest a binary nature. As for the preceding section, the large scatter of
radial velocities does not support the presence of a real cluster. Three
stars group around RV =-35 km s-1 and
mag (stars #6, #8 and #11). Star #3 shows the same radial velocity but it lies at a
closer distance.
In order to test the physical reality of an open cluster, it is common to
analyse its radial density distribution. In real clusters the density should
decrease outwardly, eventually merging with the surrounding field. Following
Odenkirchen & Soubiran (2002), we have measured the stellar
density in 0.5 arcmin2 wide annuli centered at the nominal position of NGC 6738 for three different magnitude ranges. The result
(Fig. 6) shows a stellar density distribution in agreement with
Poissonian statistics (represented by dots with error bars) except for
mag stars of the inner circle (histogram on top of
Fig. 6) and which we have spectroscopically observed and
already found not to be physically related.
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Figure 6:
Histogram of the surface density of the stars in the
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Figure 7 shows the integrated stellar density distribution in
the field of NGC 6738. The highest density is off-centered with respect to
the cluster position, and the overall pattern is very patchy. The pattern
is the mirror image of the dust emission at 100 m measured by IRAS
(cf. http://irsa.ipac.caltech.edu/applications/ISSA/): the emission
concentrates towards the areas of lowest stellar counts. This fact is indicative
of an extremely non homogeneous distribution of interstellar material over the
field of view. Coupled with a chance grouping of a few bright foreground
stars, this concentration could erroneously lead to a cluster detection.
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Figure 7:
Surface stellar density from 44 ![]() |
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Our purpose was to perform an investigation as complete as possible to verify the existence of the open cluster NGC 6738. Tycho-2 proper motions have been combined with our new deep and wide-field UBVRI photometry, radial velocities and spectral classification. We have not found evidence supporting the existence of a real cluster; the colour-magnitude and colour-colour diagrams do not show a reliable cluster main sequence; the spectro-photometric parallaxes of the 30 brightest stars show no concentration in distance.; the apparent luminosity function is one of field stars; proper motions and radial velocities do not support a common space motion of the program stars; the stellar density distribution in the field of NGC 6738 does not have a peak in the supposed location of the cluster, nor a negative gradient moving away from it, as expected if a real cluster were present, but instead it reveals the patchy structure of the interstellar absorption confirmed by the IRAS data. Finally, the concentration of stars brighter than V=12 mag toward the center of the field, from which one may infer the presence of a cluster and certainly drove the earlier investigators to pick up this object, has been shown to be a chance grouping. Concluding, it appears safe to conclude that NGC 6738 is not a real open cluster.
Acknowledgements
We thank G. Bertelli for useful discussions and B. Skiff for comments.