Photometric and Coravel observations of red giant candidates in three open clusters: membership, binarity, reddening and metallicity
A&A
Astronomy & Astrophysics
J. J. Clariá 2
J.-C. Mermilliod 1
A. E. Piatti 0
M. C. Parisi 2
0 Instituto de Astronomía y Física del Espacio , CC 67, Suc. 28, 1428 Buenos Aires , Argentina
1 Laboratoire d'Astrophysique de l'École polytechnique fédérale de Lausanne , Observatoire, 1290 Sauverny , Switzerland
2 Observatorio Astronómico , Laprida 854, 5000, Córdoba , Argentina
Aims. We present new CORAVEL radial-velocity observations and photoelectric photometry in the UBV and DDO systems for a sample of potential members of the red-giant branches of NGC 6192, NGC 6208 and NGC 6268, three open clusters projected close to the Galactic center direction. We also examine the properties of a sample of 42 inner disk open clusters projected towards almost the same direction as the three clusters. Methods. Cluster members and red field giants were discriminated by using the CORAVEL radial-velocity data and by applying two photometric criteria. Interstellar reddening and metal content of the clusters were derived from combined BV and DDO data. Results. Cluster membership for five red giants in NGC 6192, three in NGC 6208 and three in NGC 6268 has been confirmed by the analysis of the photometric and kinematic data. Photometric membership probabilities show very good agreement with those obtained from CORAVEL radial velocities. Three new spectroscopic binaries were discovered among the red giants of NGC 6192 and NGC 6208. Mean radial velocities and E(B − V ) colour excesses were derived. Conclusions. The overall metallicities were found to be nearly solar for NGC 6208 and above solar for NGC 6192 and NGC 6268. Most of the clusters located closer than 2 kpc from the Sun in the considered direction are slightly more reddened than the absorption resulting from the Baade's window absorption law.
Galaxy; open clusters and associations; general - stars; binaries; spectroscopic - techniques; photometric - techniques; radial velocities - stars; abundances
1. Introduction
In this paper we discuss NGC 6192, NGC 6208 and NGC 6268
as part of a more extensive study of red giants in open clusters
using CORAVEL radial velocity data and new photoelectric
photometry. These three poorly studied, southern, open clusters are
located near the Galactic plane and close to the Galactic
center direction. Their corresponding equatorial and Galactic
coordinates are given in Table 1, together with the
Trumpler (1930)
class according to
Archinal & Hynes (2003)
, the E(B − V) colour
excesses, distances from the Sun and ages given by the WEBDA
open Cluster Database
(Mermilliod & Paunzen 2003)
. These
clusters are interesting in themselves because of the number of
red giant candidates they contain. Neither radial-velocity
determination nor DDO photometry for any of the stars of these
three clusters has been published yet.
NGC 6192 (C1636-432) – also known as Mel 109
(Melotte
1915)
, Cr 309
(Collinder 1931)
or BH 194
(van den Bergh &
Hagen 1975)
– has been the most thoroughly studied of our
three targets. However, a remarkable disagreement about the
cluster reddening value is found in the literature and,
consequently, discrepancies in ages and distances have been reported.
Kilambi & Fitzgerald (1983
, hereafter KF83) and
King (1987)
derived a reddening of E(B − V) = 0.26, a rather old age
(∼1.0 Gyr) and a distance from the Sun of about 1 kpc, based on
photographic and CCD photometry, respectively. More recently,
Kjeldsen & Frandsen (1991
, hereafter KF91) obtained a much
higher reddening of E(B − V) = 0.68, an age of scarcely 89 Myr
and a distance of about 1.7 kpc.
Paunzen et al. (2003)
obtained
CCD photometry in the uvby Strömgren’s system to clarify the
abovementioned discrepancies. They derived [Fe/H] = −0.10 ±
0.09 and E(b − y) = 0.40, equivalent to E(B − V) = 0.54, if the
relation E(B − V) = 1.35E(b − y) given by
Crawford (1978)
is used.
This reddening value seems to support KF91’s estimate and
apparently contradicts the values previously reported by KF83 and
King (1987)
.
Loktin et al. (2001)
published a second version of
their Open Cluster Catalogue
(Loktin & Matkin 1994)
reviewing the fundamental parameters of 423 clusters. Their updated
distances are now based on a Hyades distance modulus of 3.27,
this value being 0.06 mag lower than that found from Hipparcos
parallaxes, namely V − MV = 3.33 ± 0.01
(Perryman et al. 1998)
.
By using the original data from KF91,
Loktin et al. (2001)
determined the following parameters for NGC 6192: E(B − V) =
0.637, d = 1547 pc and log t = 8.13 (Table 1).
NGC 6208 (C1645-537) – also designated Cr 313 or
BH 198 – is located ∼26◦ from the Galactic center direction
in a very rich star field in Ara, which makes its
membership and astrophysical properties rather difficult to investigate.
To the best of our knowledge, the only photometric study was
performed by
Lindoff (1972)
, who obtained UBV
photoelectric and photographic photometry for 17 and 286 stars,
respectively, (...truncated)