Ruprecht 3: An old star cluster remnant?
A&A
Astronomy & Astrophysics
D. B. Pavani 1 2
E. Bica 1 2
A. V. Ahumada 0 1
J. J. Claria´ 0 1
0 Observatorio Astrono ́mico de Co ́rdoba , Laprida, 854, 5000, Co ́rdoba , Argentina
1 Send offprint requests to: D. B. Pavani
2 Universidade Federal do Rio Grande do Sul , IF, CP 15051, Porto Alegre 01501-970, RS , Brasil
2MASS J and H photometry and integrated spectroscopy are employed to study the nature of the poorly populated compact concentration of stars Ruprecht 3, which was previously catalogued as an open cluster. The integrated spectrum remarkably resembles that of a moderately metal-rich globular cluster. The distribution of the object stars in the colour-magnitude diagram is compatible with that of a 1.5 ± 0.5 Gyr open cluster or older, depending on whether the bluer stars are interpreted as turnoff stars or blue stragglers, respectively. We derive for the object a distance from the Sun d = 0.72−+00..0034 kpc and a colour excess E(B − V ) = 0.04. Although a globular cluster remnant cannot be ruled out, the integrated spectrum resemblance to that of a globular cluster probably reflects a stochastic effect owing to the few brighter stars. The structural and photometric properties of Ruprecht 3 are compatible with what would be expected for an intermediate-age open cluster remnant.
Galaxy; open clusters and stellar associations; general - open clusters and associations; individual; Ruprecht 3 - methods; observational - techniques; spectroscopic
1. Introduction
There has been recently growing interest in investigating open
star clusters and their remnants. In the wake of this tendency,
studies based on numerical simulations
(de la Fuente Marcos
1997, 1998)
have shown that the open cluster lifetime and
stellar content depend on the initial mass function, fraction
of primordial binaries and galactocencentric distance. The
final cluster remnant is expected to be rich in binary stars.
Additionally, remnants of poorly populated clusters are
expected to be young, containing early-type stars, while remnants
of rich open clusters are expected to contain later-type stars. It
is also within reasonable expectations to assume that owing to
the effect of mass segregation, open cluster remnants should
present depletion of low Main Sequence (MS) stars by means
of encounters with more massive stars and binary ones and
evaporation due to the tidal field of the Galaxy. The depletion of
low MS stars would imply that clusters which are close to
disruption, have a core rich in compact and giant stars (Takahashi
& Portegies Zwart 2000). From the observational point of view,
an open cluster remnant can be defined as a poorly populated
concentration of stars resulting from the dynamical evolution
of an initially more massive system
(Pavani et al. 2001)
. There
is currently existing knowledge about the final destiny of open
clusters. The nature and dynamical stage of some stellar groups
have been inferred. Despite these facts, however, only a few
of them have been studied in detail and open questions still
remain. For instance, NGC 6994 (M73) has been either
interpreted as a possible remnant of an old open cluster or,
alternatively, as a random fluctuaction of the star density in the
object region
(Bassino et al. 2000; Carraro 2000; Ordenkirchen
& Soubiran 2002)
. Recently, NGC 7036 and NGC 7772 were
analysed by Carraro (2002), whose study suggests that both
objects are open cluster remnants (OCRs). NGC 7036 is a
stellar group with 17 likely members located at 1 kpc from the
Sun. It has a core radius of about 3–4 arcmin and an age of
3–4 Gyr. NGC 7772, located at about 1.5 kpc from the Sun,
was found to have 14 likely members with an age of 1.5 Gyr.
Many other candidates exist, e.g. in the study of
Bica et al.
(2001)
, where an analysis of number density contrast has been
performed. In a study of the stellar group NGC 1252,
Pavani
et al. (2001)
favoured an open cluster remnant. Likely
members populate evolutionary sequences in the colour-magnitude
diagram (CMD). In the present study we discuss the case of the
stellar group Ruprecht 3.
Ruprecht 3, also known as OCL-642
(Alter et al. 1970)
or
ESO 426SC33
(Lauberts 1982)
, is located in Canis Major at
J2000.0 α = 06h42m07s, δ = −29◦27 15 (l = 238◦.77, b =
−14◦.81). This object was classified as II1p in the Trumpler
System by
Ruprecht (1966)
, i.e. as an open cluster with
the second highest concentration degree, a small range of
brightness of its stars and a scanty population. The
direction towards Ruprecht 3 was probed in H I by
Olano et al.
(1980)
but none was found to be related to the object. We
brought Ruprecht 3 under the focus of our investigation on
account of its status as a relatively compact concentration
of stars of ≈3 , similar to those analysed in
Bica et al.
(2001)
. The object region is already available in the Two
Micron All Sky Survey (2MASS) photometric catalogue –
http://www.ipac.caltech.edu/2mass/. In Sects. 2 and 3
we present the phot (...truncated)