Optical and near-infrared photometry of old galactic clusters
Date
2018-07-18
Authors
Rosvick, Joanne Marie
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Abstract
The open clusters NGC 2141, NGC 6791, NGC 6819 and NGC 7142,
all suspected of having ages greater than 2 billion years (Gyr), were observed
at optical and near-infrared wavelengths. The images were reduced
using standard IRAF routines, and magnitudes for the stars were determined
using DAOPHOT (Stetson, 1987). These data were used to construct colour-magnitude
diagrams (CMDs) for each cluster, as well as two-colour diagrams
(J - K, V - K), (J - H, H - K) of the giants.
Colour excesses were redetermined by comparing the optical CMD main
sequences to semi-empirical ZAMS calibrations (VandenBerg and Poll, 1989;
this work) and are as follows: E{B - V) = 0.32 ± 0.04, 0.23 ± 0.03, 0.11
± 0.03 and 0.29 ± 0.04, for NGC 2141, NGC 6791, NGC 6819 and NGC
7142, respectively. Apparent distance moduli for the clusters listed above
were found to be (m - M ) v = 13.93 ± 0.13, 13.52 ± 0.13, 12.10 ± 0.13 and
12.96 ± 0.16.
The optical CMDs were compared to sets of theoretical isochrones to
ascertain ages and test whether canonical or convective overshooting models
best represent the data. It was found that isochrones which allowed for
convective overshooting provided the best fits, resulting in ages of 2.5 Gyr,
10 Gyr, 2.5 Gyr and 2.5 Gyr for NGC 2141, NGC 6791, NGC 6819 and
NGC 7142, respectively. Two sets of overshooting isochrones (Bertelli et
al., 1994; Dowler and VandenBerg, 1996) yielded ages within 0.5 Gyr. The
MAR method (Anthony-Twarog and Twarog, 1985) placed the three younger
clusters at an approximate age of 3 Gyr.
In theory, the two-colour diagrams may be used to distinguish between
cluster giants and field stars. However, in practice this is not an easy task
since the infrared observations are not always accurate enough to separate
the cluster members and field stars. This was the case for these data, since
a problem with the H magnitudes resulted in colours offset from what was
expected.
The infrared (V,V - K and K, V - K) CMDs were useful in defining the
giant branch locus based on the position of cluster members. {V - K)o colours
were computed for each giant suspected of being a member. These were used
to determine effective temperatures and bolometric luminosities which in
turn were used to produce an HR diagram for each cluster. These were
compared to HR diagrams of other open and globular clusters (Houdashelt
et al., 1992; Frogel et al., 1983), as well as evolutionary tracks (Bertelli et
al., 1994). The giant branch loci of the near-solar abundance and metal-poor
clusters were found to lie between those defined by the clusters M67 and 47
Tuc. The comparison between the cluster HR diagrams and evolutionary
tracks indicated that the theoretical temperatures may be too hot.
The new cluster results were plotted on the age-metallicity relation defined
by Houdashelt et al.’s (1992) and Friel and Janes’ (1993) sample of open
clusters, and confirmed the lack of correlation between these two quantities.
The galactocentic distances (calculated from the distances given above) for
the clusters studied here were determined and used with the cluster metallicities
to support the presence of a metallicity gradient (~ -0.09 dex kpc-1)
in the galaxy.
Description
Keywords
Stars, Open clusters, Astronomical photometry