703 research outputs found
Old open clusters: UBGVRI photometry of NGC 2506
UBGVRI photometry for the open cluster NGC 2506 is presented. From comparison
of the observed colour-magnitude diagrams with simulations based on stellar
evolutionary models we derive in a self consistent way reddening, distance, and
age of the cluster: E(B-V)=0-0.07, (m-M)o = 12.6, age = 1.5-2.2 Gyr. The
cluster shows a well definite secondary sequence, suggesting that binary
systems constitute about 20 % of the cluster members visible in the
colour-magnitude diagram.Comment: 11 pages, 7 figures, MNRAS latex style, accepte
The intermediate age open cluster NGC 2660
We present CCD UBVI photometry of the intermediate old open cluster NGC2660,
covering from the red giants region to about seven magnitudes below the main
sequence turn-off. Using the synthetic Colour - Magnitude Diagram method, we
estimate in a self-consistent way values for distance modulus ((m-M)0 ~= 12.2),
reddening (E(B-V) ~= 0.40), metallicity ([Fe/H] about solar), and age (age ~ 1
Gyr). A 30% population of binary stars turns out to be probably present.Comment: 12 pages, 8 (encapsulated) figures, to be published on MNRA
Mining SDSS in search of Multiple Populations in Globular Clusters
Several recent studies have reported the detection of an anomalous color
spread along the red giant branch (RGB) of some globular clusters (GC) that
appears only when color indices including a near ultraviolet band (such as
Johnson U or Stromgren u) are considered. This anomalous spread in color
indexes such as U-B or c_{y} has been shown to correlate with variations in the
abundances of light elements such as C, N, O, Na, etc., which, in turn, are
generally believed to be associated with subsequent star formation episodes
that occurred in the earliest few 10^{8} yr of the cluster's life. Here we use
publicly available u, g, r Sloan Digital Sky Survey photometry to search for
anomalous u-g spreads in the RGBs of nine Galactic GCs. In seven of them (M 2,
M 3, M 5, M 13, M 15, M 92 and M 53), we find evidence of a statistically
significant spread in the u-g color, not seen in g-r and not accounted for by
observational effects. In the case of M 5, we demonstrate that the observed u-g
color spread correlates with the observed abundances of Na, the redder stars
being richer in Na than the bluer ones. In all the seven clusters displaying a
significant u-g color spread, we find that the stars on the red and blue sides
of the RGB, in (g, u-g) color magnitude diagrams, have significantly different
radial distributions. In particular, the red stars (generally identified with
the second generation of cluster stars, in the current scenario) are always
more centrally concentrated than blue stars (generally identified with the
first generation) over the range sampled by the data (0.5r_{h} < r < 5r_{h}),
in qualitative agreement with the predictions of some recent models of the
formation and chemical evolution of GCs. Our results suggest that the
difference in the radial distribution between first and second generation stars
may be a general characteristic of GCs.Comment: 11 pages, 5 figures, typos adde
The old anticentre open cluster Berkeley 32: membership and fundamental parameters
We have obtained medium-low resolution spectroscopy and BVI CCD imaging of
Berkeley 32, an old open cluster which lies in the anticentre direction. From
the radial velocities of 48 stars in the cluster direction we found that 31 of
them, in crucial evolutionary phases, are probable cluster members, with an
average radial velocity of +106.7 (sigma = 8.5) km/s. From isochrone fitting to
the colour magnitude diagrams of Berkeley 32 we have obtained an age of 6.3
Gyr, (m-M)0 = 12.48 and E(B-V) = 0.10. The best fit is obtained with Z=0.008. A
consistent distance, (m-M)0 ~= 12.6 +/- 0.1, has been derived from the mean
magnitude of red clump stars with confirmed membership; we may assume (m-M)0 ~=
12.55 +/- 0.1. The colour magnitude diagram of the nearby field observed to
check for field stars contamination looks intriguingly similar to that of the
Canis Major overdensity.Comment: MNRAS, in press. Degraded resolution for Fig.
Photometric and spectroscopic study of the intermediate age open cluster NGC 3960
We present CCD UBVI photometry and high-resolution spectroscopy of the
intermediate age open cluster NGC 3960. The colour - magnitude diagrams (CMDs)
derived from the photometric data and interpreted with the synthetic CMD method
allow us to estimate the cluster parameters. We derive: age = 0.9 or 0.6 Gyr
(depending on whether or not overshooting from convective regions is included
in the adopted stellar models), distance (m-M)0 = 11.6 +/- 0.1, reddening
E(B-V) = 0.29 +/- 0.02, differential reddening Delta E(B-V) = 0.05 and
approximate metallicity between solar and half of solar. We obtained high
resolution spectra of three clump stars, and derived an average [Fe/H] = -0.12
(rms 0.04 dex), in very good agreement with the photometric determination. We
also obtained abundances of alpha-elements, Fe-peak elements, and of Ba. The
reddenings toward individual stars derived from the spectroscopic temperatures
and the Alonso et al. calibrations give further support to the existence of
significative variations across the cluster.Comment: Accepted for publication on MNRAS; fig. 3, 4, 5, 6 at degraded
resolutio
Photometric and spectroscopic study of the intermediate-age open cluster NGC 2355
In this paper we analyse the evolutionary status and properties of the old
open cluster NGC 2355, located in the Galactic anticentre direction, as a part
of the long term programme BOCCE. NGC 2355 was observed with LBC@LBT using the
Bessel , , and filters. The cluster parameters have been obtained
using the synthetic colour-magnitude diagram (CMD) method, as done in other
papers of this series. Additional spectroscopic observations with FIES@NOT of
three giant stars were used to determine the chemical properties of the
cluster. Our analysis shows that NGC 2355 has metallicity slightly less than
solar, with [Fe/H] dex, age between 0.8 and 1 Gyr, reddening
in the range 0.14 and 0.19 mag, and distance modulus of about 11 mag.
We also investigated the abundances of O, Na, Al, , iron-peak, and
neutron capture elements, showing that NGC 2355 falls within the abundance
distribution of similar clusters (same age and metallicity). The Galactocentric
distance of NGC~2355 places it at the border between two regimes of metallicity
distribution; this makes it an important cluster for the study of the chemical
properties and evolution of the disc.Comment: 20 pages, 11 figures, Accepted on MNRA
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