NGC 1883: a neglected intermediate-age open cluster located in the outskirts of the Galactic disk

We report on $BVI$ CCD photometry of a field centered in the region of the open cluster NGC 1883 down to V=21. This cluster has never been studied insofar, and we provide for the first time estimates of its fundamental parameters, namely radial extent, age, distance and reddening. We find that the cluster has a radius of about 2.5 arcmin, and shows signatures of dynamical relaxation. NGC 1883 is located in the anti-center direction, and exhibits a reddening in the range E$(B-V)= 0.23-0.35$, depending on the metal abundance. It turns out to be of intermediate-age (1 billion years old), and quite distant for an open cluster. In fact it is located 4.8 kpc from the Sun, and more than 13 kpc from the Galactic center. This results makes NGC 1883 one of the most peripheral old open clusters, with important consequences for the trend of the metallicity with distance in the outer Galactic disk.Comment: 5 pages, 6 eps figures, in press in MNRA

The anticenter old open cluster NGC 1883: radial velocity and metallicity

Having already reported on the first photometric study of the intermediate-age open cluster NGC 1883 (Carraro et al. 2003), we present in this paper the first spectroscopic multi-epoch investigation of a sample of evolved stars in the same cluster. The aim is to derive the cluster membership, velocity and metallicity, and discuss recent claims in the literature (Tadross 2005) that NGC 1883 is as metal poor as globular clusters in the Halo. Besides, being one of the few outer Galactic disk intermediate-age open clusters known so far, it is an ideal target to improve our knowledge of the Galactic disk radial abundance gradient, that is a basic ingredient for any chemical evolution model of the Milky Way. The new data we obtained allow us to put NGC 1883's basic parameters more reliable. We find that the cluster has a mean metallicity of [Fe/H] = -0.20$\pm$0.22, from which we infer an age (650$^{+70}_{-70}$ Myr) close to the Hyades one and a Galactocentric distance of 12.3$^{+0.4}_{-0.2}$ kpc. The metal abundance, kinematics, and position make NGC 1883 a genuine outer disk intermediate-age open cluster. We confirm that in the outer Galactic disk the abundance gradient is shallower than in the solar vicinity.Comment: 8 pages, 7 eps figures (some degraded in resolution), accepted for publication in MNRA

Abundance analysis of red clump stars in the old, inner disc, open cluster NGC 4337: a twin of NGC 752?

Open star clusters older than ~ 1 Gyr are rare in the inner Galactic disc. Still, they are objects that hold crucial information for probing the chemical evolution of these regions of the Milky Way. We aim at increasing the number of old open clusters in the inner disc for which high-resolution metal abundances are available. Here we report on NGC 4337, which was recently discovered to be an old, inner disc open cluster. We present the very first high-resolution spectroscopy of seven clump stars that are all cluster members. We performed a detailed abundance analysis for them. We find that NGC 4337 is marginally more metal-rich than the Sun, with [Fe/H]=+0.12$\pm$0.05. The abundance ratios of $\alpha$-elements are generally solar. At odds with recent studies on intermediate-age and old open clusters in the Galactic disc, Ba is under-abundant in NGC 4337 compared with the Sun. Our analysis of the iron-peak elements (Cr and Ni) does not reveal anything anomalous. Based on these results, we estimate the cluster age to be 1.6$^{+0.1}_{-0.1}$ Gyr, and derive a reddening E(B-V)=0.23$\pm$0.05, and an apparent distance modulus $(m-M)_{V}=12.80\pm0.15$. Its distance to the Galactic centre is 7.6 kpc. With this distance and metallicity, NGC 4337 fits the metallicity gradient for the inner Galactic disc fairly well. The age and metallicity we measured make NGC 4337 a twin of the well-known old open cluster NGC 752. The red clumps of these two clusters bear an amazing resemblance. But the main sequence of NGC 752 is significantly more depleted in stars than that of NGC 4337. This would mean that NGC 752 is in a much more advanced dynamical stage, being on the verge of dissolving into the general Galactic field. Our results make NGC 4337 an extremely interesting object for further studies of stellar evolution in the critical turn-off mass range 1.1-1.4 solar masses.Comment: 7 pages, 8 eps figures, in press in Astronomy and Astrophysics. arXiv admin note: text overlap with arXiv:1401.156

Radial velocities and metallicities of red giant stars in the old open cluster NGC 7762

We present and discuss radial velocity and the very first metallicity measurements for nine evolved stars in the poorly known old open cluster NGC 7762. We isolated eight radial velocity cluster members and one interloper. Radial velocities are in good agreement with previous studies. NGC 7762 turns out to be of solar metallicity within the uncertainties ([Fe/H]=0.04$\pm$0.12). For this metallicity, the cluster age is 2.5$\pm$0.2 Gyr, and falls in a age range where only a few old open clusters are known. With respect to previous studies, we find a larger distance, implying the cluster to be located at 900$^{+70}_{-50}$ pc from the Sun. For most of the elements we measure solar-scaled abundance ratios. We searched the literature for open clusters of similar age in the solar vicinity and found that NGC 7762 can be considered a twin of Ruprecht 147, a similar age cluster located at only 300 pc from the Sun. In fact, beside age, also metallicity and abundance ratios are very close to Ruprecht 147 values within the observational uncertainties.Comment: 8 pages, 5 figures, accepted for publication in the Astronomical Journa

Bimodality of light and s-elements in M4 (NGC 6121)

All Globular Clusters (GCs) studied in detail so far host two or more populations of stars. Theoretical models suggest that the second population is formed from gas polluted by processed material produced by massive stars of the first generation. However the nature of the polluter is a matter of strong debate. Several candidates have been proposed: massive main-sequence stars (fast rotating or binaries), intermediate-mass AGB stars, or SNeII. We studied red giant branch (RGB) stars in the GC M4 (NGC 6121) to measure their chemical signature. We confirm the presence of a bimodal population, first discovered by Marino et al. (2008). The two groups have different C,$^{12}$C/$^{13}$C,N,O,Na content, but share the same Li,C+N+O,Mg,Al,Si,Ca,Ti,Cr,Fe,Ni,Zr,Ba and Eu abundance. Quite surprisingly the two groups differ also in their Y abundance. The absence of a spread in $\alpha$-elements, Eu and Ba makes SNeII and AGB stars unlikely as polluters. On the other hand, massive main-sequence stars can explain the bimodality of Y through the weak s-process. This stement is confirmed independently also by literature data on Rb and Pb. Observations suggest that the mass of the polluters is between 20 and 30 M$_{\odot}$. This implies a formation time scale for the cluster of 10$\div$30 Myrs. This result is valid for M4. Other clusters like NGC 1851, M22, or $\omega$ Cen have different chemical signatures and may require other kinds of polluter.Comment: 14 pages, 7 figures, accepted for publication on A&

The Metallicity of the Open Cluster Tombaugh 2

We investigate the nature of the chemical composition of the outer disc open cluster Tombaugh 2, that a recent study by Frinchaboy et al. (2008) suggested to possess an intrinsic metal abundance dispersion. We aim to investigate such claims by high resolution spectra obtained for a number of stars in the Tombaugh 2 field, together with independent UBVIc photometry. The spectra, together with input atmospheric parameters and model atmospheres, are used to determine detailed chemical abundances for a variety of elements in 13 members having good spectra. We find the mean metallicity to be [Fe/H]=-0.31+-0.02 with no evidence for an intrinsic abundance dispersion, in contrary to the recent results of Frinchaboy et al. (2008). We find Ca and Ba to be slightly enhanced while Ni and Sc are solar. The r-process element Eu was found to be enhanced, giving an average [Eu/Ba]=+0.17. The Li abundance decreases with Teff on the upper giant branch and maintains a low level for red clump stars. The mean metallicity we derive is in good agreement with that expected from the radial abundance gradient in the disc for a cluster at its Galactocentric distance. The surprising result found by Frinchaboy et al. (2008), that is the presence of 2 distinct abundance groups within the cluster, implying either a completely unique open cluster with an intrinsic metallicity spread, or a very unlikely superposition of a cold stellar stream and a very distant open cluster, is not supported by our new result.Comment: 11 pages, 5 eps figures, in press on A&

Chemical abundance analysis of the old, rich open cluster Trumpler 20

Trumpler 20 is an open cluster located at low Galactic longitude, just beyond the great Carina spiral arm, and whose metallicity and fundamental parameters were very poorly known until now. As it is most likely a rare example of an old, rich open cluster -- possibly a twin of NGC 7789 -- it is useful to characterize it. To this end, we determine here the abundance of several elements and their ratios in a sample of stars in the clump of Trumpler 20. The primary goal is to measure Trumpler 20 metallicity, so far very poorly constrained, and revise the cluster's fundamental parameters. We present high-resolution spectroscopy of eight clump stars. Based on their radial velocities, we identify six bona fide cluster members, and for five of them (the sixth being a fast rotator) we perform a detailed abundance analysis. We find that Trumpler 20 is slightly more metal-rich than the Sun, having [Fe/H]=+0.09$\pm$0.10. The abundance ratios of alpha-elements are generally solar. In line with recent studies of clusters as old as Trumpler 20, Ba is overabundant compared to the Sun. Our analysis of the iron-peak elements (Cr and Ni) does not reveal anything anomalous. Based on these results, we re-estimate the cluster age to be 1.5$^{+0.2}_{-0.1}$ Gyr. Its distance to the Galactic centre turns out to be 7.3 kpc. With this distance and metallicity, Trumpler 20 fits fairly well in the metallicity gradient for the galactic inner disc. With this new study, the characterization of Trumpler~20 is now on much more solid ground. Further studies should focus on the estimate of the binary fraction and on its main sequence membership.Comment: 13 pages, 6 eps figures, in press in Astronomy and Astrophysic

NGC 1883: a neglected intermediate-age open cluster located in the outskirts of the Galactic disc

We report on BVI CCD photometry of a field centred in the region of the open cluster NGC 1883, down to V = 21. This cluster has never been studied so far; we provide, for the first time, estimates of its fundamental parameters - namely radial extent, age, distance and reddening. We find that the cluster has a radius of about 2.5 arcmin, and shows signatures of dynamical relaxation. NGC 1883 is located in the anticentre direction, and exhibits a reddening in the range E(B-V) = 0.23-0.35, depending on the metal abundance. It turns out to be of intermediate age (1 Gyr old), and is quite distant for an open cluster. In fact, it is located 4.8 kpc from the Sun, and more than 13 kpc from the Galactic centre. This results makes NGC 1883 one of the most peripheral old open clusters, with important consequences for the trend of the metallicity with distance in the outer Galactic disc.Facultad de Ciencias Astronómicas y GeofísicasInstituto de Astrofísica de La Plat

NGC 1883: a neglected intermediate-age open cluster located in the outskirts of the Galactic disc

We report on BVI CCD photometry of a field centred in the region of the open cluster NGC 1883, down to V = 21. This cluster has never been studied so far; we provide, for the first time, estimates of its fundamental parameters - namely radial extent, age, distance and reddening. We find that the cluster has a radius of about 2.5 arcmin, and shows signatures of dynamical relaxation. NGC 1883 is located in the anticentre direction, and exhibits a reddening in the range E(B-V) = 0.23-0.35, depending on the metal abundance. It turns out to be of intermediate age (1 Gyr old), and is quite distant for an open cluster. In fact, it is located 4.8 kpc from the Sun, and more than 13 kpc from the Galactic centre. This results makes NGC 1883 one of the most peripheral old open clusters, with important consequences for the trend of the metallicity with distance in the outer Galactic disc.Facultad de Ciencias Astronómicas y GeofísicasInstituto de Astrofísica de La Plat

Abundance analysis of a sample of evolved stars in the outskirts of Omega Centauri

The globular cluster $\omega$ Centauri (NGC 5139) is a puzzling stellar system harboring several distinct stellar populations whose origin still represents a unique astrophysical challenge. Current scenarios range from primordial chemical inhomogeneities in the mother cloud to merging of different sub-units and/or subsequent generations of enriched stars - with a variety of different pollution sources- within the same potential well. In this paper we study the chemical abundance pattern in the outskirts of Omega Centauri, half-way to the tidal radius (covering the range of 20-30 arcmin from the cluster center), and compare it with chemical trends in the inner cluster regions, in an attempt to explore whether the same population mix and chemical compositions trends routinely found in the more central regions is also present in the cluster periphery.We extract abundances of many elements from FLAMES/UVES spectra of 48 RGB stars using the equivalent width method and then analyze the metallicity distribution function and abundance ratios of the observed stars. We find, within the uncertainties of small number statistics and slightly different evolutionary phases, that the population mix in the outer regions cannot be distinguished from the more central regions, although it is clear that more data are necessary to obtain a firmer description of the situation. From the abundance analysis, we did not find obvious radial gradients in any of the measured elements.Comment: 22 pages, 8 eps figures, in press in New Astronom