The effect of Main-Sequence stars on age estimates of open clusters

Existing methods for age estimation of open clusters are discussed. Most of the observed open clusters (except extremely young and old) contain 50–90% unevolved Main-Sequence stars. Possible difficulties encountered in estimating the actual age of an unevolved Main-Sequence star are discussed. For a relatively reliable cluster age estimation only a small percentage of cluster members are suitable. The effect of unevolved Main-Sequence stars on open cluster age estimates using the modified method of isochromes is analyzed.On leave of absence from the Department of Physics, Kumaon University, Naini Tal, India.     

Integrated luminosity distribution of Galactic open clusters

The integrated magnitudes of 221 Galactic open clusters have been used to derive the luminosity function. The completeness of the data has also been discussed. In the luminosity distribution the maximum frequency of clusters occurs nearI (M_v) = −3 _m ^. 5, and some plausible reasons for a sharp cut-off atI (M_v) = −2^m. 0 have been discussed. It is concluded that the paucity of the clusters fainter thanI (M _v) = −2 _m ^.0 is not purely due to selection effects. The surface density of the clusters for different magnitude intervals has. been obtained using the completeness radius estimated from the logN- logd plots. A relation betweenI (M^v) and surface density has been obtained which yields a steeper slope than that obtained by van den Bergh & Lafontaine (1984).     

On the lifetimes of galactic clusters

From the observed age distribution of galactic clusters within 1 kpc we deduce that the typical total lifetime of a galactic cluster is about 2×10⁸ yr. The individual lifetimes vary between 10⁸ and 10¹⁰ yr. The observed lifetimes are compared with the evaporation times which are found from numerical experiments with star cluster models. These models contain up to 250 stars with a realistic mass spectrum. The effect of the galactic tidal field is taken into account and enhances the rate of escape significantly. Escapers are identified by using the Jacobian integral. We give the evaporation time in years as a function of the median radius for different values of the total mass of a cluster. The agreement between the resulting theoretical lifetimes and the observed values is sufficiently good. We estimate that the tidal field of passing interstellar clouds should be in most cases less efficient in dissolving a galactic cluster than the internal evaporation process combined with the effect of the general galactic field.     

Note on the spatial distribution of old open clusters

Using the sample of 74 old open clusters compiled by Friel (1995) the increase of the mean distance from the galactic plane with cluster age has been determined. The result is in a qualitative agreement with the hypothesis of scattering of the clusters during encounters with massive molecular clouds and by spiral arms. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Young open star clusters: The spatial distribution of metallicity in the solar neighborhood

We perform a comparative analysis of the spatial distribution of young (<50 Myr) open star clusters and field Cepheids with different metallicities. A significant fraction of young clusters are shown to have low metallicities atypical of field Cepheids. Both types of objects exhibit approximately equal (in magnitude) negative radial metallicity gradients, while their azimuthal metallicity gradients differ outside the error limits and have opposite signs. Among the stellar complexes identified by young clusters, the most metal-poor clusters are grouped in the Perseus complex. It is the clusters of this complex that are responsible for the radial and azimuthal metallicity gradients among young clusters. The described properties are indicative of a weak mixing of interstellar matter before the onset of star formation there. Significant differences between the spatial distributions of open clusters and field stars with different metallicities suggest different conditions required for the formation of these types of objects.     

Radial distribution of the integrated light and photometric colours in open star clusters

Mass segregation in the form of preferential concentration of more massive stars in the central regions of a number of open star clusters has been known for some time. In this paper, integratedUBV colours in concentric zones have been estimated for 12 nearby open clusters using the observations of individual cluster members. It is found that the clusters showing pronounced mass segregation also show significant radial variations in the integrated colours. However, the effects of stochastic fluctuations around the massive portion of the mass distribution function on the integrated colours should be taken into consideration, if they are present.     

The problem of stellar membership in open clusters

Open clusters become a powerful astrophysics laboratory once the essential problem of stellar membership has been solved. In this paper different methods to compute membership probabilities using kinematical variables, mainly proper motions, are presented.Presented at the 2nd UN/ESA Workshop, held in Bogotá, Colombia, 9–13 November, 1992.     

The galactic distribution of globular clusters

The spatial distribution of galactic globular clusters has been studied by means of Walsh-Hadamard transforms. Consistent indications have been collected suggesting the existence, at least in a first approximation, of two disting populations of clusters — the metal poor clusters showing a spherical distribution whereas metal rich clusters are flattened along the disk.Simulated cluster-halos have been computed in order to study the dependence of observed radial velocities on the eccentricity of cluster orbits, showing that only a general high value of eccentricity seems able to reproduce the observational features.     

The orbits of open clusters in the Galaxy

We present and analyse the kinematics and orbits for a sample of 488 open clusters (OCs) in the Galaxy. The velocity ellipsoid for our present sample is derived as  (σ _U , σ _V , σ _W ) = (28.7, 15.8, 11.0) km s⁻¹  which represents a young thin-disc population. We also confirm that the velocity dispersions increase with the age of a cluster subsample. The orbits of OCs are calculated with three Galactic gravitational potential models. The errors of orbital parameters are also calculated considering the intrinsic variation of the orbital parameters and the effects of observational uncertainties. The observational uncertainties dominate the errors of derived orbital parameters. The vertical motions of clusters calculated using different Galactic disc models are rather different. The observed radial metallicity gradient of clusters is derived with a slope of   b =−0.070 ± 0.011   dex kpc⁻¹. The radial metallicity gradient of clusters based on their apogalactic distances is also derived with a slope of   b =−0.082 ± 0.014   dex kpc⁻¹. The distribution of derived orbital eccentricities for OCs is very similar to that derived for the field population of dwarfs and giants in the thin disc.     

The light-weighted age and metallicity of Galactic globular clusters

Spectra synthesis is entering a new era with the availability of large and high-quality spectroscopic data and full spectrum fitting methods. Establishing the reliability whether the STARLIGHT code (a full spectrum fitting code, Cid Fernandes et al. (Mon. Not. R. Astron. Soc. 358:363, 2005)) is suitable to study the properties of various types of galaxies is vital to galaxy study. In testing this possibility, Galactic globular clusters (GCs) provide an excellent medium. Taking the 40 Galactic GCs as an example in our work, we analyze their integrated spectra from Schiavon et al. (Astrophys. J. Suppl. 160:163, 2005) and obtain their mixtures of stellar populations that best represents their composite nature. Through analyzing the distributions of stellar ages and metallicities respectively, we find that the stellar metallicity distribution is centered at a low value and the stellar age distribution is very scattered. To compare with the literature, we also calculate the light-weighted ages and metallicities of GCs, and we find that the light-weighted metallicities agree with the literature, while the light-weighted ages are lower than those of the literature. This demonstrates that STARLIGHT can be used to obtain the metallicities of GCs.     

The OPD photometric survey of open clusters II. robust determination of the fundamental parameters of 24 open clusters

In the second paper of the series we continue the investigation of open cluster fundamental parameters using a robust global optimization method to fit model isochrones to photometric data. We present optical UBVRI CCD photometry (Johnsons-Cousins system) observations for 24 neglected open clusters, of which 14 have high quality data in the visible obtained for the first time, as a part of our ongoing survey being carried out in the 0.6 m telescope of the Pico dos Dias Observatory in Brazil. All objects were then analyzed with a global optimization tool developed by our group which estimates the membership likelihood of the observed stars and fits an isochrone from which a distance, age, reddening, total to selective extinction ratio R_V (included in this work as a new free parameter) and metallicity are estimated. Based on those estimates and their associated errors we analyzed the status of each object as real clusters or not, finding that two are likely to be asterisms. We also identify important discrepancies between our results and previous ones obtained in the literature which were determined using 2MASS photometry.     

uvby photometry of open clusters

Für insgesamt 40 Stern im Gebiet des verfärbten offenen Haufens NGC 6871 erhieken wir eine uvby Photometric. 21 dieser Sterne klassifizierten definitely as members. From the photometric results we derived 1.2 × 10⁷ years as the cluster age, 2440 pc as its distance, and E(b - y)) = 0.348 mag as mean reddening of the cluster. From the scatter around this mean colour excess and the deviation of a subgroup of stars by more than twice the standard deviation a varible intracluster reddening in order of 0.1 mag is indicated. The value of the colour excess ratio E(b - y)/E(B - V) = 0.705 for the cluster stars leads us to the conclusion that the very broadband structure (VBS) in the spectra of the cluster stars must be very weak. Für insgesamt 40 Stern im Gebiet des verfärbten offenen Haufens NGC 6871 erhielten wir eine uvby Photometrie. 21 dieser Sterne klassifizieren wir definitiv als Haufenmitglieder. Aus den photometrischen Ergebnissen leiteten wir ein Haufenalter von 1.2 × 10⁷ Jahren, eine Entfernung von 2440 pc und eine Verfärbung E (b - y) = 0.348 mag ab. Aus der Streuung um diesen mittleren Farbexzeß und der Tatsache, daß eine Gruppe von Sternen um mehr als die doppelte Standardabweichung von diesem Wert differiert, läßt sich eine Verfärbung innerhalb des Haufens in der Größenordnung von 0.1 mag vermuten. Aus der Größe des Farbexzeßverhältnisses E(b - y )/E(B - V) = 0.705 schließen wir, daß die Breitbandstruktur (VBS) in den Spektren der Haufensterne nur sehr schwach ausgeprägt sein kann.     

Two populations of open clusters

The paper proposes a division of the local open clusters into two different disklike subpopulations on the base of the metallicity enhancement history, spatial distribution in the Galaxy and morphology of their C-M diagrams. The thin disk population objects exhibit higher metallicities and are more closely concentrated to the center and the disk of the Galaxy than the thick disk clusters. The former contain also frequently blue stragglers while the later exhibit the presence of Clump giant members, typical for the thick disk globular clusters. Kinematical properties of the two considered open cluster subpopulations remain unknown due to the lack of observational data.     

Yttrium and barium in open clusters

Based on the VLT high-resolution spectra for stars of five open clusters: Ruprecht 4, Ruprecht 7, Berkeley 25, Berkeley 73, and Berkeley 75 (G. Carraro et al., 2007) we determined the yttrium and barium abundances using the synthetic-spectrum method. Barium abundance was calculated under a non-LTE approximation. We analyzed the correlation between yttrium and barium abundances and the ages of open clusters and stars of the Galactic thin disk.     

Optical polarimetric study of open clusters: distribution of interstellar matter towards NGC 654

We present new B ,  V and R linear polarimetric observations for 61 stars towards the region of the young open cluster NGC 654. In this study we found evidence for the presence of at least two layers of dust along the line of sight to the cluster. The distances to the two dust layers are estimated to be ∼200 pc and ∼1 kpc which are located much closer to the Sun than the cluster (∼2.4 kpc). Both the dust layers have their local magnetic field orientation nearly parallel to the direction of the Galactic plane. The foreground dust layer is found to have a ring morphology with the central hole coinciding with the centre of the cluster. The foreground dust grains are suggested to be mainly responsible for both the observed differential reddening and the polarization towards the cluster.     

Integrated magnitudes and colors of open clusters

The integratedUBV characteristics of 50 galactic clusters, computed for the clusters' physical members only (segregated on the basis of the proper motions or radial velocities) are discussed.On the basis of a comparison between the empirically-obtained dependences and their theoretical counterparts (Searleet al., 1973) a suggestion has been made that the number of the massive stars (with masses>10M ) in the initial mass function must be considerably greater.Evolutionary effects on integrated parameters and their dependences have been discussed. An attempt at an evolutionary interpretation of the integrated luminosity function for galactic clusters, obtained by Starikova (1962) in the light of the self-obtained dependences has been made.     

Dynamics of very rich open clusters

The oldest open clusters in our Galaxy set the lower limit to the age of the Galactic Disk (9–10 Gyr). Although they appear to be very rich now, it is clear that their primordial populations were much larger. Often considered as transitional objects, these populous open clusters show structural differences with respect to globular clusters so their dynamics and characteristic evolutionary time scales can also be different. On the other hand, their large membership lead to different dynamical evolution as compared with average open clusters. In this paper, the differential features of the evolution of rich open clusters are studied using N-body simulations, including several of the largest (10⁴ stars) published direct collisional N-body calculations so far, which were performed on a CRAY YMP. The disruption rate of rich open clusters is analysed in detail and the effect of the initial spatial distribution of the stars in the cluster on its dynamics is studied. The results show that cluster life-time depends on this initial distribution, decreasing when it is more concentrated. The effect of stellar evolution on the dynamical evolution of rich clusters is an important subject that also has been considered here. We demonstrate that the cluster's life-expectancy against evaporation increases because of mass loss by evolving high-mass stars. This revised version was published online in July 2006 with corrections to the Cover Date.