Daniela Pavani

Dissolving star cluster candidates

We present a list of 34 neglected entries from star cluster catalogues located at relatively high galactic latitudes (

Open clusters or their remnants: B and V photometry of NGC 1901 and NGC 1252

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NGC 2180: A disrupting open cluster?

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The relationship between the Sérsic law profiles measured along the major and minor axes of elliptical galaxies

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Ruprecht 3: An old star cluster remnant?

) which appear to be candidate late stages of star cluster dynamical evolution. Although underpopulated with respect to usual open clusters, they still present a high number density contrast as compared to the galactic field. This was verified by means of (i) predicted model counts from different galactic subsystems in the same direction, and (ii) Guide Star Catalog equal solid angle counts for the object and surrounding fields. This suggests that the objects are physical systems, possibly star clusters in the process of disruption or their fossil remains. The sample will be useful for followup studies in view of verifying their physi cal nature.

Determination of astrophysical parameters of open clusters using classical and semi-automated isochrone-fitting methods

Photometry in theB andV bands is presented for the southern stellar groups NGC 1901 and NGC 1252. NGC 1901 is often described as an open cluster while NGC 1252 consists of a concentration of about 20 stars centered20 0 north of the original New General Catalogue coordinates, and at the southwest edge of the large region previously assigned to this object in the literature. NGC 1901 has a clear main sequence and shares similarities with the Hyades. We derive a reddening value E(B V )=0 :04, a distance from the Sund =0 :45 kpc (Z = 0:23 kpc) and an age 0.6 0.1 Gyr. NGC 1901 is conclusively a physical system, dynamically comparable to or more evolved than the Hyades. The colour-magnitude diagram of NGC 1252 suggests a turno and main sequence, and a total of 12 probable members. We simulated the Galactic eld colour-magnitude diagram in the same direction and found it to be a poor match to NGC 1252, suggesting that NGC 1252 is not a eld fluctuation. Isochrone tting to the probable members is consistent with E(B V )=0 :02, d =0 :64 kpc (Z = 0:46 kpc) and an age 3 1 Gyr. NGC 1252 cannot be ruled out as a physical group with the available data. If so, evidence is found that it is not a classical open cluster, but rather an open cluster remnant.

Further developments in star cluster spectral libraries

The spatial dependence of luminosity and mass functions of evolved open clusters is discussed in this work using J and H 2MASS photometry, which allows a wide spatial coverage and proper background determination. The target objects are the intermediate-age open cluster NGC 3680 (� = 286.76 ◦ and b = 16.92 ◦ ), which has been reported as being in an advanced state of dissolution, and NGC 2180 (� = 203.85 ◦ , b = −7.01 ◦ ), a possible dynamically evolved open cluster. At the 1 − σ level, NGC 2180 presents a radial distribution of stars that can be represented by a King law. We conclude that, although in an advanced dynamical state (mass segregated), NGC 3680 does not present strong signs of dissolution, having luminosity and mass functions very similar to those of the τage ≈ 3.2 Gyr open cluster M 67. On the other hand, NGC 2180 appears to have flat, eroded luminosity functions throughout its structure, suggesting that in addition to mass segregation, Galactic tidal stripping has been effective in depleting this object. Accordingly, NGC 2180 may be the missing link between evolved open clusters and remnants. For NGC 2180 we infer a distance to the Sun d� = 0.91 ± 0.15 kpc, an age τage ≈ 710 Myr and an observed stellar mass of mobs ∼ 47 M� . For NGC 3680 we derive an age τage ≈ 1. 6G yr,E(B − V) = 0. 0a ndd� = 1.00 ± 0.09 kpc, confirming previous estimates. The observed stellar mass mobs ≈ 130 Magrees with previous values. We study both clusters in the context of dynamical states estimated from diagnostic diagrams involving photometric and structural parameters. Both objects appear to be dynamically evolved systems. In particular, NGC 2180 is closer to open cluster remnants than NGC 3680.

Diagnostic tool to analyse colour–magnitude diagrams of poorly populated stellar concentrations

In this paper we discuss the reason why the parameters of the Sersic model best-fitting the major axis light profile of elliptical galaxies can differ significantly from those derived for the minor axis profile. We show that this discrepancy is a natural consequence of the fact that the isophote eccentricity varies with the radius of the isophote and present a mathematical transformation that allows the minor axis Sersic model to be calculated from the major axis model, provided that the elliptical isophotes are aligned and concentric and that their eccentricity can be represented by a well behaved, though quite general, function of the radius. When there is no variation in eccentricity only the effective radius changes in the Sersic model, while for radial-dependent eccentricity the transformation, which allows the minor axis Sersic model to be calculated from the major axis model is given by the Lerch Φ transcendental function. The proposed transformation was tested using photometric data for 28 early-type galaxies.

On open cluster physical parameters from 2MASS data

MASS J and H photometry and integrated spectroscopy are employed to study the nature of the poorly populated compact concentration of stars Ruprecht3, which was previously catalogued as an open cluster. The integrated spectrum re- markably 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 +0.04 −0.03 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 Ruprecht3 are compatible with what would be expected for an intermediate-age open cluster remnant.

A vida na Terra e as culturas sob a influência de solstícios e equinócios

When we use optical isochrone-fitting solutions from the literature to 2 mass CMDs, they are often not the best solutions in the infrared domain. We analysed 10 open clusters with 2 mass , nine of them previously studied with optical photometry (NGC 1245, NGC 1342, NGC 1502, NGC 2104, NGC 2204, NGC 2243, NGC 2281, NGC 6709 and NGC 744) and one using integrated spectroscopy (BH 132). The study involved the classical (by eye) and a semi-automated method of isochrone fitting. We used the solutions of the first method as input for the second, looking for refined solutions. The semi-automated method uses a synthetic color–magnitude diagram (CMD), based on different Padova isochrones, to compare with the observed CMDs by means of likelihood statistics. The derived astrophysical parameters are age, distance and reddening values. The present results show better fits than those implied by the optical values. We also show that the semi-automated method decreases the parameter uncertainties.