Gianni Marconi

UBVI photometry of the intermediate-age open cluster NGC 6939

ABSTRACT We present CCD UBVI photometry of the nearby, intermediate age open clusterNGC 6939. Using the synthetic Colour - Magnitude Diagrams technique we estimatethe following parameters: age between 1.3 and 1.0 Gyr (depending on whether or notovershooting is considered), reddening 0.34 ≤ E(B−V) ≤ 0.38 and distance modulus11.3 ≤ (m−M) 0 ≤ 11.4.Key words: Hertzsprung-Russell (HR) diagram – open clusters and associations:general – open clusters and associations: individual: NGC 6939 1 INTRODUCTIONA vast amount of information on the formation and evolu-tion of our Galaxy can be gathered from the study of openclusters (OCs). In particular, old OCs may be useful to de-rive information not only on the present day situation, butalso on the time evolution of the disc, since their ages coverthe whole existence of the disc, reaching to about 10 Gyr(Friel 1995). In order to fully exploit the information pro-vided by OCs we must derive for them in a homogeneousway: accurate absolute ages (and a consistent age ranking),distances, reddenings, and metal content. With this aim weare studying a number of old OCs (see Bragaglia & Tosi2003 and Sandrelli et al. 1999 for references), and we nowadd another cluster to our sample.Given its relative proximity, NGC 6939 (C2030+604)has been the target of several studies in the past: the firstbibliographic entry is 80 years ago (Kustner 1923) but, sur-prisingly, the first CCD data appeared only in 2002 (Rosvick& Balam 2002, hereafter RB02). As usual, the cluster pa-rameters found in literature do not agree with each other,and we present in this paper new and improved determi-nations for this intermediate age open cluster located atRA(2000) = 20:31:32, DEC(2000) = +60:39:00, or l = 95.88,b = 12.30.Photometry has been previously presented by severalauthors, but old photographic photometry only reachedabout one magnitude below the main sequence Turn-Off.Mermilliod, Huestamendia, & del Rio (1994) took UBV pho-toelectric photometry of 37 member stars all in the redclump phase, with the intent of discriminating between dif-ferent evolutionary models (with or without overshooting)by comparison with theoretical isochrones. It turned outthat most of the bright stars lying in the cluster directionare indeed cluster members (their Table 1 lists only 4 non-members among 45 objects). The recent work by Rosvick &Balam (2002) has presented the first deep BVI CCD data.They used the 1.85m Dominion Astrophysical Observatorytelescope, covering the central part of NGC 6939 (more orless our field A, see later). Their CMDs show considerablescatter, which they do not attribute to contamination fromfield stars (even if they could not prune their diagrams onthe main sequence, since no proper motion survey on thiscluster has reached so deep), but to differential reddening[E(B−V) = 0.29 to 0.41], which also influences the distancederivation [(m−M)

VLT FORS-1 observations of NGC 6397: Evidence for mass segregation

We present (V, V − I) VLT-FORS1 observations of the Galactic Globular Cluster NGC 6397. We derive accurate color-magnitude diagrams and luminosity functions (LFs) of the cluster Main Sequence (MS) for two fields extending from a region near the centre of the cluster out to � 10 � . The photometry of these fields produces a narrow MS extending down to V � 27, much deeper than any previous ground based study on this system and comparable to previous HST photometry. The V, V − I CMD also shows a deep white dwarf cooling sequence locus, contaminated by many field stars and spurious objects. We concentrate the present work on the analysis of the MSLFs derived for two annuli at different radial distance from the center of the cluster. Evidence of a clear-cut correlation between the slope of the observed LFs before reaching the turn-over, and the radial position of the observed fields inside the cluster area is found. We find that the LFs become flatter with decreasing radius (x � 0.15 for 1 � < R1 < 5.5 � ; x � 0.24 for 5.5 � < R2 < 9.8 � ;c ore radius,rc = 0.05 � ), a trend that is consistent with the interpretation of NGC 6397 as a dynamically relaxed system. This trend is also evident in the mass function.

Berkeley 22, an old and distant open cluster towards the Galactic anticentre

We present deep CCD BV I photometry of the distant, old open cluster Berkeley 22, covering from the red giants branch (RGB) to about 6 magnitudes below the main sequence (MS) turn-off. Using the synthetic Colour - Magnitude Diagram method with three different types of stellar evolutionary tracks, we estimate values and theoretical

The distance to NGC 5904 (M 5) via the subdwarf main sequence fitting method

We present a determination of the distance modulus of the globular cluster NGC 5904 (M 5), obtained by means of the subdwarf main-sequence fitting on the (V,V −I) color-magnitude diagram. The subdwarf sample has been selected from the HIPPARCOS catalog in a metallicity range homogeneous with the cluster ((Fe/H) �− 1.1). Both the cluster and the subdwarfs have been observed with the same telescope+instrument+filters setup (namely, ESO-NTT equipped with the SUSI2 camera), in order to preserve homogeneity and reduce systematic uncertainties. A set of archival HST data has then been used to obtain a deep and precise ridge line. These have been accurately calibrated in the ground photometric system by using the NTT data and used to fit the cluster distance modulus. By adopting the most commonly accepted values for the reddening, E(B − V) = 0.035 and 0.03, we obtain respectively µ0 = 14.44± 0.09± 0.07 and µ0 = 14.41± 0.09± 0.07, in agreement with recent determinations.

Using globular clusters to test gravity in the weak acceleration regime: NGC 6171

As part of an ongoing program to test Newtons law of gravity in the low acceleration regime using globular clusters, we present here new results obtained for NGC 6171. Combining VLT spectra for 107 stars with data from the literature, we were able to trace the velocity dispersion profile up to 16 pc from the cluster center, probing accelerations of gravity down to 3.5e-9 cm/s/s. The velocity dispersion is found to remain constant at large radii rather than follow the Keplerian falloff. Similar results were previously found for the globular clusters

Chemical abundances in Terzan 7

\omega

The metallicity of NGC 6253

Centauri and M15. We have now studied three clusters and all three have been found to have a flat dispersion profile beyond the radius where their internal acceleration of gravity is ~ 1e-8 cm/s/s. Whether this indicates a failure of Newtonian dynamics or some more conventional dynamical effect (e.g., tidal heating) is still unclear. However, the similarities emerging between globular clusters and elliptical galaxies seem to favor the first of the two possibilities.