UBV(RI) C photometry and spectroscopy of the young open cluster Haffner 18

UBV(RI) _C CCD photometry of a 2.1 × 3.3 arcmin² field centred on the young open cluster Haffner 18 is presented and discussed. Spectroscopic classification of seven stars is also provided. 44 cluster members are identified, the earliest type being O6. The distance to the cluster is found to be 6.3 kpc, corresponding to a galactocentric distance of 12.7 kpc (for a Sun galactocentric distance of 8.5 kpc). An excellent fit to the observed main sequence is achieved by a solar composition isochrone of 2 × 10⁶ yr reddened by E(B − V)  = 0.62 mag. Differential reddening of intracluster origin is present. Pre-main-sequence members are likely to be present over the 6 mag range explored by our observations (reaching down to earliest A spectral types). The presence of differential reddening and pre-main-sequence members agrees with the evidence for a bright parent nebulosity embedding the whole cluster. The radial velocity of the cluster is consistent with the Hron model of Galactic rotation.     

Transforming observational data and theoretical isochrones into the ACS/WFC Vega-mag system

We propose a zero-point photometric calibration of the data from the Advanced Camera for Surveys (ACS) Wide Field Channel (WFC) on board the Hubble Space Telescope , based on a spectrum of Vega and the most up-to-date in-flight transmission curves of the camera. This calibration is accurate at the level of a few hundredths of a magnitude. The main purpose of this effort is to transform the entire set of evolutionary models into a simple observational photometric system for ACS/WFC data, and to make them available to the astronomical community. We provide the zero-points for the most used ACS/WFC bands, and give basic recipes for calibrating both the observed data and the models. We also present the colour–magnitude diagram from ACS data of five Galactic globular clusters, spanning the metallicity range  −2.2 <[Fe/H] < −0.04  , and we provide fiducial points representing their sequences from several magnitudes below the turn-off to the red giant branch tip. The observed sequences are compared with the models in the newly defined photometric system.     

CCD uvbyβ photometry of the young open cluster NGC 663

NGC 663 is a young, moderately rich open cluster, known to contain one of the largest fractions of Be stars among all galactic clusters. In this work we present CCD u v by β photometry for stars in its central area. We have used these data to obtain the main cluster physical parameters. We find that the reddening is highly variable, with values ranging from   E ( b − y ) = 0.639 ± 0.032  in the central part to   E ( b − y ) = 0.555 ± 0.038  in the south-east. The distance modulus is found to be  11.6 ± 0.1 mag (2.1 kpc)  and the age  log  t = 7.4 ± 0.1 yr (25⁺⁷₋₅ Myr)  . The age obtained is consistent with the interpretation of the Be phenomenon as an evolutionary effect.     

Old open clusters: the interesting case of Berkeley 21

We present CCD BVI photometry of the old open cluster Berkeley 21, one of the most distant clusters in the Galactic anticentre direction, and possibly the lowest metallicity object in the open clusters sample. Its position and metal abundance make it very important for the study of the Galactic disc. Using the synthetic colour–magnitude diagram method, we estimate values for the distance modulus ( m  − M )₀ = 13.4–13.6, reddening E ( B  −  V ) = 0.74–0.78 (with possible differential absorption), and age = 2.2–2.5 Gyr.     

The radio structure of S5 1803+784

We present deep CCD BVI photometry of the distant, old open cluster Berkeley 22, covering from the red giant branch (RGB) to about six 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 uncertainties of the distance modulus ( m − M )₀, reddening E ( B − V ), age τ and approximate metallicity. The best fit to the data is obtained for  13.8 ≤ ( m − M )₀≤ 14.1,  0.64 ≤ E ( B − V ) ≤ 0.65,  2.0 ≤τ≤ 2.5 Gyr  (depending on the amount of overshooting from convective regions adopted in the stellar models) and solar metallicity.     

The young open cluster NGC 2129

The first charge-coupled device   UBV ( RI )_C  photometric study in the area of the doubtful open cluster NGC 2129 is presented. Photometry of a field offset 15 arcmin northwards is also provided, to probe the Galactic disc population towards the cluster. Using star counts, proper motions from the UCAC2 catalogue, colour–magnitude and colour–colour diagrams, we demonstrate that NGC 2129 is a young open cluster. The cluster radius is 2.5 arcmin, and across this region we find evidence of significant differential reddening, although the reddening law seems to be normal towards its direction. Updated estimates of the cluster fundamental parameters are provided. The mean reddening is found to be   E ( B − V ) = 0.80 ± 0.08  and the distance modulus is  ( m − M )₀= 11.70 ± 0.30  . Hence, NGC 2129 is located at 2.2 ± 0.2 kpc from the Sun inside the Local spiral arm. The age derived from 37 photometrically selected members is estimated to be approximately 10 Myr. These stars are used to provide new estimates of the cluster absolute proper-motion components.     

BVI CCD photometry of the distant open star clusters Berkeley 81, Berkeley 99, NGC 6603 and NGC 7044

We present CCD observations for the distant northern open star clusters Berkeley 81, Berkeley 99, NGC 6603 and NGC 7044 in B V I photometric passbands. A total of 9 900 stars have been observed in fields of about 6 × 6 arcmin² of the sky around the clusters. Colour–magnitude diagrams in V , ( B  −  V ) and V , ( V  −  I ) have been generated down to V  = 22 mag and, for the first time, such diagrams have been produced for the clusters Berkeley 81 and Berkeley 99. The data serve as a base for the study of mass functions and for comparison with theoretical models. Analysis of the radial distribution of stellar surface density indicates that the radius values for Berkeley 81, Berkeley 99, NGC 6603 and NGC 7044 are 2.7, 2.8, 2.8 and 2.2 arcmin respectively. By fitting the latest convective core overshooting isochrones to the colour–magnitude diagram and using its morphological features, reddenings, distances and ages of the star clusters have been determined. Broad but well-defined main sequences with stellar evolutionary effects in the brighter stars are clearly visible in colour–magnitude diagrams of all the clusters under study. Some blue stragglers along with well-developed giant branches and red giant clumps are also clearly seen in all of them. The clusters studied here are located at a distance of ∼ 3 kpc, except for Berkeley 99 which is located at a distance of 4.9 kpc. Their linear sizes lie between 3.8 and 8.0 pc; E ( B  −  V ) values range from 0.3 to 1.0 mag, while their ages are between 0.5 and 3.2 Gyr. Thus the star clusters studied here are of intermediate and high age but are compact and distant objects.     

NGC 6404 and 6583: two neglected intermediate-age open clusters located in the Galactic Centre direction

We report on VI charge-coupled device photometry of two fields centred in the region of the open clusters NGC 6404 and 6583 down to   V = 22.0  . So far these clusters have never been studied, and we provide for the first time estimates of their fundamental parameters, namely, radial extent, age, distance and reddening. We find that the radius of NGC 6404 is 2.0 arcmin, as previously proposed, while the radius of NGC 6583 is 1.0 arcmin, significantly lower than previous estimates. Both clusters turn out to be of intermediate age (0.5–1.0 Gyr old), and located inside the solar ring, at a Galactocentric distance of about 6.5 kpc. These results make these objects very interesting targets for spectroscopic follow-up to measure their metallicity. In fact, they might allow us to enlarge by more than 1 kpc the baseline of the radial abundance gradient in the Galactic disc towards the Galactic Centre direction. This baseline is currently rather narrow especially for clusters of this age.