OVz stars, a subclass of O-type dwarfs characterized by having HeII{lambda}4686 stronger in absorption than any other helium line in their blue-violet spectra, have been suggested to be on or near the zero-age main sequence (ZAMS). If their youth were confirmed, they would be key objects with which to advance our knowledge of the physical properties of massive stars in the early stages of their lives. We test the hypothesis of OVz stars being at a different (younger) evolutionary stage than are normal O-type dwarfs. We have performed the first comprehensive quantitative spectroscopic analysis of a statistically meaningful sample of OVz and OV stars in the same star-forming region, exploiting the large number of OVz stars identified by the VLT-FLAMES Tarantula Survey in the 30 Doradus region of the Large Magellanic Cloud (LMC). We obtained the stellar and wind parameters of 38 OVz stars (and a control sample of 46 OV stars) using the FASTWIND stellar atmosphere code and the IACOB-GBAT, a grid-based tool developed for automated quantitative analysis of optical spectra of O stars. In the framework of a differential study, we compared the physical and evolutionary properties of both samples, locating the stars in the logg vs. logT_eff_, logQ vs. logT_eff_, and logL/L_{sun}_ vs. logT_eff_ diagrams. We also investigated the predictions of the FASTWIND code regarding the OVz phenomenon.
We present a catalog of the 48 Voronoi Galaxy Cluster Finder (VGCF) cluster detections. We provide a cluster ID, signal to noise ratios (SNR) in the B, V, R and I bands, positions, angular radius of the cluster, photometric redshift estimates (derived from color-magnitude diagrams (CMD)) and PDCS redshift estimate (Postman et al., 1996AJ....111..615P), if available. Table 5 contains cluster detections in F0028+0515, F0027+0555, F0228+0115 and F0226+0026 fields and other two regions that overlap PDCS 0h and 2h fields.
Charge-coupled device (CCD) photometry in the Johnson V, Kron-Cousins I and Washington CMT1 systems is presented in the field of the poorly known open cluster NGC 2627.
We have obtained CCD photometry in the Johnson V, Kron-Cousins I and CT_1_ Washington systems for NGC 2324, a rich open cluster located ~35{deg} from the Galactic anticentre direction. We measured V magnitudes and V-I colours for 2865 stars and T_1_ magnitudes and C-T_1_ colours for 1815 stars in an area of 13.6'x13.6' (pixel=0.4")
We present the results of the DART imaging and CaII triplet (CaT) survey of the Fornax dwarf spheroidal galaxy (dSph) at ESO. We collected hundreds of low resolution spectra of Red Giant Branch (RGB) stars in the CaT region, using VLT/FLAMES in Medusa mode, with the GIRAFFE spectrograph (grating LR8, resolving power R~6500). We list the results for the targets with a S/N per Angstrom >10 and an error in velocity <5km/s, because we found this to be the minimum for an accurate determination of velocity and equivalent width. We checked the sample to weed out any spurious object (e.g., broken fibres, background galaxies, foreground stars, spectra not consistent with RGBs). We use a 2.5sigma cut from the system velocity to derive which stars are probable velocity members of the Fornax dSph. We transform the equivalent width of the CaT lines of probable velocity members into metallicity ([Fe/H]) using the relation derived by Rutledge et al. (1997PASP..109..907R) and combining the CaT lines as in Tolstoy et al. (2001MNRAS.327..918T). The errors in [Fe/H] are based on S/N computations and adjusted to agree with repeated measurements.
We present VI CCD photometric observations of about 5500 stars up to a limiting magnitude of V~20 in an area of ~16'x16' around the cluster NGC 6631. The cluster parameters determined for the first time by fitting the theoretical isochrones in the V, (V-I) diagram of the cluster stars are E(V-I)=0.60+/-0.05mag, distance=2.6+/-0.5kpc, age=400+/-100Myr and metallicity, Z=0.05. The cluster diameter determined from the radial density profile is 4.8+-0.5pc. The mass function of the cluster has a slope of 2.1+/-05.
Using H{alpha} grism spectroscopy and optical and Two-Micron All-Sky Survey photometry of the fields containing NGC 1893 and two emission nebulae, Sim 129 and Sim 130, we show the presence of a number of H{alpha} and near-infrared excess sources towards the region between HD 242935 and the two emission nebulae. Among them a large majority of these pre-main-sequence candidate sources are concentrated towards regions closer to Sim 129 and Sim 130.
We present CCD VI_KC_ photometry down to V~21mag in the field of the rich open cluster NGC 2194, which is projected towards the Galactic anticentre direction. We measured V magnitudes and V-I colours for a total of 2515 stars in a field of 13.6x13.6arcmin^2^, supplemented with CCD photometry in the C, M and T_1_ filters of the Washington system and photoelectric CMT_1_T_2_ photometry of 20 red giant candidates, using the 0.9-m telescope at the Cerro Tololo Inter-American Observatory (CTIO), Chile, on 1997 December 23-24.
We report deep (V~24.0) VI CCD photometry of three fields centred in the regions of the old open clusters Saurer A, B and C. In the case of Saurer A, which is considered one of the oldest known open clusters, we also provide a comparison field.
Attention is given to a population of 110 stars in the NGC 6611 cluster of the Eagle nebula that have prominent near-infrared excess and optical colours typical of pre-main-sequence (PMS) stars older than 8Myr. At least half of those for which spectroscopy exists have a H{alpha} emission line profile revealing active accretion. In principle, the V-I colours of all these stars would be consistent with those of young PMS objects (<1Myr) whose radiation is heavily obscured by a circumstellar disc seen at high inclination and in small part scattered towards the observer by the back side of the disc. However, using theoretical models it is shown here that objects of this type can only account for a few percent of this population. In fact, the spatial distribution of these objects, their X-ray luminosities, their optical brightness, their positions in the colour-magnitude diagram and the weak Li absorption lines of the stars studied spectroscopically suggest that most of them are at least eight times older than the ~1Myr-old PMS stars already known in this cluster and could be as old as ~30Myr. This is the largest homogeneous sample to date of Galactic PMS stars considerably older than 8Myr that are still actively accreting from a circumstellar disc and it allows us to set a lower limit of 7 percent to the disc frequency at ~16Myr in NGC 6611. These values imply a characteristic exponential lifetime of ~6Myr for disc dissipation.
