The discovery of several clusters of red supergiants towards l=24{deg}-30{deg} has triggered interest in this area of the Galactic plane, where lines of sight are very complex and previous explorations of the stellar content were very preliminary. We attempt to characterise the stellar population associated with the HII region RCW 173 (=Sh2-60), located at l=25.3{deg}, as previous studies have suggested that this population could be beyond the Sagittarius arm.
Photoelectric UBV photometry and star counts are presented for stars in the previously unstudied open cluster Ruprecht 91, supplemented by observations for stars in adjacent regions surrounding the Cepheids SX Car and VY Car, including new observations for the latter. Ruprecht 91 is typical of groups associated with Cepheids, with an evolutionary age of ~8x10^7^yr, but it is only 980+/-8pc distant, much closer than the Cepheids.
UBV CCD photometry for about 2500 stars in the Galactic globular cluster Ruprecht 106 has been performed yielding the first color-magnitude diagram (CMD) for this object. The CMD extends down to about 2 mag fainter than the main-sequence turnoff (TO). The cluster possesses a remarkably flat horizontal branch which lies completely to the red of the instability strip. From the analysis of the CMD the following basic parameters have been determined: [Fe/H] = -1.09 and primordial helium abundance Yp = 0.20 0.05. A sizable, highly centrally concentrated population of blue stragglers has also been detected. The location and morphology of the mean ridge lines in the CMD of Ruprecht 106 have then been differentially compared with that of the metal-rich cluster 47 Tuc, NGC 362, a cluster of intermediate metallicity, and with two well-studied metal-poor clusters, M68 and NGC 6397. Significant differences in the positions of the TOs are apparent in the comparison with the metal-poor clusters. These differences can be explained if Ruprecht 106 is 4-5 Gyr younger than NGC 6397 and M68, although alternative, less straightforward explanations cannot be ruled out. The horizontal branch morphology fits perfectly into this picture using models with non-solar-scaled abundances ([O/Fe]>0.5), showing that two second parameters are at work in this cluster, i.e., age (the dominant) and O enhancement. If these findings are confirmed, Ruprecht 106 will turn out to be the first young metal-poor Galactic globular cluster discovered, with quite strong implications for the study of the collapse and enrichment of the Galactic halo.
Three-colour photographic photometry of the young open cluster NGC 7510 based upon UBV observations made with the 122cm and 182cm Asiago reflectors is presented here. A mean colour excess E(B-V)=1.12, a distance of 3.09kpc could be achieved for this cluster and 31 stars could be identified as photometric physical members. An age of 10^7^years has been derived by comparison with standard isochrones. The discrepancy between the photometric and spectroscopic distances obtained for some stars is discussed and comparisons with recent investigations of the same object are also given.
Photoelectric UBV observations were made in two associations in the Large Magellanic Cloud, in the magnitude range 10.6 to 16.2. The observations were made at the Cerro Tololo Inter-American Observatory with the 36-inch telescope in the nights of 1-5 December 1967. The positions were computed by B. Skiff (Lowell Obs.) in 2009.
It is generally believed that expanding superbubbles and mechanical feedback from massive stars trigger star formation, because there are numerous examples of superbubbles showing secondary star formation at their edges. However, although these systems show an age sequence, they do not provide strong evidence of a causal relationship. The W3/W4 Galactic star-forming complex suggests a three-generation hierarchy: the supergiant shell structures correspond to the oldest generation; these triggered the formation of IC 1795 in W3, the progenitor of a molecular superbubble that in turn triggered the current star-forming episodes in the embedded regions W3-North, W3-Main, and W3-OH. We present UBV photometry and spectroscopic classifications for IC 1795, which show an age of 3-5 Myr. This age is intermediate between the reported 6-20 Myr age of the supergiant shell system and the extremely young ages (10^4^-10^5^yr) for the embedded knots of the ultracompact HII regions, W3-North, W3-Main, and W3-OH. Thus, an age sequence is indeed confirmed for the entire W3/W4 hierarchical system. This therefore provides some of the first convincing evidence that superbubble action and mechanical feedback are indeed a triggering mechanism for star formation.
Two new Galactic Wolf-Rayet stars were found in open clusters: a WN4 star in the O9 cluster Sher 1 and a WN7 star in the O7 cluster Westerlund 2. This confirms a previous trend, namely that fainter, hotter WN stars tend to be older than brighter, cooler WN stars. This may be a consequence of evolution via extreme mass loss.
The results of photoelectric UBV photometry of 234 AB stars in the southern galactic halo are presented. The stars were chosen from the Curtis Schmidt objective-prism survey of Preston and Shectman and comprise a sample of halo objects selected without kinematic bias. The majority of the stars lie in the region of the (U-B, B-V) plane occupied by field and globular cluster blue horizontal-branch stars.
Magnitudes in V and B-V and U-B colors observed by the 91-cm telescope at Okayama are presented for 109 stars including both classical and marginal barium stars. The two-color diagram shows a fair amount of spread. This can be interpreted by interstellar reddening and variable amounts of line blocking effect. Both classical and marginal barium stars form a fairly homogeneous group.
A summary of results of the systematic UBV photoelectric monitoring of bright northern Be stars carried out at the Hvar Observatory between 1972 and 1990 is presented. Altogether, 76 Be stars of all luminosity classes were observed and 13848 UBV measurements secured. Simultaneously, 9648 UBV measurements of 48 check stars (most of them of early spectral types) were obtained. A careful transformation of all observations into the standard Johnson system allowed detection and monitoring of even very mild long-term light and colour variations of these objects. Almost all early-type Be stars in the sample turned out to be variable. For several stars phase-locked light variations related to their binary nature were established. Sudden brightenings, on a time scale of a few days, were detected for o Cas and QR Vul.
