We combine Stromgren-Crawforduvby beta photometry with Gaia-DR2 parallaxes for a sample of 880 supergiants of O-B9 spectral types to explore the potential of these data in providing physical parameters of such stars. We obtained reddening corrections and photometric absolute magnitudes via the existinguvby beta calibrations. We calculated Gaia-based absolute magnitudes applying individual photometric reddening corrections for each star. Theuvby beta photometry is particularly useful in this case, since it provides color excess independent of spectral classification, thus overcoming possible MK classification inconsistencies. After evaluating and correcting the effect of luminosity class mis-classifications on the photometric absolute magnitudes, we found a reasonable agreement between the photometric and Gaia-based absolute magnitudes for this sample. At this point, the color-magnitudeM(V)versus (b-y)(0)diagrams based on photometry and Gaia show slight discrepancies, which have to be addressed in the light of the next Gaia data releases. The derived precise reddening corrections should further help to refine the physical parameters of these stars and address their connection to OB-associations and regions of star formation.

AbstractWe present new CCD uvbyβ photometry of about 50 stars located in the central region of the open cluster NGC 6913. The individual stellar distances and color excesses of established cluster’s members are analyzed and new estimate of the cluster’s distance is provided.

Photometry-based determination of physical parameters of classical Be stars is useful to clarify their evolutionary status and their presence in star-forming complexes and open clusters. We use a sample of nearly 600 stars with complete uvby beta data to evaluate various methods for providing estimates of absolute magnitude and color excess for 0- and B-type emission-line objects.

In any photometric system, the relationship between the spectral type and the intrinsic colors is fundamental for the photometric stellar classification and for a reliable derivation of stellar parameters. Such relationships are being constantly verified and improved based on larger and better samples. The capability of the Stromgren-H beta photometry to provide reddening-free quantities allows the classification of reddened stars over the entire spectral sequence. However, a detailed consistent tabulation of the uvby beta quantities for the O-B9 part of the main sequence is currently warranted. We combine spectral types and photometry from the literature to provide a new empirical tabulation of the intrinsic color b - y, color differences c(1), m(1), [c(1)] and [m(1)] and the H beta index for main-sequence (luminosity class V) stars of spectral types O4.5 through B9.5 in terms of spectral sub-types. Our survey is based on more than 1400 stars. We use the established c(1) versus b - y intrinsic line and the accepted reddening slopes to obtain dereddened quantities and then calculate their median values as a function of the spectral sub-type. The classification ability of the system in this spectral range is reliable and the photometric quantities are not affected by the stellar rotation.

We present the results of low-resolution optical spectroscopy with OSIRIS/GTC (Optical System for Imaging and Low Resolution Integrated Spectroscopy/Gran Telescopio Canarias) for a sample of ultracool dwarfs. For a subsample of seven objects, based on 2 Micron Sky Survey (2MASS) NIR photometric colours, a 'photometric' spectral type is determined and compared to the results of the optical spectroscopy. For the stars, showing Ha line in emission, equivalent widths were measured, and the ratio ofH alpha to bolometric luminosity were calculated. We find that two dwarfs show the presence of magnetic activity over long periods, LP 326-21 - quasi-constant-like, and 2MASS J17071830+6439331 - variable.

We report observations of the flickering variability of the symbiotic recurrent nova RS Oph at quiescence in five bands (UBVRI). We find evidence of a correlation between the peak-to-peak flickering amplitude (Delta F) and the average flux of the hot component (F-av). The correlation is highly significant, with a correlation coefficient of 0.85 and a p-value of similar to 10(-20). Combining the data from all wavebands, we find a dependence of the type Delta F alpha F-av(k), with power-law index k = 1.02 +/- 0.04 for the UBVRI flickering of RS Oph. Thus, the relationship between the amplitude of variability and the average flux of the hot component is consistent with linearity. The rms amplitude of flickering is on average 8 per cent (+/-2 per cent) of F-av. The detected correlation is similar to that found in accreting black holes/neutron stars and cataclysmic variables. The possible reasons are briefly discussed. The data are available upon request from the authors.

We summarize results of precision photometry studies of prominent Galactic star-forming regions. The reliable uvbyβ photometry-based parallaxes we utilize for our purpose not only provide a significant revision of the OB-star distribution, but also help to identify previously undetected OB-groups.

The distribution of OB stars along with that of H$\alpha$, $^{12}$CO, dust infrared emission, and neutral hydrogen is carried out in order to provide a more complete picture of interactions of the young massive stars and the observed supershell GSH 305+01-24. The studied field is located between $299^\circ \le l \le 311^\circ$ and $-5^\circ \le b \le 7^\circ$. The investigation is based on nearly 700 O-B9 stars with $uvby\beta$ photometry currently available. The derived stellar physical parameters were used to establish a homogeneous scale for the distances and extinction of light for major apparent groups and layers of foreground and background stars in Centaurus and study the interaction with the surrounding interstellar medium. The distance to the entire Centaurus star-forming complex is revised and a maximum of the OB-star distance distribution is found at 1.8$\pm$0.4 (r.m.s) kpc. The massive star component of GSH 305+01-24 is identified at about 85-90 % completeness up to 11.5-12 mag. The projected coincidence of the OB stars with the shell and the similarities between the shell's morphology and the OB-star distribution indicate a strong interaction of the stellar winds with the superbubble material. We demonstrate that these stars contribute a sufficient wind injection energy in order to explain the observed size and expansion velocity of the supershell. The derived stellar ages suggest an age gradient over the Coalsack Loop. A continuous star-formation might be taking place within the shell with the youngest stars located at its periphery and the open cluster NGC 4755 being the oldest. A layer of very young stars at 1 kpc is detected and its connection to both GSH 305+01-24 and the foreground GSH 304-00-12 H I shells is investigated.

A photometric study in the uvbyβ system of a 20˚ × 20˚ field in direction of the Cas OB6 and Per OB1 associations is presented.All currently available uvbyβ photoelectric data are used to obtain homogeneous color excesses and distances of nearly 230 stars of spectral types O-B9.The double cluster h & χ Per, NGC 663 and NGC 1502 are well represented in our sample.The sample also contains the brightest members of the young open clusters IC 1805, IC 1848, St 2, St 7 and ASCC9.We found that, within the errors, h & χ Per, NGC 663, IC 1805, IC 1848 and ASCC9, together with the Per OB1 association are located at very similar distance moduli between 11.0 to 11.3 mag.Our results indicate that the distance spread among these objects is less than the previously estimated, suggested that they could represent starforming complexes located at the same distance.

The Cherenkov Telescope Array (CTA) is a new observatory for very high-energy (VHE) gamma rays. CTA has ambitions science goals, for which it is necessary to achieve full-sky coverage, to improve the sensitivity by about an order of magnitude, to span about four decades of energy, from a few tens of GeV to above 100TeV with enhanced angular and energy resolutions over existing VHE gamma-ray observatories. An international collaboration has formed with more than 1000 members from 27 countries in Europe, Asia, Africa and North and South America. In 2010 the CTA Consortium completed a Design Study and started a three-year Preparatory Phase which leads to production readiness of CTA in 2014. In this paper we introduce the science goals and the concept of CTA, and provide an overview of the project.

We present a photometric investigation in the uvbyβ system of a 20° × 20° field centered at galactic coordinates (l, b) = (105°, 5°). This region contains a relatively isolated structure of numerous HII regions in the direction of some of the OB associations in Cepheus and Cassiopeia. Photoelectric uvbyβ data of more than 220 stars of spectral types O-B9 are available for this field and are used to obtain homogeneous distance estimates for a number of prominent stellar groups, among which are the Cep OB2 and Cep OB3 associations and the open clusters NGC 7243 and NGC 7654.

In this contribution we present our investigation of a sample of Large Magellanic Cloud star clusters. This galaxy is the closest neighbour of the Milky Way. In our sample we selected three clusters with similar ages of 10 Gyr, namely NGC 1754, NGC 2005 and NGC 2019. We construct the radial profiles of the clusters, derive structural parameters and study the distribution of the stars within the clusters through the variation of the core radius with magnitude of the stars. Indication of stellar stratification is found in NGC 1754 and NGC 2005.