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We outline the impact of a small two-band UV-photometry satellite mission on the field of stellar physics, magnetospheres of stars, binaries, stellar clusters, interstellar matter, and exoplanets. On specific examples of different types of stars and stellar systems, we discuss particular requirements for such satellite missions in terms of specific mission parameters such as bandpass, precision, cadence, and mission duration. We show that such a mission may provide crucial data not only for hot stars that emit most of their light in UV, but also for cool stars, where UV traces their activity. This is important, for instance, for exoplanetary studies, because the level of stellar activity influences habitability. While the main asset of the two-band UV mission rests in time-domain astronomy, an example of open clusters proves that such a mission would be important also for the study of stellar populations. Properties of the interstellar dust are best explored when combining optical and IR information with observations in UV. It is well known that dust absorbs UV radiation efficiently. Consequently, we outline how such a UV mission can be used to detect eclipses of sufficiently hot stars by various dusty objects and study disks, rings, clouds, disintegrating exoplanets or exoasteroids. Furthermore, UV radiation can be used to study the cooling of neutron stars providing information about the extreme states of matter in the interiors of neutron stars and used for mapping heated spots on their surfaces., Comment: Submitted to Space Science Reviews

We investigate the dynamic evolution of gaseous region around FS~CMa post-mergers. Due to the slow rotation of a central B-type star, the dynamics is driven mainly by the magnetic field of the central star. Recent observations have allowed us to set a realistic initial conditions such as, the magnetic field value ($B_\star\approx6\times10^{3}G$), the mass of the central star ($M_\star=6M_\odot$), and the initial disc density $\rho_{d0}\in[10^{-13}\mathrm{g\,cm^{-3}},10^{-11}\mathrm{g \, cm^{-3}}] $. We use the PLUTO code to perform 2.5D-MHD simulations of thin and thick discs models. Especially relevant for the interpretation of the observed properties of FS~CMa post-mergers are the results for low-density discs, in which we find formation of a jet emerging from inner edge of the disc, as well as the formation of the so called "hot plasmoid" in the corona region. Jets are probably detected as discrete absorption components in the resonance lines of FS~CMa stars. Moreover, the magnetic field configuration in the low-density plasma region, favors the appearance of magnetocentrifugal winds from the disc. The currents toward the star created by the magnetic field may explain accidentally observed material infall. The disc structure is significantly changed due to the presence of the magnetic field. The magnetic field is also responsible for the formation of a hot corona as observed in several FS~CMa stars through the Raman lines. Our results are valid for all magnetic stars surrounded by a low density plasma, i.e., some of stars showing the B[e] phenomenon., Comment: 12 pages, 6 figures

A reliable determination of the basic physical properties and variability patterns of hot emission-line stars is important for understanding the Be phenomenon and ultimately, the evolutionary stage of Be stars. This study is devoted to one of the most remarkable Be stars, V1294 Aql = HD 184279. We collected and analysed spectroscopic and photometric observations covering a time interval of about 25000 d (68 yr). We present evidence that the object is a single-line 192.9 d spectroscopic binary and estimate that the secondary probably is a hot compact object with a mass of about 1.1-1.2 solar masses. We found and documented very complicated orbital and long-term spectral, light, and colour variations, which must arise from a combination of several distinct variability patterns. Attempts at modelling them are planned for a follow-up study. We place the time behaviour of V1294 Aql into context with variations known for some other systematically studied Be stars and discuss the current ideas about the nature of the Be phenomenon., Comment: 15 pages, 15 figures, accepted for publication in Astronomy and Astrophysics

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Context. The velocity field gradient in the radiative transfer in disc models of cataclysmic variables (CVs) is usually neglected; however, the geometry of the system and the value of Keplerian velocity suggest that it can be important for high inclination angles. Aims. We investigate the influence of the Keplerian velocity gradient on the line formation in CV discs. Methods. The vertical structure of the disc was determined using the NLTE accretion disc code AcDc, where the hydrostatic equation, the energy balance equation, the radiative transfer equation, the rate equations, and the equations of charge and particle conservation are consistently solved using the accelerated lambda iteration. NLTE opacities and emissivities are interpolated onto a 2D grid, where the radiative transfer equation was then solved with the velocity field taken into account. Results. We show line-profile changes and limb-darkening dependences for the Hα and Hγ lines in a model of SS Cyg, along with the HeI 4923 A° line of a model representing a typical AM CVn system. Both systems are considered in the quiescent phase. Conclusions. The line-profile changes due to the velocity gradient in the disc are small enough for most CV discs to allow the classical approach, where the radiative transfer is solved through the static disc and the velocity field is only taken into account in the final flux integration. However, the exact solution must be performed for CVs, where the disc rim plays an important role (inclination angles almost 90°). Extremely cool and relatively transparent discs under high inclinations should also be investigated with detailed radiative transfer models. © 2011 ESO.

We show the influence of the Keplerian velocity shear on the line profiles of cataclysmic variable discs. The complete disc structure is taken into account. The radial disc structure follows the alpha disc approximation. Based on this assumption, the vertical structure is computed using the detailed non-LTE code AcDc. The obtained opacities and source functions are interpolated in the 2D grid, where the radiative transfer is calculated with the inclusion of the velocity field gradient. © International Astronomical Union 2012.

In this work we provide a brief insight into two aspects of Be/X-ray binaries, which are probably involved in production of X-ray outbursts: the evolution of the Be star disk, in particular of its size, and the binary geometry which drives gravitational interaction. Simultaneous X-ray and optical data will aid our investigation of the evolution of Be stars in binaries and the X-ray outburst mechanism.