Search for new nearby stars withDENIS

As already known, the Third Catalogue of Nearby Stars by Gliese and Jahreiss is incomplete and the missing stars are all faint, red dwarfs. We underline that there are more stars missing in the South than in the North, for both the CNS3 and the NLTT.     

Search for variability in hydrogen-poor stars

The evolutionary status of hydrogen-poor stars is analyzed.Photoelectric observations of six objects are reported as a first step of a long-term project devoted to search for variability of a large sample of hydrogen-poor stars.The observed stars show phenomena of microvariability with an amplitude of the order of 0^m. 1 or less.Two extreme helium stars have been examined: a period in the range of 0^d.162-0^d.164 has been found for BD+ 10⁰2179, andP=0^d.1079962 for BD+13⁰3324.The mass-losing O subdwarf (sdO) BD+37⁰ 443 presents short-term fluctuations with a time-scale of several minutes and long-term variations on a scale of months. The standard sdO BD+75⁰325 is probably non-variable, although light variations of very small amplitude (m0.03) with a time-scale of about 1 hr might be present. The high gravity sdO BD+25⁰4655, which is very close to the white dwarf stage, also presents variability on a time-scale of about 13 minutes, and might be an analog of the recently discovered pulsating sdO, or hot white dwarf, PG 1159-035.The variability of the intermediate helium star HD 37776 is finally confirmed.     

Light polarization of Mira stars

The results of polarimetric observations of long-period Mira variables obtained over the past 30 years at the Byurakan observatory are presented. Analysis of these observations shows that there is a real correlation between the degree of light polarization and the brightness of these stars. The maximum degree of polarization is observed at the middle of the ascending branch of the light curve. The degree of polarization is correlated with the period of brightness variation. The average degree of polarization of Mira stars increases with an increase in their period up toP=380 days.Translated fromAstrofizika, Vol. 39, No. 4, pp. 541–551, November, 1996.     

Polarization of intrinsic radiation of tidally distorted stars with electron-scattering atmospheres

Linear polarization of radiation emitted by tidally distorted stars as a function of the binary system phase is computed, taking into account true absorption and the scattering of light on free and bound electrons within hot stellar atmospheres. Computations are made both for the linear distribution of true sources across the atmospheres and for radiative-stable model atmospheres presented by Kurcuzet al. (1974) and Kurucz (1979). Polarization variability was investigated as a function of wavelength . In a number of cases, polarization variability was found to be at an observable level. The most marked variability was expected in the ultraviolet range adjacent to the boundaries of the spectral series for H and He. Near the Lyman limit of approximately =912 Å for stars with an effective temperatureT _eff35 000 K and near the ionization boundary for HeII 226 Å for stars withT _eff>35 000 K, the amplitude of polarization variability is greater than in the case of pure electron atmospheres, sometimes reaching the level of 0.5–1%. For fairly long waves where the limb-darkening coefficient falls below a certain critical valueu _cr0.5, the plane of polarization is found to be turned by 90° as compared to the case of a pure electron atmosphere. For limb-darkening coefficients far from the value ofu _cr; the form of the polarization phase curves, as well as dependence on the parameters of a binary system, remain approximately the same as those in the case of pure electron scattering.     

Search for young stars with ir surveys: Regions of massive star formation

A near-IR survey of a selected number of OB star forming regions using large format array cameras is in progress at Las Campanas observatory. The results relative to the HII/molecular cloud complexes NGC 3576 and NGC 6334I-II are presented.     

Search for exotic matter from gravitational microlensing observations of stars

We consider small-scale spheroidal clusters of weakly interacting massive particles in our Galaxy as non-compact gravitational microlenses and predict the appearance of caustics in the plane of a lensed source. The crossing of these caustics by a lensed star can produce a large variety of light curves, including some observed in actual microlensing events that have been interpreted as manifestations of binary gravitational lenses. We consider also observable effects during the gravitational microlensing of stars of non-zero angular size with a given brightness distribution across their disks by such an exotic objects as natural wormholes and objects whose space-time environment is described with the NUT metric. We demonstrate that, under certain conditions, the microlensing light curves, chromatic and polarizational effects due to the properties of the lens and the star disk brightness distributions can differ considerably from those observed for a Schwarzschild gravitational lens, so that their analysis can facilitate the identification of such objects.     

Search for spectral line polarization in the solar vacuum ultraviolet

An instrument designed to record polarization in the region 120–150 nm of the solar spectrum was launched on the satellite Intercosmos-16, July 27, 1976. The aim was to search for resonance-line polarization that is caused by coherent scattering. Oblique reflections at gold- and aluminium-coated mirrors in the instrument were used to analyze the polarization. The average polarization of the L solar limb was found to be less than 1%. It is indicated how future improved VUV polarization measurements may be a diagnostic tool for chromospheric and coronal magnetic fields and for the three-dimensional geometry of the emitting structures.On leave from Lund Observatory, S-22224 Lund, Sweden.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

Center-to-limb variation of the continuum intensity and linear polarization of stars with transiting exoplanets

The limb darkening and center-to-limb variation of the continuum polarization is calculated for a grid of one-dimensional stellar model atmospheres and for a wavelength range between 300 and 950 nm. Model parameters match those of the transiting stars taken from the NASA exoplanet archive. The limb darkening of the continuum radiation for these stars is shown to decrease with the rise in their effective temperature. For the λ = 370 nm wavelength, which corresponds to the maximum of the Johnson–Cousins UX filter, the limb darkening values of the planet transiting stars lie in a range between 0.03 and 0.3. The continuum linear polarization depends not only on the effective temperature of the star but also on its gravity and metallicity. Its value decreases for increasing values of these parameters. In the UX band, the maximum linear polarization of stars with transiting planets amounts to 4%, while the minimum value is approximately 0.3%. The continuum limb darkening and the linear polarization decrease rapidly with wavelength. At the R band maximum (λ = 700 nm), the linear polarization close to the limb is in fact two orders of magnitude smaller than in the UX band. The center- to-limb variation of the continuum intensity and the linear polarization of the stars with transiting planets can be approximated, respectively, by polynomials of the fourth and the sixth degree. The coefficients of the polynomials, as well as the IDL procedures for reading them, are available in electronic form. It is shown that there are two classes of stars with high linear polarization at the limb. The first one consists of cold dwarfs. Their typical representatives are HATS-6, Kepler-45, as well as all the stars with similar parameters. The second class of stars includes hotter giants and subgiants. Among them we have CoRoT-28, Kepler-91, and the group of stars with effective temperatures and gravities of approximately 5000 K and 3.5, respectively.     

Search for transiting exoplanets and variable stars in the open cluster NGC 7243

We report results of the first five observing campaigns for the open stellar cluster NGC 7243 in the frame of project Young Exoplanet Transit Initiative (YETI). The project focuses on the monitoring of young and nearby stellar clusters, with the aim to detect young transiting exoplanets and to study other variability phenomena on time‐scales from minutes to years. After five observing campaigns and additional observations during 2013 and 2014, a clear and repeating transit‐like signal was detected in the light curve of J221550.6+495611. Furthermore, we detected and analysed 37 new eclipsing binary stars in the studied region. The best fit parameters and light curves of all systems are given. Finally, we detected and analysed 26 new, presumably pulsating variable stars in the studied region. The follow‐up investigation of these objects, including spectroscopic measurements of the exoplanet candidate, is currently planned. (© 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Intrinsic polarization and extinction features of early-type stars with emission lines due to X-ray irradiation

The explanation of intrinsic polarization and extinction of early-type stars with emission due to X-ray irradiation of silicate grains is given.     

Emergent polarization structure in spectral lines from rotating stars of spectral type B

The emergent polarization structure within spectral lines from rapidly rotating stars of spectral type B5V has been computed for the Mgii 4481 Å doublet and H for several angles of inclination and for rotational speeds of 0.5 and 0.9 of the critical rotational speed. It is found in this study that the emergent polarization within the Mgii 4481 Å doublet is currently unobservable with modern polarimeters. However, the polarization within H should be observable.     

Intrinsic polarization changes and the Hα and Caii emission features in T-Tauri stars

On grounds of the correlation between polarization and emission features observed in some T-Tauri stars, it is concluded that flaring effects associated with UV and/or X-ray irradiation and with increased magnetic field are the cause of the intrinsic polarization changes in T-Tauri stars.     

Variational principle for stars with a phase transition

The variational principle for stars with a phase transition has been investigated. The term outside the integral in the expression for the second variation of the total energy of a star is shown to be obtained by passage to the limit from the integration over the region of mixed states in the star. The form of the trial functions ensuring this passage has been found. All of the results have been generalized to the case where general relativity is applicable. The known criteria for the dynamical stability of a star when a new phase appears at its center are shown to follow automatically from the variational principle. Numerical calculations of hydrostatically equilibrium models for hybrid stars with a phase transition have been performed. The form of the trial functions for the second variation of the total energy of a star that describes almost exactly the stability boundaries of such stellar models is proposed.