We present results of numerical simulations of flux and linear polarization variations in transiting exoplanetary systems, caused by the host star disk symmetry breaking. We consider different configurations of planetary transits depending on orbital parameters. Starspot contribution to the polarized signal is also estimated. Applying the method to known systems and simulating observational conditions, a number of targets is selected where transit polarization effects could be detected. We investigate several principal benefits of the transit polarimetry, particularly, for determining orbital spatial orientation and distinguishing between grazing and near-grazing planets. Simulations show that polarization parameters are also sensitive to starspots, and they can be used to determine spot positions and sizes.

We present the results of modelling the polarization produced during planetary transits in the systems HD 189733, TrES‐3, Wasp‐4 and Wasp‐25, using the Monte Carlo method. Polarization maxima at the limb are calculated to be ∼0.022 per cent for HD 189733 with stellar polarization according to Chandrasekhar. The polarization for the system HD 189733 of ∼0.022 per cent is close to that previously published, although this was attributed to scattering of starlight, rather than produced in transit. Using three‐dimensional modelling data for the linear polarization of the Sun’s continuous spectrum, the limb polarization of the solar‐type stars Wasp‐25 was calculated to be ∼0.00018 per cent, ∼0.00024 per cent for TrES‐3 and ∼0.00016 per cent for Wasp‐4 in the B band. It is noted that observations of the Sun‐like stars in the Ti I 4536 Å spectral line are particularly suitable for distinguishing between different contributions to the polarization. Also, the shape of the polarization curves, at the near limb transits, can be used for obtaining the inclination of the planet orbit, as a good alternative to standard transit methods.

Aims. We analyzed the resolved stellar population of the C component of the extremely metal-poor dwarf galaxy I Zw18 in order to evaluate its distance and star formation history as accurately as possible. In particular, we aimed at answering the question of whether this stellar population is young. Methods. We developed a probabilistic approach to analyzing high-quality photometric data obtained with the Advanced Camera for Surveys of the Hubble Space Telescope. This approach gives a detailed account of the various stochastic aspects of star formation. We carried out two successive models of the stellar population of interest, paying attention to how our assumptions could affect the results. Results. We found a distance to the C component of I Zw 18 as high as 27 Mpc, a significantly higher value than those cited in previous works. The star formation history we inferred from the observational data shows various interesting features: a strong starburst that lasted for about 15 Myr, a more moderate one that occurred ≈100 Myr ago, a continuous process of star formation between both starbursts, and a possible episode of low level star formation at ages over 100 Myr. The stellar population studied is likely ≈125 Myr old, although ages of a few Gyr cannot be ruled out. Furthermore, nearly all the stars were formed in the last few hundreds of Myr.

Context. To properly interpret photometric and polarimetric observations of exoplanetary transits, accurate calculations of centerto-limb variations of intensity and linear polarization of the host star are needed. These variations, in turn, depend on the choice of geometry of stellar atmosphere. Aims. We want to understand the dependence of the flux and the polarization curves during a transit on the choice of the applied approximation for the stellar atmosphere: spherical and plane-parallel. We examine whether simpler plane-parallel models of stellar atmospheres are good enough to interpret the flux and the polarization light curves during planetary transits, or whether more complicated spherical models should be used. Methods. Linear polarization during a transit appears because a planet eclipses a stellar disk and thus breaks left-right symmetry. We calculate the flux and the polarization variations during a transit with given center-to-limb variations of intensity and polarization. Results. We calculate the flux and the polarization variations during transit for a sample of 405 extrasolar systems. Most of them show higher transit polarization for the spherical stellar atmosphere. Our calculations reveal a group of exoplanetary systems that demonstrates lower maximum polarization during the transits with spherical model atmospheres of host stars with effective temperatures of T eff = 4400−5400 K and surface gravity of log g = 4.45−4.65 than that obtained with plane-parallel atmospheres. Moreover, we have found two trends of the transit polarization. The first trend is a decrease in the polarization calculated with spherical model atmosphere of host stars with effective temperatures T eff = 3500−5100 K, and the second shows an increase in the polarization for host stars with T eff = 5100−7000 K. These trends can be explained by the relative variation of temperature and pressure dependences in the plane-parallel and spherical model atmospheres. Conclusions. For most cases of known transiting systems the plane-parallel approximation of stellar model atmospheres may be safely used for calculation of the flux and the polarization curves because the difference between two models is tiny. However, there are some examples where the spherical model atmospheres are necessary to get proper results, such as the systems with grazing transits, with Earth-size planets, or for the hot host stars with effective temperatures higher than 6000 K.