Talk

12 660 spotted stars towards the OGLE Galactic bulge fields

Patryk Iwanek, Warsaw University Observatory

Since 1992, OGLE variability sky survey has been conducting an intense monitoring of one of the most interesting, but also the most difficult to explore due to strong and highly non-uniform interstellar extinction, region of the Milky Way - the Galactic bulge. Every night we observe over a billion stars in this direction, providing time-series data for many interesting objects. Our extensive experience in variable stars selections has allowed us to detect more than 20 000 spotted stars towards the Galactic bulge. Recently, we have proposed a new dereddening procedure for individual stars, thanks to which we are able to divide spotted stars into dwarfs and giants. We have found well-defined correlations between rotating period, luminosity and variability amplitude for the giants. Moreover, our two-band, high-cadence photometry has allowed us to find an evident correlation between the changes in brightness and changes in color of the stars. The positive correlation is characteristic for stars that are cooler when fainter, which results from the variable coverage of the stellar surface with spots similar to sunspots. The variability of stars that are cooler when brighter (negative correlation) can be characterized by chemical spots with overabundance of heavy elements inside and variable line-blanketing effect, which is observed in chemically peculiar stars. The null correlation may result from a very high level of the magnetic activity with rapidly variable magnetic field. This division is clearly visible on the color–magnitude diagram, which suggests that it may depend on the radius of the star. In my presentation, I will present the latest results and I will introduce future plans for the next research in the area of stellar magnetic activity.