• Stellar Activity

I present a study of four eclipsing RS Canes Venaticorum (RS CVn) binary systems AR Lac, Z Her, V711 Tau and UX Ari, using high resolution visible-range (3700-9000 Å) observations, following their orbital cycles by means of the HEROS spectrograph on the TIGRE telescope (La Luz,Mexico).
RS CVn stars are active binary systems (separations of a few stellar radii but not massexchanging) with tidal synchronization of spin and angular motion, with orbital periods from 1 day to a few weeks, with at least one of the components having an outer convective envelope. Magnetic fields arise through the alphaOmega-dynamo effect, forming cooler atmospheric regions, called starspots. These magnetic fields are strong enough to suppress the overturning convective motion. Consequently, starspots are cooler than the rest of the photosphere. As the stars orbit, these dark spots vary in the projected area so the rotation causes a modulation of the total brightness, shown in the photometric light curve.
The photospheric spots are structurally linked (magnetic arcs/ filaments/waves) to activity in the chromosphere, a nonthermally heated overlying geometrically thin outer atmosphere. These active regions can be studied through the emission in the photospheric spectral absorption lines core, that are the indicators of solar chromospheric activity, especially CaII, H-K (3933.66¡3968.47 Å), Halpha (6562.81 Å) and the CaII IR triplet (8498.02¡8542.09¡8662.14 Å). The line core shows intense emission in these binaries, especially in UX Ari and V711 Tau, and the wings in absorption, (photospheric contribution).
A comparison between velocity profiles fromphotospheric and chromospheric lines is necessary to understand the activity as a function of orbital phase (stellar longitude and latitude) and how the chromosphere influences the entire stellar atmosphere.
Computing the radial velocity profiles for all of the targets, I found the component masses and possible structures in the chromosphere. I have separated the two stellar contributions and studied their activities. In details, AR Lac and ZHer show a weaker activity with a relative emission strength in CaII,H-K. Therefore the velocity profiles from chromospheric lines are in agreement with the photospheric lines. UX Ari and V711 Tau are very active, the first is the most of my targets, and there is a shift between the chromospheric and photospheric velocity curves. Comparing the profile fromthe CaII,K line and that of the second of the CaII, IRT I found a possible extended structure, which moves differetly than the center of mass and likely sited at high latitude, an arc filament. When a filament emerges from the photosphere, it transports cooling plasma through the chromosphere.
Vortex plasma flows down along the magnetic field lines. From the observations of the Sun, some of these arch filament systems are more luminous than the surrounding chromospheric.
This behavior suggests that an arch filament appears to have a own temperature and velocity distribution due to the strength and structure of the active region magnetic field. Thus, comparing the velocity fromthe emission lines with those from the photospheric absorption lines is the key point to map the outer atmospheric events and study the atmospheric structure.
To study the atmospheric abundances I computed synthetic spectra with PHOENIX code.
I started with single solar type models, giving different effective temperature and optical depth in input, according to the systems. Then, to reproduce the observed spectrum due to both components of the binary, I wrote a new pipeline to combine the models, based on a disentangle method. Taking the spectra at opposite elongations, I obtained an excellent match with the composite synthetic models. This last is influenced by the contribution of the orbital motion along the line of sight and the luminosity ratio relative to the system, according to the different temperature between the two components.
From this comparison I could confirm the computed radial velocities with the derived orbital parameters and the considerations about surface structure.
In conclusion, my thesis project provides informations about the enhanced dynamo action in the stellar atmosphere and how it can be account for spectroscopic diagnostic in the visible wavelength region. This will be very useful for further analysis, for instance:
1. contemporaneous observations in different wavelength intervals, with the chance to get activity event (as flare), more studied in Radio;
2. comparison between this system and FK Com-type, that is a single chromospherical active star, with the same but more intense features, that are thought to be very related to the RS CVn-type stars.