3D radiative transfer effects in parametrized starspots

¹ Hamburger Sternwarte, Gojenbergsweg 112, 21029 Hamburg, Germany
e-mail: aberkner@hs.uni-hamburg.de; yeti@hs.uni-hamburg.de
² Homer L. Dodge Department of Physics and Astronomy, University of Oklahoma, 440 W Brooks, Rm 100, Norman, OK 73019-2061, USA
e-mail: baron@ou.edu

Received: 9 October 2012
Accepted: 20 December 2012

Abstract

Aims. We use our 3D radiative transfer framework to investigate how the presence of a parametrized starspot affects radiative transfer in stellar atmospheres in general, and molecular CO lines in a stellar spectrum, in particular.

Methods. The equation of state is solved for a given temperature structure including a simple parametrized spot model and the 3D scattering problem for line transfer is solved via an operator splitting technique. The formal solution is based on a full characteristics solution. We have used both a LTE model and a test model with a 2 level atom, simulating a single spectral line in NLTE.

Results. We present the resulting CO band spectra showing both surface resolved and an integrated total emergent flux for the star and compare the umbral, penumbral, and quiet spectrum for a solar type star, which clearly shows the presence of spots as an increased CO line-depth. Furthermore, we show that the opacity structure of the spot has a significantly different angular variation than the quiet plane parallel star and its visible shape is strongly influenced by scattering, where strong scattering disconnects the lateral intensity profile from the temperature profile of the spot.

Conclusions. Even a simple, small scale parametrized model shows significant 3D effects, in both the resolved and in the surface integrated spectrum. The 3D model allows for a much more detailed treatment than simple mixing of spectra with different effective temperatures.