Modelling of the ultraviolet and visual SED variability in the hot magnetic Ap star CU Virginis
Authors:
J. Krticka, Z. Mikulasek, T. Luftinger, D. Shulyak, J. Zverko, J. Ziznovsky and N. A. Sokolov
Image & caption:
Please click on image thumbnail to get more info:
Image caption::
Upper plot: the emergent flux from a reference model atmosphere with εHe = −1.0,
εSi = −3.75, εCr = −5.9, and εFe = −4.4.
Lower plot: the emergent flux from the model atmospheres with modified abundance of individual elements minus the flux from a reference model. All fluxes were smoothed by a Gaussian filter with a dispersion of 10Å to show the changes in continuum, which are important for SED variability.
The passbands of the uvby photometric
system are also shown
in the graph (grey areas).
Description:
We show that most of the light variations in the vby filters of the Strömgren photometric system are a result of the uneven surface distribution of silicon, chromium, and iron. Our models are only able to explain a part of the variability in the u filter, however.
The observed UV flux distribution is very well reproduced, and the models are able to explain most of the observed features in the UV light curve, except for the region 2000−2500 Å, where the amplitude of the observed light variations is higher than predicted.
The variability observed in the visible is merely a faint gleam of that in the UV. While the amplitude of the light curves reaches only several hundredths of magnitude in the visual domain, it reaches about 1mag in the UV.
Conclusions: The visual and UV light variability of CU Vir is caused by the flux redistribution from the far UV to near UV and visible regions, inhomogeneous distribution of the elements and stellar rotation. Bound-free transitions of silicon and bound-bound transitions of iron and chromium contribute the most to the flux redistribution. This mechanism can explain most of the rotationally modulated light variations in the filters centred on the Paschen continuum and on the UV continuum of the star CU Vir. However, another mechanism(s) has to be invoked to fully explain the observed light variations in the u filter and in the region 2000−2500 Å.
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