Influence of UV Radiation Fields on Density Diagnostics with He-like Triplets
Jan-Uwe Ness, Rolf Mewe, Juergen Schmitt, & Anton Raassen

Institute: Hamburg University
Institute: Space Research Organization of the Netherlands
Institute: Astronomical Institute Amsterdam

Contact Email: jness@hs.uni-hamburg.de

Abstract: Spectroscopic density diagnostics based on He-like triplets are routinely used to investigate the solar corona. With the new instrumentation onboard Chandra and XMM this method of analysis can also be applied to stellar coronae. In collision-dominated plasmas the forbidden line, f, disappears at high densities, and the intercombination line, i, increases at higher densities at the expense of the forbidden line. Therefore, the ratio f/i is used as a sensitive indicator of electron density. However, depopulation of the forbidden line compared to the intercombination line, is not always an indicator for high densities, it might also indicate that the depopulation of the forbidden line level is due to a UV radiation field instead of the collisions in a high-density plasma. We illustrate this effect with IUE measurements of Capella, Procyon, Algol and Alpha Cen A and Alpha Cen B and a simulation showing the trend of the radiation fields when regarding stars with different surface temperatures. Focusing on the triplets of C V, N VI, O VII and Ne IX, we show that the radiation fields can have significant influence on the density analysis of the low-Z He-like ions of C, N and O. We present Chandra LETGS measurements and calculate the densities accounting for the measured radiation fields and neglecting them. The sources of the UV radiation are assumed to be the respective stellar surfaces, but in the case of Algol the radiation is supplied by the companion B star. A detailed investigation of whether the observed part of Algol's corona is actually illuminated by the radiation field of the B star, is necessary.

Influence of UV Radiation Fields on Density Diagnostics with He-like Triplets [PDF - Type 1 fonts]

Influence of UV Radiation Fields on Density Diagnostics with He-like Triplets [PS]

Index Keywords: Algol ; Capella ; Procyon ; LETGS ; He-like triplets

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Manuscript submitted: 2001-Sep-5
"The Future of Cool-Star Astrophysics", 2003, Eds. A. Brown, G. M. Harper, & T. R. Ayres. Proceedings of 12th Cambridge Workshop on Cool Stars, Stellar Systems, & The Sun,
© 2003 University of Colorado.