Astronomy and Astrophysics – Astrophysics
Scientific paper
Oct 2003
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003csss...12..265n&link_type=abstract
The Future of Cool-Star Astrophysics: 12th Cambridge Workshop on Cool Stars , Stellar Systems, and the Sun (2001 July 30 - Augus
Astronomy and Astrophysics
Astrophysics
Algol, Capella, Procyon, Letgs, He-Like Triplets
Scientific paper
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 (1s2s 3S1 rightarrow 1s2 1S0), disappears at high densities, and the intercombination line i (1s2p 3P2,1 rightarrow 1s21S0) 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 (1s2s 3S1 rightarrow 1s2p 3P2,1) 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 α Cen A and α 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.
Mewe Rolf
Ness Jan-Use
Raassen A. J. J.
Schmitt Juergen H. M. M.
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