|Ph.D Student||Nordon Raanan|
|Subject||Coronal X-Ray Flares on Active Stars|
|Department||Department of Physics||Supervisor||Professor Ehud Behar|
|Full Thesis text|
Stellar coronae are the hot (kT>0.1 keV) tenuous regions in the outer atmospheres of cool-stars. Stellar coronae have been researched for many years, and yet they are poorly understood. In particular, the deviation of coronal chemical composition from photospheric elemental abundances is a long standing mystery. In the solar case, this was labeled the first ionization potential (FIP) effect. While some stellar coronae show a solar-like FIP effect, others show no FIP effect, or an inverse effect, although difficulties in measuring stellar photospheric abundances cast some doubt on these results. A correlation between coronal activity and abundance patterns led to a suggestion that flares affect coronal abundances. However, different variations were observed during flares, with no clear pattern emerging.
We investigate a full sample of X-ray flares on stellar coronae from the archives of XMM-Newton and Chandra space observatories. We develop a method for reconstructing emission measure distribution, EMD(T), and abundances that is optimized to reduce systematic uncertainties. We measure variations of coronal abundances during flares, relative to quiescence abundances. This measurement is independent of the photospheric abundances and their related uncertainties. A theoretical analysis of the EMD(T) degeneracy problem is also presented.
We find excess emission during flares originates predominantly from temperatures of kT>2 keV, while the low-T emission is very close to quiescence. This result cannot be reconciled with pure radiative-cooling or simple conductive-cooling. Evaporation from low dense regions into higher, thinner corona may aid in explaining this observed behavior. We define a relative measure for the FIP bias and compare the FIP bias of flare vs. quiescence with that of quiescence vs. photospheric (solar). We discovered a general trend where the relative FIP bias during flares is opposite to the quiescence FIP bias, meaning that the flares tend to neutralize the abundance differences between the corona and the photosphere. We found that solar flares also follow this pattern.
Our preferred interpretation is that material from the footprints of magnetic loops is being evaporated into the corona (chromospheric evaporation). Such a scenario, which is supported by radio timing evidence, means that the quiescence inverse-FIP composition of highly active coronae is genuine and is not an artifact of uncertain photospheric abundances. Since large flares tend to neutralize the quiescence FIP bias, they are not the direct cause for the inverse-FIP effect, which remains to be discovered.