The Density of Coronal Plasma in Active Stellar Coronae

Abstract

We have analyzed high-resolution X-ray spectra of a sample of 22 active stars observed with the High Energy Transmission Grating Spectrometer on {\em Chandra} in order to investigate their coronal plasma density. Densities where investigated using the lines of the He-like ions O VII, Mg XI, and Si XIII. While Si XIII lines in all stars of the sample are compatible with the low-density limit, Mg XI lines betray the presence of high plasma densities ($> 10^{12}$ cm$^{-3}$) for most of the sources with higher X-ray luminosity ($> 10^{30}$ erg/s); stars with higher $L_X$ and $L_X/L_{bol}$ tend to have higher densities at high temperatures. Ratios of O VII lines yield much lower densities of a few $10^{10}$ cm$^{-3}$, indicating that the ``hot'' and ``cool'' plasma resides in physically different structures. Our findings imply remarkably compact coronal structures, especially for the hotter plasma emitting the Mg XI lines characterized by coronal surface filling factor, $f_{MgXI}$, ranging from $10^{-4}$ to $10^{-1}$, while we find $f_{OVII}$ values from a few $10^{-3}$ up to $\sim 1$ for the cooler plasma emitting the O VII lines. We find that $f_{OVII}$ approaches unity at the same stellar surface X-ray flux level as solar active regions, suggesting that these stars become completely covered by active regions. At the same surface flux level, $f_{MgXI}$ is seen to increase more sharply with increasing surface flux. These results appear to support earlier suggestions that hot $10^7$ K plasma in active coronae arises from flaring activity, and that this flaring activity increases markedly once the stellar surface becomes covered with active regions.Comment: 53 pages, 19 figures, accepted for publication in Astrophysical Journal. A version of the paper with higher quality figures is available from http://www.astropa.unipa.it/Library/preprint.htm

Fe Kα and Hydrodynamic Loop Model Diagnostics for a Large Flare on II Pegasi

“…This value is compatible with density diagnostics from line ratios of hot He-like triplets, such as Mg xi, observed during flaring and quiescence on II Peg (Huenemoerder et al 2001;Testa et al 2004). Similarly high densities have also been reported by Güdel et al (2002) during an X-ray flare in Proxima Centauri.…”

The Coronae of AB Doradus and V471 Tauri: Primordial Angular Momentum versus Tidal Spin‐up

“…These arguments are qualitative, and we have insufficient spectroscopic diagnostic information to investigate abundance variations with temperature in a rigorous fashion. We note, however, that such abundance variations might not be unreasonable in the light of a recent survey of coronal densities by Testa et al (2004) that clearly demonstrates that the cooler T $ 10 6 K corona resides in quite different structures to the hotter, T $ 10 7 K plasma that appears to be universally characterized by much higher pressures. Feldman & Laming (2000) found that for the solar corona the FIP effect is larger at T $ 10 5:9 K than at T $ 10 6:15 K.…”

“…We also computed synthetic spectra using emissivities from the APED database ) and the DEM and abundances derived from line set 1 for comparison. For these simulations, we have use an electron density of n e ¼ 5 ; 10 11 cm À3 for AB Dor and n e ¼ 2 ; 10 10 cm À3 for V471 Tau (Testa et al 2004), with an interstellar medium column density of n H ¼ 1 ; 10 18 cm À2 . The exact density and very low ISM column make no practical difference to the synthetic spectra.…”

On the observability of T Tauri accretion shocks in the X-ray band

“…This component could be produced by the shock-heated plasma at the base of the accretion column, the plasma density being much higher than that (n e ≤ 10 10 cm −3 ) measured for coronae of active stars (Testa et al 2004). The discovery of optical depths effects on the X-ray spectrum of MP Mus further supports this hypothesis (Argiroffi et al 2009).…”