On the sizes of stellar X-ray coronae

Abstract

Abstract. Spatial information from stellar X-ray coronae cannot be assessed directly, but scaling laws from the solar corona make it possible to estimate sizes of stellar coronae from the physical parameters temperature and density. While coronal plasma temperatures have long been available, we concentrate on the newly available density measurements from line fluxes of X-ray lines measured for a large sample of stellar coronae with the Chandra and XMM-Newton gratings. We compiled a set of 64 grating spectra of 42 stellar coronae. Line counts of strong H-like and He-like ions and Fe  lines were measured with the CORA single-purpose line fitting tool by Ness & Wichmann (2002). Densities are estimated from He-like f /i flux ratios of O  and Ne  representing the cooler (1-6 MK) plasma components. The densities scatter between log n e ≈ 9.5−11 from the O  triplet and between log n e ≈ 10.5−12 from the Ne  triplet, but we caution that the latter triplet may be biased by contamination from Fe  and Fe  lines. We find that low-activity stars (as parameterized by the characteristic temperature derived from H-and He-like line flux ratios) tend to show densities derived from O  of no more than a few times 10 10 cm −3 , whereas no definitive trend is found for the more active stars. Investigating the densities of the hotter plasma with various Fe  line ratios, we found that none of the spectra consistently indicates the presence of very high densities. We argue that our measurements are compatible with the low-density limit for the respective ratios (≈5 × 10 12 cm −3 ). These upper limits are in line with constant pressure in the emitting active regions. We focus on the commonly used Rosner et al. (1978) scaling law to derive loop lengths from temperatures and densities assuming loop-like structures as identical building blocks. We derive the emitting volumes from direct measurements of ionspecific emission measures and densities. Available volumes are calculated from the loop-lengths and stellar radii, and are compared with the emitting volumes to infer filling factors. For all stages of activity we find similar filling factors up to 0.1.

The disk-bearing young star IM Lupi

“…The f /i ratio of the Ne ix triplet in IM Lup is statistically compatible at the 1σ level with the typical values of coronal sources (Ness et al 2004) at the low-density limit; however, a lower f /i value is allowed for IM Lup at the 90% confidence level due to the low count statistic. The low-density limit is also observed in WTTS (TWA 4 Kastner et al 2004; TWA 5 (Huenemoerder et al 2007) all show significant deviations from the low-density limit.…”

“…Well-studied examples of young main-sequence (MS) stars such as AU Mic, Speedy Mic and AB Dor (Ness et al 2004) also show triplets indicating low densities similar to older MS objects. Five of 83 WTTS observed by Padgett et al (2006) show an IR-excess caused by dust within a few AU of the star, detected by Spitzer, thus there is a matter reservoir for accretion.…”

The Origin of T Tauri X‐Ray Emission: New Insights from the Chandra Orion Ultradeep Project

“…Although the high X-ray luminosities of active MS stars and TTSs cannot be reproduced by simply filling the available coronal volume with solar-like active regions, coronal structures with higher plasma density 20 can explain the high levels of stellar X-ray activity. Evidence for higher than solar plasma densities is found in high-resolution X-ray and EUV spectra for many active stars (e.g., Sanz-Forcada et al 2003;Ness et al 2004). Furthermore, once the stellar coronae get nearly completely filled with active regions, the magnetic interaction of the active regions should lead to increased flaring in the most active stars, boosting their X-ray luminosities even further .…”

Observations of accretion shocks

“…Measurements of the temperature sensitive O vii/O viii line ratio (tracing gas between 2 × 10 6 K and 4 × 10 6 K) suggest that as a group CTTs have larger amounts of cool gas than WTTs (Telleschi et al 2007a) as expected from the accretion shock. If the emission is due to post-shock gas, the density of this cool plasma should be larger in CTTs, compared to the same emission in naked atmospheres (∼ 10 9 cm −3 to ∼ 10 11 cm −3 , see Ness et al 2004). Observations of density-sensitive forbidden (for) and intercombination (int) lines of He-like ions (like Ne ixand O vii) reveal for/int values smaller than ∼ 0.5 in CTTs, suggesting plasma densities larger than ∼ 10 11 cm −3 (see for example Kastner et al in this volume, and references therein).…”