Issue |
A&A
Volume 427, Number 2, November IV 2004
|
|
---|---|---|
Page(s) | 667 - 683 | |
Section | Stellar atmospheres | |
DOI | https://doi.org/10.1051/0004-6361:20040504 | |
Published online | 28 October 2004 |
On the sizes of stellar X-ray coronae*
1
Universität Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany e-mail: jness@hs.uni-hamburg.de
2
Paul Scherrer Institut, Würenlingen & Villingen, 5232 Villingen PSI, Switzerland
3
Columbia Astrophysics Laboratory, 550 West 120th Street, New York, NY 10027, USA
Received:
24
March
2004
Accepted:
5
July
2004
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 xxi lines were
measured with the CORA single-purpose line fitting tool by [CITE].
Densities are estimated from He-like flux ratios of O vii and
Ne ix representing the cooler (1–6 MK) plasma
components. The densities scatter between
from the
O vii triplet and between
from the Ne ix triplet, but we caution that the latter triplet may be biased by
contamination from Fe xix and Fe xxi 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 vii of no more than a few times 1010 cm-3,
whereas no definitive trend is found for the more active stars. Investigating
the densities of the hotter plasma with various Fe xxi 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 (≈
cm-3). These
upper limits are in line with constant pressure in the emitting active regions.
We focus on the commonly used [CITE] 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 ion-specific 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.
Key words: X-rays: stars / stars: coronae / stars: late-type / stars: activity / techniques: spectroscopic
© ESO, 2004
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