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
∼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.