Halpha line-profile variations of the two
active binary systems UX For and AG Dor
Introduction
A large piece of information on the solar chromosphere comes from
observations of the Balmer H_alpha line. H_alpha contains
information about the photosphere, the chromosphere and about
activity tracers such as plages, prominences and macroscopic velocity
fields.
The generally accepted explanation of the optical variability of RS
CVn stars is that they are covered with large, dark spotted areas.
In analogy to the Sun we expect that stellar plages
are seen in the H_alpha line as emission
component while prominences will show up
as absorption components.
Only a few chromospherically active binary systems show H_alpha
as a pure emission line.
Commonly this feature is seen in absorption but partly filled in by
chromospheric emission when compared with appropriate M-K standards.
This is also true for UX For and AG Dor.
Spectrum Synthesis
We found, that both systems, UX For and AG Dor,
are double-lined spectroscopic binaries. In order to eliminate the
photospheric H_alpha part of each component of the two binary systems
we synthesize
the observed spectra with two spectra of non-active standard stars
of the same M-K classification. Radial velocity shift, rotational Doppler
broadening and relative intensities of the two spectra are adjusted.
The combined standard star spectra are then subtracted from our
observed spectra presumably representing the chromospheric H_alpha part.
Residual H_alpha variations
In the case of UX For a second run of the spectrum synthesis
technique was applied in order to show the variations in more details.
For this purpose we choose a residual spectrum
where the two H_alpha emission peaks are well separated and appear
weakest and presumably represent the least active phase.
After the seperated subtraction of the individual H_alpha emission
profiles only the short-term variations remain (Fig.1).
We clearly see some variations from the stronger-emission component.
These variations may be due to additional emission
from plages or unresolved flares. There is also
evidence
for additional emission in-between the two H_alpha components
(the zero-velocity center), which could be caused by material
left over from coronal mass ejections. No phase dependence
of the H_alpha variations could be found though, and it is thus
unlikely that H_alpha in UX For is rotationally modulated.
An A&A paper presenting these results (and additionally the
first Doppler image of AG Dor) was published in April 2001.
Please find more in formation here.
(update: May 16, 2001)
