Halpha line-profile variations of the two active binary systems UX For and AG Dor


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)