Doppler Imaging - Criteria For Doppler Imaging

Criteria For Doppler Imaging

In order to be able to use the Doppler imaging technique the star needs to fulfill some specific criteria.

• The stellar rotation needs to be the dominating effect broadening spectral lines, .

The projected equatorial rotational velocity should be at least, . If the velocity in lower, spatial resolution is degraded, but variations in the line profile can still give information of areas with higher velocities. For very high velocities, ., lines become too shallow for recognizing spots.

• The inclination angle, i, should preferably be between 20˚-70˚.

When i =0˚ the star is seen from the pole and therefore there is no line-of-sight component of the rotational velocity, i.e. no Doppler effect. When seen equator-on, i =90˚ the Doppler image will get a mirror-image symmetry, since it is impossible to distinguish if a spot is on the northern or southern hemisphere. This problem will always occur when i ≥70˚; Doppler images are still possible to get but harder to interpret.