Click on the H-alpha line profile plot at left to see a movie of the H-alpha line profile variations in the T Tauri star Sz 68. See the page on SU Aur for the details of the movie construction ad appearance. Sz 68 is a relatively early type T Tauri star, K2IV, hence the star itself is intrinsically bright. Therefore, the H-alpha emission line appears relatively weak against this bright star despite the fact the H-alpha luminosity of Sz 68 is quite high. Sz 68 is often classified as a weak T Tauri star because the H-alpha equivalent width is relatively low, but Sz 68 is an actively accreting T Tauri star with a close circumstellar disk.
The data used to make the H-alpha movie and for all the analysis discussed here was collected with the 82-inch Struve telescope of McDonald Observatory. In addition to studying the emission line variations present in Sz 68, variations in the line profiles of several photospheric absorption lines are also used to construct a Doppler image of the surface of Sz 68. The resulting image is shown below.
The dominant feature aparent in the surface image is the large starspot centered very close to star's rotation pole. Starspots, like sunspots, are regions of strong magnetic field on the surface of the star. Due to the relatively low resolution of the Doppler image of Sz 68, it is impossible to tell if the polar spot on this star is one large spot, or represents many pairs closely spaced pairs of spots as is usually seen on the Sun. If it is, in fact, one large spot near the stellar pole, the magnetic field topology is very dipolar in structure (such as produced by a bar magnet), as suggested by a number of current theories regarding the nature of Classical T Tauri stars. Large scale organized magnetic field such as those of a dipole should produce measureable amounts of circular polarization in many of the spectral lines formed in these stars. Such polarization has been detected in the emission lines of several Classical T Tauri stars, but circular polarization indicative of these globally organized fields has not been seen in the photospheric absoprtion lines of any T Tauri star. While the search continues with ever more sensitive measurements, the current situation is somewhat baffling as to what is exactly happening with the magnetic fields on these stars.
The results of this work on Sz 68 has appeared in Johns-Krull and Hatzes 1997.
In addition to Doppler imaging Sz 68, we have recently Doppler imaged the Naked (diskless) T Tauri star V830 Tau. This image is shown below. The image of this star is also dominated by a large starspot; however, this time the spot is not centered on the rotation pole, but sits near a latitude of 45 degrees. If this spot traces a dipole magnetic field on V830 Tau, then the magnetic axis is completely misalinged with the rotation axis. Perhaps the most important thing about this image, though, is simply the fact that the starspot does not appear centered on the rotation pole. Doppler images of many rapidly rotating T Tauri stars as well as other types of stars have found pole-centered starspots, prompting many to say there is some fundamenatl flaw in the Doppler imaging technique. However, the appearance of images such as that on V830 Tau indicate that there is no such flaw that always produces a spot at the rotation pole, giving additional confidence that the star with polar spots really do have them.
