Walter et al. (1988) suggested that was higher for nTTs than for cTTs.
The measurement of is a spectroscopic observation of the Doppler broadening of the stellar spectral lines by the projected rotational velocity. It is fairly straightforward to measure, given sufficient resolution and signal to noise. Since only a single observation of a star is required to measure , the quantity is known for many stars. However, since only the projected velocities are measured, random projection effects can skew the results of any given star. Young stars may not have randomly distributed rotational axes. Heyer et al. (1987) found that the rotational axes of dark clouds are aligned with the local magnetic field.
If this alignment continues down to the stellar level, the term introduces a systematic bias. However, this finding is not supported by the recent work of Goodman et al. (1992, 1995)
The first attempts to directly measure periodic modulation of T Tauri stars occurred in the early 1980s. Rydgren & Vrba (1983) observed four PMS stars using photoelectric photometry over a period of seven consecutive photometric nights. They found rotation periods ranging from 1.9 to 4.1 days and amplitudes of about 0.1 magnitudes at V. This confirmed the results of the spectrophotometric study; The TTs are relatively fast rotators, but are not close to their breakup velocities. They further concluded that there was no correlation between rotation period and line emission. Over the next 6 years, rotation periods were measured for about 20 TTs. In general, periods of between one and ten days were found. The slow rate of progress was essentially due to the inherently strict requirements of observing rotation periods which demand runs of at least 10 nights with photometric conditions on most of those nights. Further, even when working in a cluster, the efficiency of photoelectric photometry is limited by the single element nature of the detector.
The development of inexpensive, highly linear, large format charge--coupled devices (CCDs) has made it possible to directly measure the rotation periods of a much larger number of stars, at a much wider expanse of ages, than earlier spectrophotographic or photoelectric techniques. Using wide field CCDs, many nTTs can be observed at a single time and field stars can be used as comparison objects. The multiplexing nature of the CCD is most efficient when used in regions where there are many target stars, along with many possible comparison objects. This allows differential measurements to be made among the target and several comparisons objects, all observed through the same atmospheric conditions.
Detailed models by Bouvier and Bertout (1989) show that large fractions (> 10%) of the photospheres of nTTs are covered with cool starspots. The observational work by Wolk and Walter (1996) verifies these predictions. The work of Vrba et al. (1988) indicates that on active stars, spot lifetimes may exceed several years. The large covering factor means that the spots can have a noticeable effect on the star's magnitude. Because the spot lifetime is much longer than the rotational period, the observed effect will be a modulation of the light from the star synchronously with the rotation period. Thus, the rotation period can be measured photometrically. Figure 1 demonstrates this technique.