Relations

Mindful of these biases, I can now explore the relations between the measured rotation periods and various other observables.

The hypothesis behind monitoring stellar variability is that active stars have asymmetric spot distributions on their photospheres. It has been argued (Bouvier 1990) that if the magnetic field is a function of a dynamo, then the faster rotators should be more active. Taking this to its logical extension, the stronger the activity, the greater the spot coverage, and thus the stronger the observed variability should be, so long as the spot filling factor does not exceed 50%. Choi & Herbst (1996) were surprised to note the opposite effect. They found that only stars with rotation periods greater than seven days show variations greater than 0.4 magnitudes at I. Bouvier et al. (1995) found that wTTs they observed did not vary by more than half a magnitude in B or V while the cTTs varied by over a magnitude. In Figure 2, a plot of rotation period versus observed variability is shown. V--band is plotted because of its greater sensitivity to spots. Almost 30% of the slow rotators show variability of greater than 0.4 magnitudes at V-band, while only 2 rapidly rotating stars show such variability. Both of those stars had very high FAPs. Overall, the mean value of the V-band modulation of the stars with rotation periods < 4 days is 0.14 0.11 magnitudes, while the modulation for the slower rotators is 0.32 0.18 magnitudes. If I assume that the stars which are more slowly rotating still have or recently lost their disks, the trend in these results, while not as strong, is in agreement with analysis of Herbst et al. (1994). Among PMS stars without disks, they found that the observed variability had a mean of 0.32 0.2 magnitudes in the V-band. They find the cTTs with larger amplitude variations, a mean 0.89 0.66 magnitudes.

I did not observe objects as variable as those in the study of Herbst et al. (1994). This may be due to selection effects. The optical sources of variability on cTTs seem to be related to the disk and accretion (Herbst et al. 1994). X--ray selection not only selects out the most X--ray active stars, it also singles out the more massive, G and K stars, ignoring the M stars. Prosser et al. (1993a) demonstrated a relation between X--ray flux and spectral type in ZAMS stars. As shown in Figure 3, this dependence does not translate itself into a simple relation between rotation period and spectral type for the PMS stars. Since spectral data for most of the stars with rotation periods are not available, R-I color was taken to be a proxy for spectral type (Figure 4). R-I color was chosen since it was available for all stars and it is least affected by both from reddening extinction effects (which attenuate blue wavelengths more strongly than red wavelengths) and by confusion caused by an active chromosphere (which is a high temperature effect and thus is more apparent in blue wavelengths). There is no apparent direct relation between rotation and spectral type.

That the highest mass stars are rotating most quickly may be an evolutionary effect. Such stars evolve more quickly than low mass stars. Thus, they may lose their disks more quickly, so that there are no slowly rotating high mass stars left to observe by the age of Orion OB1b. However, Figure 4 does not confirm this. The bias towards monitoring only X--ray active sources confuses any possible interpretation. Note that in the Pleiades, while stars later than about K0 appear to have saturated X--ray luminosities, in earlier types the relative X--ray luminosity decreases as mass increases (Prosser et al. 1993). In Orion OB1a and b, the most evolved of these stars are starting on their radiative tracks, so they are no longer fully convective.