Evolutionary Effects

Edwards et al. (1993) noted that stars with near--IR flux excess have longer rotation periods than stars with more normal photospheric IR colors. They attributed this to a mechanism which regulated stellar angular momentum through the accretion disk. More recently, Shu et al. (1994) have incorporated this into their model for bipolar flows from disk winds. In Figure 5, rotation periods are plotted against H-K color for the stars near Orionis. The result here is similar to that of Figure 2. While none of the rapidly rotating stars have H-K greater than 0.4, over 3 of the 9 slower rotators do. The lack or presence of disks seems to be primarily responsible for the bimodal distribution of rotation periods first noted by Attridge & Herbst (1992).

Although many slow rotators show H-K excesses, many do not. This is true also in the results presented by Edwards et al. (1993). This means that while the existence of optically thick disk material might be linked to slow rotation periods, it is not necessary. Since the IR data presented here have large measurement errors, any estimate of evolutionary rates would be extremely rough. One can say that at an age of 2 Myr, 70% of the slow rotators do not have large disk signatures. The data presented here also show a dearth of periods from 3--7 days. While some of this may be due to sampling effects, the data seem to indicate that after a star loses its disk, either it continues its slow rotation for a period of time and then a rapid increase in rotation rate commences or spin up does not necessarily follow the loss of the disk.

If stars maintain their disks for a very long time, then by the time they lose their disks they have achieved their final internal configuration, and no additional spin--up occurs. There are two competing effects to consider here: contraction which leads to an increased rotation rate and core formation which can act as a brake. In the case of contraction, the star gets smaller and the rotation rate must increase to maintain the star's angular momentum. This assumes that the star is rigidly rotating. Once a star forms a relatively massive core, the envelope is freed from this constraint. If the core contains the bulk of the mass, it will spin--up to maintain angular momentum, but the stellar photosphere, which is what is observed, does not have to follow suit since it contains relatively little mass and thus little angular momentum. If the star loses its disk after core formation, it may be too late for photospheric spin--up to occur. Angular momentum will still be transferred from the core to the photosphere, but the rate at which the photosphere accelerates is a strong function of the strength of the coupling between the core and the envelope, which is not well determined.

When one compares the histograms shown in Figure , one is comparing three temporal epochs, < 1 Myr, 2 Myr and 10 Myr. It seems likely that the difference among the three plots is evolutionary in nature. It follows that younger stars are more likely to have disks, and as the stars evolve, they lose their disks and spin--up. There is a great deal of evidence that stars lose their disks as function of time. While evidence for disks is observed around all cTTs, no low mass main sequence star has ever been observed to possess an optically thick disk. The transient nature of disks around stars was first noted by Herbig (1978) who suggested that there should exist a population of stars between the main sequence and what are now called cTTs. More recently the data of Sterzik et al. (1995) show the ratio of nTTs to cTTs increases with increasing distance from dark clouds. The difference in IR colors between the FR and SR is indicative of disk involvement. The change in the ratio of FR to SR may be a manifestation of young stars losing their disks in time.