Rotation of T Tauri and cluster stars

Clues to the initial angular momentum distribution of solar-type stars are mainly gathered from observation of much younger objects such as T Tauri stars, which are low-mass pre-main-sequence stars of age less than 10 Myr. Rotation periods and projected rotational velocities are available for more than one hundred of these stars. Figure 7.3 illustrates the histogram of rotation periods for stars belonging to the Orion Nebula cluster. One readily sees that this frequency distribution is distinctly bimodal, confirming the i i i i | i i i i | i i i i | i i i i | i i i i | i i i i | i i i i

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Period (days)

Fig. 7.3. Frequency distribution of rotation periods of T Tauri stars in the Orion Nebula cluster. Source: Choi, P. I., and Herbst, W., Astron. J.., 111,283, 1996.

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Period (days)

Fig. 7.3. Frequency distribution of rotation periods of T Tauri stars in the Orion Nebula cluster. Source: Choi, P. I., and Herbst, W., Astron. J.., 111,283, 1996.

discovery of Attridge and Herbst (1992). About one third of the stars are rapid rotators with a median period of 2.55 days and a dispersion of 0.7 days. The others are slow rotators with a median period of 8.30 days, a dispersion of 3.8 days, and a sparsely populated tail of very slow rotators extending to 34.5 days. It is important to note that this bimodal distribution of periods is not restricted to the Orion Nebula cluster since it is also apparent in Figure 1.9, which depicts the histogram of rotation periods for T Tauri stars in other clusters and associations. According to Choi and Herbst (1996), there is little doubt that 4-5 day periods are rare among T Tauri stars and so this bimodal period distribution is real.

Edwards et al. (1993) have also measured infrared color excesses for a sample of thirty-four T Tauri stars with photometrically derived rotation periods and spectral types later than K5. Their main conclusion is that the observed periods appear to be related to the presence or absence of a circumstellar accretion disk. Those stars that they infer to be surrounded by accretion disks (i.e., the classical T Tauri stars) are slow rotators with periods larger than 4 days, with a most probable period of 8.5 days, while those that lack accretion disk signatures (i.e., the weak-line T Tauri stars) cover a wide range of rotation periods, ranging from 1.5 to 16 days, including a significant number of objects with periods smaller than 4 days. This result was confirmed by Bouvier et al. (1993), who made a detailed study of T Tauri stars belonging to the Taurus-Auriga cloud. Their analysis shows that the mean rotation period is about 4 days for the weak-line T Tauri stars and about 8 days for the classical T Tauri stars. This apparent bimodality will be interpreted in Section 7.4.1.

Other clues to understanding the late pre-main-sequence/early main-sequence evolution of solar-type stars have been obtained from the study of late-type stars in the a Persei cluster (age ~ 50 Myr), the Pleiades cluster (age ~ 70 Myr), and the Hyades cluster (age ~ 600 Myr). Figure 7.4 illustrates the rotation periods of low-mass stars

Fig. 7.4. Observations of stellar rotation periods in the open clusters a Persei, the Pleiades, and the Hyades. Source: Barnes, S., and Sofia, S., Astrophys. J., 462, 746, 1996.

belonging to these three clusters. It is immediately apparent that there is a significant increase in rotation period between the ages of a Persei and the Hyades (see also Figures 1.7 and 1.8). In Figure 7.5 we display the observed v sin i distributions of solar-type stars in the (B — V) color range 0.55-0.85, corresponding to the mass range 0.8-1.OM0. Again note the considerable spread in projected rotational velocities for the stars in a Persei.

The salient features of these observations have been summarized by Stauffer (1994). These are:

1. Very rapid rotators (v sin i > 100 km s—are present at all spectral types in a Persei.

2. Relatively rapid rotators (v sin i > 50 km s—are still present in the Pleiades among the K- and M-dwarfs but are nearly absent among the G dwarfs.

3. All of the G- and K-dwarfs in the Hyades are slow rotators (v sin i < 10 km s—although there are still one K8 dwarf and some M dwarfs with moderate rotation (v sin i ^ 15-20 km s—in the cluster.

4. In all three clusters, for all spectral types later than G0, more than half of the stars are slow rotators, with v sin i < 10 km s—1.

With the adopted ages for these clusters, the spin-down time during the early main-sequence evolution is a few 10 Myr for the G dwarfs, several 10 Myr for the K dwarfs, and a few 100 Myr for the M dwarfs.

Fig. 7.5. Observed v sin i distributions for the open clusters a Persei, the Pleiades, and the Hyades. Only stars with (B — V) color between 0.55 and 0.85 (or mass between 0.8M0 and 1.0M0) are shown. Source: Keppens, R., MacGregor, K. B., and Charbonneau, P., Astron. Astrophys., 294, 469, 1995.

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