These efforts to develop complete YSO samples are compromised by the likelihood that many of the older WTT and post-T Tauri stars have kinematically dispersed from the cloud vicinity (Feigelson 1996). Unless corrections for dispersion are made, all existing T Tauri samples, even with X-ray discovered Class III stars, show a steep decline in star formation rates for stars older than 78#78 Myr. Stars inheriting velocities around 1 km s-1, which is the characteristic velocity dispersion within molecular clouds on small-scales, produce populations disperse over 10-20 degrees if star formation continues for ten Myr in nearby star formation regions ( 79#79 pc).
This hypothesis is supported by the discoveries of what appear to be many Class III stars at large distances from active star forming clouds (Figure 10). The ROSAT All-Sky Survey (RASS) has revealed thousands of X-ray sources associated with 9-15-magnitude stars at low Galactic latitudes, many of which are late-type stars with strong Li 670.7nm lines (0.01-0.05 nm equivalent widths) characteristic of WTT stars (e.g. Neuhäuser et al 1995a, Alcalá et al 1995, Sterzik et al 1995a, Wichmann et al 1996). A vigorous debate has grown over these findings. Some argue that the stars are 50-100 Myr old ZAMS and not 1-30 Myr old stars on the Hayashi tracks (Briceño et al 1997, Favata et al 1997). Many recent studies are extending the survey to the entire celestial sphere and measuring various properties of these stars.
X-ray surveys have also uncovered a previously unrecognized nearby pre-main sequence star cluster, the 80#80 Cha cluster (Mamajek et al 1999), and have located new members of the TW Hya association (Webb et al 1999). These are very nearby (d = 50-100 pc) open clusters about 10 Myr old, lying far from significant molecular material and exhibiting high proper motions. They are distinctly different in one respect: TW Hya stars are spread over a 30 degree (30 pc) region, while the known 80#80 Cha members lie within 0.5 degree (1 pc) region. Both the 80#80 Cha cluster and RASS stars spread throughout the Chamaeleon region have proper motion similar to that of the Sco-Cen association. It thus seems possible that the Sco-Cen star forming region extended tens of parsecs further into the southern sky than previously thought.
Although our understanding of these widely dispersed magnetically active Li-rich young stars is still incomplete, some results are emerging. Observationally, several recent studies indicate that the RASS stars are a mixture of older ZAMS stars and younger but widely dispersed WTT stars, in which the ratio between the types varies across the sky (Martín 1997, Covino et al 1997, Motch et al 1997, Alcalá et al 1998). These studies however depend on the subtle discrimination of WTT, post-T Tauri, and ZAMS stars from their location on the Teff-lithium equivalent-width diagram. A cross-correlation between the Tycho parallax and RASS X-ray catalogs produced a sample of nearly 9,000 stars, showing that a considerable fraction of the RASS stars appears to be distributed in a disk associated with Gould's Belt (Guillout et al 1998), a well-known ring of star-forming clouds, OB associations and open clusters lying between 150 and 500 pc from the Sun. As the RASS is severely flux-limited, detecting only 81#81% of the youthful stellar population at 79#79 pc, thousands of low-mass stars may be dispersed throughout Gould's Belt.
Astrophysically, several possible origins of `isolated' WTT stars lying far from a star-forming cloud are viable. First, they may be dynamically ejected at high velocity from star forming regions due to gravitational scattering from close binaries (Sterzik & Durisen 1995b). While energetically feasible, the efficiency of ejection would have to be implausibly high to explain the large observed population of dispersed WTT stars. Second, stars may inherit high velocities from cloudlets within giant molecular clouds (Feigelson 1996). Molecular spectroscopy shows that cloud complexes typically exhibit supersonic turbulence with large-scale velocity dispersions of order 10 km/s, which is sufficient to project stars tens of parsecs while still on the Hayashi tracks. However, the highest velocity cloudlets are typically not associated with the densest star forming cloud cores. Third, a starburst may occur near the end of molecular cloud lifetimes (perhaps star formation is delayed by ambipolar diffusion) so that the molecular material dissipates soon after star formation resulting in groups of isolated WTT stars (Palla & Galli 1997).
However incomplete the current samples are, X-ray surveys are leading to a resolution of the long-standing mystery of the missing post-T Tauri stars (Herbig 1978) and are simultaneously uncovering a widely distributed populations of low-mass young stars associated with Gould's Belt and/or the Local Association (Eggen 1999). As the properties of the dispersed magnetically active young stars are elucidated, significant insights into the history and dynamics of star formation in the solar neighborhood over the past 108 yrs may emerge.