In the last ten years, a number of stars have been discovered that share many of the same properties as T Tauri stars (stars that are relatively low mass, like the Sun, but which are much younger, roughly 1 million years old) except that they are located far from known regions of star formation. T Tauri stars are typically found in or near clouds of molecular gas, the material from which they formed, but these isolated young stars are not near any such clouds.
These stars are interesting because they represent a missing link of sorts - they are at a stage of their evolution (with ages of some tens of millions of years) that places them between the ages of T Tauri stars and main sequence stars like the Sun. In the T Tauri stars, we see that many stars harbor disks of gas and dust that presumably is the raw material for planet formation. Around the main sequence stars (with ages of 100 million years up to billions of years) we find no such disks, but astronomers are starting to have success finding planets around main sequence stars. The logical presumption is that the (abundant) disks form the planets we see later, but it would be nice to see what's happening in between, at ages of around 10-100 million years. That's why these isolated young stars are of interest.
Unfortunately, the fact that these stars are not associated with big clouds of molecular gas makes these stars much harder to find. You don't have a big signpost like the Orion Nebula to tell you "Hey, there are young stars over here!"
A good introduction to the problem of finding and studying young, nearby stars can be found in the proceedings of the meeting Young Stars Near Earth: Progress and Prospects. I wrote a contribution to those proceedings, "Can Post T Tauri Stars be Found? Yes!", which gives an overview of the problem. You can read the abstract or get the entire piece from my publications page.
My collaborators and I are in the process of a survey to look for such isolated, young, nearby stars. We have just completed observations of the last candidate young stars in the southern sky, and preliminary results were presented at the January 2002 American Astronomical Society meeting in Washington, D.C. The poster from the AAS meeting is available on my publications page.
This work was supported in part by grant AST-9996278 from the National Science Foundation. Any opinions, findings and conclusions, or recommendations expressed in this material are those of the author and do not necessarily reflect the views of the National Science Foundation (NSF).