New Stars on the Block

Groups of young stars located in the solar neighborhood have recently received a lot of attention. These stars, which are ∼ < 10 million years old, can provide insights into the birth of stars and planetary systems. For example, the TW Hydrae Association is just at the age when planets are believed to form, while MBM12 represents an earlier stage. In this Perspective, I briefly discuss recent research in this field. Much glory in astronomy comes from probing the farthest reaches of the universe for the most exotic beasts. But poking around closer to home for more prosaic objects has its own rewards–such as finding small groups of relatively isolated young stars destined for the limelight. Over the past few years, one such group, known as the TW Hydrae Association, has attracted quite a following, as evidenced by the high attendance of a special session during the recent American Astronomical Society meeting. The reason for this widespread interest is that the TW Hydrae group, and others like it in the solar neighborhood, may tell us a lot about the birth of stars and planetary systems. The TW Hydrae Association, in particular, appears to be at the age at which planet formation is believed to occur. It may even be possible to take a picture of a newborn planet around one of these stars. The star TW Hydrae first caught astronomers' attention back in 1978 (1). George Herbig noted that it had the earmarks of a young low-mass star, or so-called T Tauri star, including variability in brightness, a strong Hα emission line in its spectrum, and a high abundance of lithium (an element that is easily fused in nuclear reactions and thus does not survive in older stars). Most T Tauri stars are found in clouds of gas and dust, the presumed sites of their birth, such as the Orion nebula. Curiously, TW Hydrae is not. Subsequent work added further evidence for TW Hydrae's youth and indications for the presence of a circumstellar disk and revealed four other stars in the same region of the sky with similar characteristics (2, 3). Three years ago, Kastner et al. (4) suggested on the basis of strong x-ray emission from all five systems that the group forms a physical association at a distance of roughly 150 light-years. Since then, at least seven more stars have been identified as candidate members of the TW …