Star Forming Regions and Near-IR Surveys

Abstract

The traditional way to find young, low-mass stars has been to do deeper and deeper near-IR surveys of molecular clouds, and to look for objects with excess IR flux over a blackbody photosphere. We thus expect to find many low-luminosity young objects with surveys such as DENIS and 2MASS. However, taken in isolation, such surveys cannot be conclusive in two cases: (i) young objects without near-IR excess (such as the “weak-line T Tauri stars”: WTTS); (ii) background cool main-sequence stars or red giants, which dominate the faint population of IR sources. One of the best ways to select young stars without IR excess is to use their X-ray emission properties. As X-ray observations done with ROSAT show, many WTTS are present at a distance from star-forming regions. The spectroscopic determination of their age then allows in particular to trace the past history of star formation of these regions. In this context, I will also discuss the new possibilities offered by the next generation of X-ray satellites, AXAF and XMM