11 - The formation and evolution of prestellar cores

Summary

Introduction: dense cores and the origin of the IMF

Stars form from the gravitational collapse of dense cloud cores in the molecular interstellar medium of galaxies. Studying and characterizing the properties of dense cores is thus of great interest to gain insight into the initial conditions and initial stages of the star formation process.

Our observational understanding of low-mass dense cores has made significant progress in recent years, and three broad categories of cores can now be distinguished within nearby molecular clouds, which possibly represent an evolutionary sequence: starless cores, prestellar cores and ‘Class 0’ protostellar cores. Starless cores are possibly transient concentrations of molecular gas and dust without embedded young stellar objects (YSOs), typically observed in tracers such as C¹⁸O (Onishi et al. 1998), NH₃ (Jijina et al. 1999) or dust extinction (Alves et al. 2007), and which do not show evidence of infall. Prestellar cores are also starless (M_* = 0) but represent a somewhat denser and more centrally concentrated population of cores which are self-gravitating, hence unlikely to be transient. They are typically detected in (sub)millimetre dust continuum emission and dense molecular gas tracers such as NH₃ or N₂H⁺ (Benson & Myers 1989; Ward-Thompson et al. 1994; Caselli et al. 2002), are often seen in absorption at mid- to far-infrared wavelengths (Bacmann et al. 2000; Alves et al. 2001) and frequently exhibit evidence of infall motions (Gregersen & Evans 2000). Conceptually, all prestellar cores are starless but only a subset of the starless cores evolve into prestellar cores; the rest are presumably ‘failed’ cores that eventually disperse and never form stars.