stitute only a small fraction of the volume and mass of a typical molecular cloud. Knowing how these dense regions form and evolve is vital to understanding the initiation of star formation, and it has implications for galactic and cosmic evolution. Yet the mechanisms controlling these processes are not well understood. To make further progress in characterizing the internal dynamical states of molecular clouds over a wide range of spatial scales and environments, the panel recommends the following:
Extensive dust and molecular-line emission surveys of massive giant molecular clouds spanning spatial scales from 100 to 0.1 parsec (pc) at distances greater than 5 kiloparsec (kpc), and
Complementary studies of the young stellar populations spawned in these regions, conducted by means of infrared surveys with spatial resolution at least 0.1 arcsec to reduce source confusion in clusters, with probing sufficiently faint to detect young brown dwarfs.
In the next stage of star formation, the dense structures in molecular clouds fragment into self-gravitating “cores” that are the direct progenitors of stars. There is mounting evidence from nearby star-forming regions that the distribution of core masses may be directly related to the resulting distribution of stellar masses, although some subsequent fragmentation likely produces binaries and very low mass objects. This may occur especially during the final stage of star formation through disk accretion. To explore this evolution and to improve core-mass spectra and characterize the core properties that may lead to subsequent fragmentation into stars, the panel recommends the following:
Deep surveys of cores down to sizes of 0.1 pc at millimeter and submillimeter wavelengths in diverse star-forming environments out to distances of several kiloparsecs, using both interferometers and large single-dish telescopes, far-infrared imaging and spectroscopy from spaceborne telescopes, and polarimetry to determine the role of magnetic fields.
An essential test of the understanding of star formation requires a definitive answer to the question of whether the initial mass function (IMF)—that is, the relative frequency with which stars of a given mass form—is independent of environment. This is a topic of great importance to an understanding of the development of galaxies and the production of heavy elements over cosmic time (see the discussion in Chapter 2, “Report of the Panel on Galaxies Across Cosmic Time”) in this volume. Initial investigations of massive young clusters using the Hubble Space Telescope (HST) and other large instruments have suggested that the IMF may be “top-heavy” (with larger fractions of massive stars) in very dense regions,