The Earliest Phases of Star Formation (EPoS): A Herschel Key Program -
  The precursors to high-mass stars and clusters
  (Abridged) We present an overview of the sample of high-mass star and cluster
forming regions observed as part of the Earliest Phases of Star Formation
(EPoS) Herschel Guaranteed Time Key Program. A sample of 45 infrared-dark
clouds (IRDCs) were mapped at PACS 70, 100, and 160 micron and SPIRE 250, 350,
and 500 micron. In this paper, we characterize a population of cores which
appear in the PACS bands and place them into context with their host cloud and
investigate their evolutionary stage. We construct spectral energy
distributions (SEDs) of 496 cores which appear in all PACS bands, 34% of which
lack counterparts at 24 micron. From single-temperature modified blackbody fits
of the SEDs, we derive the temperature, luminosity, and mass of each core.
These properties predominantly reflect the conditions in the cold, outer
regions. Taking into account optical depth effects and performing simple
radiative transfer models, we explore the origin of emission at PACS
wavelengths. The core population has a median temperature of 20K and has masses
and luminosities that span four to five orders of magnitude. Cores with a
counterpart at 24 micron are warmer and bluer on average than cores without a
24 micron counterpart. We conclude that cores bright at 24 micron are on
average more advanced in their evolution, where a central protostar(s) have
heated the outer bulk of the core, than 24 micron-dark cores. The 24 micron
emission itself can arise in instances where our line of sight aligns with an
exposed part of the warm inner core. About 10% of the total cloud mass is found
in a given cloud's core population. We uncover over 300 further candidate cores
which are dark until 100 micron. These are candidate starless objects, and
further observations will help us determine the nature of these very cold
cores.
