  Aims: Our aim is to understand the evolutionary sequence of high-mass star
formation from the earliest evolutionary stage of high-mass starless cores, via
high-mass cores with embedded low- to intermediate-mass objects, to finally
high-mass protostellar objects. Methods: Herschel far-infrared PACS and SPIRE
observations are combined with existing data at longer and shorter wavelengths
to characterize the spectral and physical evolution of massive star-forming
regions. Results: The new Herschel images spectacularly show the evolution of
the youngest and cold high-mass star-forming regions from mid-infrared shadows
on the Wien-side of the spectral energy distribution (SED), via structures
almost lost in the background emission around 100mum, to strong emission
sources at the Rayleigh-Jeans tail. Fits of the SEDs for four exemplary regions
covering evolutionary stages from high-mass starless cores to high-mass
protostellar objects reveal that the youngest regions can be fitted by
single-component black-bodies with temperatures on the order of 17K. More
evolved regions show mid-infrared excess emission from an additional warmer
component, which however barely contributes to the total luminosities for the
youngest regions. Exceptionally low values of the ratio between bolometric and
submm luminosity additionally support the youth of the infrared-dark sources.
Conclusions: The Herschel observations reveal the spectral and physical
properties of young high-mass star-forming regions in detail. The data clearly
outline the evolutionary sequence in the images and SEDs. Future work on larger
samples as well as incorporating full radiative transfer calculations will
characterize the physical nature at the onset of massive star formation in even
more depth.
