  To understand evolutionary and environmental effects during the formation of
high-mass stars, we observed three regions of massive star formation at
different evolutionary stages that reside in the same natal molecular cloud.
Methods. The three regions S255IR, S255N and S255S were observed at 1.3 mm with
the Submillimeter Array (SMA) and followup short spacing information was
obtained with the IRAM 30m telescope. Near infrared (NIR) H + K-band spectra
and continuum observations were taken for S255IR with VLT-SINFONI to study the
different stellar populations in this region. The combination of millimeter
(mm) and near infrared data allow us to characterize different stellar
populations within the young forming cluster in detail. While we find multiple
mm continuum sources toward all regions, their outflow, disk and chemical
properties vary considerably. The most evolved source S255IR exhibits a
collimated bipolar outflow visible in CO and H2 emission, the outflows from the
youngest region S255S are still small and rather confined in the regions of the
mm continuum peaks. Also the chemistry toward S255IR is most evolved exhibiting
strong emission from complex molecules, while much fewer molecular lines are
detected in S255N, and in S255S we detect only CO isotopologues and SO lines.
Also, rotational structures are found toward S255N and S255IR. Furthermore, a
comparison of the NIR SINFONI and mm data from S255IR clearly reveal two
different (proto) stellar populations with an estimated age difference of
approximately 1 Myr. A multi-wavelength spectroscopy and mapping study reveals
different evolutionary phases of the star formation regions. We propose the
triggered outside-in collapse star formation scenario for the bigger picture
and the fragmentation scenario for S255IR.
