Far-infrared observations of a massive cluster forming in the Monoceros
  R2 filament hub
  We present far-infrared observations of Monoceros R2 (a giant molecular cloud
at approximately 830 pc distance, containing several sites of active star
formation), as observed at 70 {\mu}m, 160 {\mu}m, 250 {\mu}m, 350 {\mu}m, and
500 {\mu}m by the Photodetector Array Camera and Spectrometer (PACS) and
Spectral and Photometric Imaging Receiver (SPIRE) instruments on the Herschel
Space Observatory as part of the Herschel imaging survey of OB young stellar
objects (HOBYS) Key programme. The Herschel data are complemented by SCUBA-2
data in the submillimetre range, and WISE and Spitzer data in the mid-infrared.
In addition, C18O data from the IRAM 30-m Telescope are presented, and used for
kinematic information. Sources were extracted from the maps with getsources,
and from the fluxes measured, spectral energy distributions were constructed,
allowing measurements of source mass and dust temperature. Of 177 Herschel
sources robustly detected in the region (a detection with high signal-to-noise
and low axis ratio at multiple wavelengths), including protostars and starless
cores, 29 are found in a filamentary hub at the centre of the region (a little
over 1% of the observed area). These objects are on average smaller, more
massive, and more luminous than those in the surrounding regions (which
together suggest that they are at a later stage of evolution), a result that
cannot be explained entirely by selection effects. These results suggest a
picture in which the hub may have begun star formation at a point significantly
earlier than the outer regions, possibly forming as a result of feedback from
earlier star formation. Furthermore, the hub may be sustaining its star
formation by accreting material from the surrounding filaments.
