The historical record of massive star formation in Cygnus
  The Cygnus region, which dominates the local spiral arm of the Galaxy, is one
of the nearest complexes of massive star formation. Its massive stellar
content, regions of ongoing star formation, and molecular gas have been studied
in detail. However, little is known of the history of the region beyond the
past 10 Myr. The brightness and spectroscopic characteristics of red
supergiants make it easy to identify them and build up a virtually complete
sample of such stars at the distance of the Cygnus region, thus providing a
record of massive star formation extending several tens of Myr into the past, a
period inaccessible through the O and early B stars observable at present. We
have made a selection of a sample of bright, red stars in an area of 84 square
degrees covering the whole present extension of the Cygnus region. We have
obtained spectroscopy in the red visible range allowing an accurate,
homogeneous spectral classification as well as a reliable separation between
supergiants and other cool stars. Our data are complemented with Gaia Data
Release 2 astrometric data. We have identified 29 red supergiants in the area,
17 of which had not been previously classified as supergiants. Twenty-four of
the 29 most likely belong to the Cygnus region and four of the remaining to the
Perseus arm. We have used their derived luminosities and masses to infer the
star formation history of the region. Intense massive star formation activity
is found to have started approximately 15 Myr ago, and we find evidence for two
other episodes, one taking place between 20 and 30 Myr ago and another one
having ended approximately 40 Myr ago. There are small but significant
differences between the kinematic properties of red supergiants younger or
older then 20 Myr, hinting that stars of the older group were formed outside
the precursor of the present Cygnus complex, possibly in the Sagittarius-Carina
arm.
