Interferometric view of the circumstellar envelopes of northern FU
  Orionis-type stars
  FU Orionis-type objects are young, low-mass stars with large outbursts in
visible light that last for several years or decades. They are thought to
represent an evolutionary phase during the life of every young star when
accretion from the circumstellar disk is enhanced during recurring time
periods. These outbursts are able to rapidly build up the star while affecting
the circumstellar disk and thus the ongoing or future planet formation. In many
models infall from a circumstellar envelope seems to be necessary to trigger
the outbursts. We observed the J=1$-$0 rotational transition of $^{13}$CO and
C$^{18}$O towards eight northern FU Orionis-type stars (V1057 Cyg, V1515 Cyg,
V2492 Cyg, V2493 Cyg, V1735 Cyg, V733 Cep, RNO 1B and RNO 1C) and derive
temperatures and envelope masses and discuss the morphology and kinematics of
the circumstellar material. We detected extended CO emission associated with
all our targets. Smaller scale CO clumps were found to be associated with five
objects with radii of 2000$-$5000 AU and masses of 0.02$-$0.5 $M_{\odot}$;
these are clearly heated by the central stars. Three of these envelopes are
also strongly detected in the 2.7 mm continuum. No central CO clumps were
detected around V733 Cep and V710 Cas but there are many other clumps in their
environments. Traces of outflow activity were observed towards V1735 Cyg, V733
Cep and V710 Cas. The diversity of the observed envelopes enables us to set up
an evolutionary sequence between the objects. We find their evolutionary state
to range from early, embedded Class I stage to late, Class II-type objects with
very low-mass circumstellar material. The results reinforce the idea of FU
Orionis-type stars as representatives of a transitory stage between embedded
Class I young stellar objects and classical T-Tauri stars.
