Protostellar accretion traced with chemistry. High resolution C18O and
  continuum observations towards deeply embedded protostars in Perseus
  Context: Understanding how accretion proceeds is a key question of star
formation, with important implications for both the physical and chemical
evolution of young stellar objects. In particular, very little is known about
the accretion variability in the earliest stages of star formation.
  Aims: To characterise protostellar accretion histories towards individual
sources by utilising sublimation and freeze-out chemistry of CO.
  Methods: A sample of 24 embedded protostars are observed with the
Submillimeter Array (SMA) in context of the large program "Mass Assembly of
Stellar Systems and their Evolution with the SMA" (MASSES). The size of the
C$^{18}$O emitting region, where CO has sublimated into the gas-phase, is
measured towards each source and compared to the expected size of the region
given the current luminosity. The SMA observations also include 1.3 mm
continuum data, which are used to investigate whether a link can be established
between accretion bursts and massive circumstellar disks.
  Results: Depending on the adopted sublimation temperature of the CO ice,
between 20% and 50% of the sources in the sample show extended C$^{18}$O
emission indicating that the gas was warm enough in the past that CO sublimated
and is currently in the process of refreezing; something which we attribute to
a recent accretion burst. Given the fraction of sources with extended C$^{18}$O
emission, we estimate an average interval between bursts of 20000-50000 yr,
which is consistent with previous estimates. No clear link can be established
between the presence of circumstellar disks and accretion bursts, however the
three closest known binaries in the sample (projected separations <20 AU) all
show evidence of a past accretion burst, indicating that close binary
interactions may also play a role in inducing accretion variability.
