摘要：Context. Understanding how protostars accrete their mass is a central
question of star formation. One aspect of this is trying to understand whether the time
evolution of accretion rates in deeply embedded objects is best characterised by a smooth
decline from early to late stages or by intermittent bursts of high accretion.
Aims. We create synthetic observations of deeply embedded protostars in
a large numerical simulation of a molecular cloud, which are compared directly to real
observations. The goal is to compare episodic accretion events in the simulation to
observations and to test the methodology used for analysing the observations.
Methods. Simple freeze-out and sublimation chemistry is added to the
simulation, and synthetic C18O line cubes are created for a large number of simulated
protostars. The spatial extent of C18O is measured for the simulated protostars and compared
directly to a sample of 16 deeply embedded protostars observed with the Submillimeter
Array. If CO is distributed over a larger area than predicted based on the protostellar
luminosity, it may indicate that the luminosity has been higher in the past and that CO is
still in the process of refreezing.
Results. Approximately 1% of the protostars in the simulation show
extended C18O
emission, as opposed to approximately 50% in the observations, indicating that the
magnitude and frequency of episodic accretion events in the simulation is too low relative
to observations. The protostellar accretion rates in the simulation are primarily
modulated by infall from the larger scales of the molecular cloud, and do not include any
disk physics. The discrepancy between simulation and observations is taken as support for
the necessity of disks, even in deeply embedded objects, to produce episodic accretion
events of sufficient frequency and amplitude.
关键词：stars: formation;stars: protostars;ISM:
molecules;astrochemistry;magnetohydrodynamics
(MHD);radiative transfer
