Protostellar accretion traced with chemistry: Comparing synthetic C18O maps of embedded protostars to real observations

TL;DR

This study uses synthetic C18O maps from simulations to compare episodic protostellar accretion with observations, revealing the need for disk physics to explain observed accretion variability.

Contribution

It introduces a method to compare simulated and observed protostellar accretion via chemical tracers, highlighting the importance of disks in episodic accretion.

Findings

Simulations show fewer extended C18O emissions than observed, indicating lower episodic accretion activity.

Most simulated accretion is driven by large-scale infall, lacking disk physics.

Discrepancies suggest disks are essential for realistic episodic accretion modeling.

Abstract

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 C$^{18}$O line cubes are created for a large number of simulated protostars. The spatial extent of C$^{18}$O 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 C$^{18}$O 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.