Assessing Self-Absorbed Molecular Lines as Tracers of Gravitational Collapse
Lillian Y. Cai, Felix D. Priestley, Sarah E. Ragan
TL;DR
The paper addresses the reliability of infall-velocity estimates from redshifted self-absorption in prestellar cores, highlighting biases when using constant-velocity slab models. Using SPH collapse simulations, time-dependent gas-grain chemistry, and radiative transfer, the authors produce synthetic line profiles for multiple tracers and compare the inferred infall velocity $V_{in}$ (via the Myers $1996$ two-slab method) to the true mass-weighted infall velocity $V_{mw}$. They find that $V_{in}$ often underestimates $V_{mw}$ by up to an order of magnitude, with CS providing the closest but still partial recovery and HCO$^+$ performing poorly due to larger peak separations; optically-thin tracers’ non-thermal dispersion $\sigma_{bulk}$ tracks $V_{mw}$ within a factor of ~2. The findings imply that mass accretion rates derived from traditional line diagnostics are biased low, underscoring the need for model-informed diagnostics or alternative proxies when diagnosing collapse in prestellar cores.
Abstract
Redshifted self-absorption features in molecular lines are commonly interpreted as signatures of gravitational collapse in pre- and protostellar cores. The shape of the line profile then encodes information on the dynamics of the collapse. There exist well-established observational techniques to estimate infall velocities from these profiles, but these have historically been calibrated on constant-velocity slab models, whereas more realistic simulations of gravitational collapse produce highly non-uniform radial velocity profiles. We produce synthetic line observations of a simulated collapsing prestellar core, including a treatment of the time-dependent chemical evolution. Applying observational techniques to the synthetic line profiles, we find that the estimated infall velocities are significantly and systematically lower than the mass-weighted infall velocities from the simulation. This is primarily because the self-absorption features tend to originate from the outer regions of the core, well beyond the location of the peak infall velocity. Velocities and mass accretion rates measured via these techniques are likely to underestimate the true values.