Investigating the Role of Protostellar Variability with PRIMA Using Monte Carlo Simulations

TL;DR

The paper addresses whether episodic accretion bursts can supply the bulk of protostellar mass, i.e., whether $M_{ m burst} \geq 0.5 M_{*}$. It uses Monte Carlo-based synthetic ensembles drawn from Herschel 70 μm fluxes to simulate PRIMA-like far-infrared monitoring of evolving protostars, combining steady accretion with bursts. Two scenarios are explored—FU Ori-dominated bursts and multiple equally important burst types—and the results indicate that about 2000 protostars are needed for PRIMA to constrain the burst contribution to mass assembly. These findings inform planning of deep far-IR time-domain surveys and place quantitative limits on the role of episodic accretion in the early evolution of stars.

Abstract

Evidence suggests that protostellar outbursts likely play a critical role in the stellar mass assembly process, but the extent of this contribution is not well understood. Using the proposed observing program of PRIMA, a concept far-IR observatory (PRIMA GO Case #43 in Moullet et al. 2023), we examine the probe's ability to unambiguously determine whether or not variable accretion events dominate the stellar mass assembly process ($M_{\rm burst}\geq0.5M_{*}$). To do this, we construct multiple protostellar ensembles using Herschel 70$μ$m flux data and evolve them using a toy Monte Carlo simulation through steady-state and high magnitude accretion events. Ensembles are observed at various epochs in the evolution process to conclude how many large amplitude outbursts are observationally recoverable during the proposed program. Based on our synthetic observations and our simulation specifications, we determine that observing a protostellar ensemble of at least 2000 protostars using PRIMA's proposed program is sufficient for determining the importance of protostellar outbursts in the stellar mass assembly process.

Figures (3)

• Figure 1: A schematic illustrating the steps used in this simulation. In the first panel, Monte Carlo simulation parameters are set using outburst amplitude and duration from Fischer2023 and assuming outbursts contribute 50% of a star's mass. In situations in which multiple outbursts types are significant mass contributors, we assume each outburst type is equally important. In the second panel, synthetic ensembles are created by assigning 70$\mu$m Herschel flux measurements from Furlan2016 to each source in the ensemble, where ensembles can have between 50 and 5000 protostars. In panel three, 50 ensembles are evolved for 300 years and synthetically observed using the proposed PRIMA program (Battersby et al. Moullet2023). In panel four, data retrieval and analysis is conducted on the number of detected outbursts by PRIMA and the associated mass contribution is calculated. Credit: Protostellar Images: T. Pyle (Caltech/IPAC); Spacecraft: NASA/JPL-Caltech; Background: ESO/S. Brunier
• Figure 2: The duration-amplitude relationship of accretion outbursts considered in this paper: FU Ori, V1647 Ori, Ex Lup, and routine variability events (RV) of long and short duration. Central values and ranges of the different burst types (shaded regions) were obtained from Fischer2023. In our simulations, only the central values of these regions are used (see Table \ref{['tab:burst_properties']} for quantities).
• Figure 3: (Top:) Example light curves of our simulated data showing the true behavior of three protostars within our ensemble over the full 25 year observing program assuming all five burst types can occur. The grey lines indicate the timesteps in which an observation occurs. It can be seen that all burst types can be detected by the end of the program where the medium duration bursts are much more frequently and more easily observed than the larger, longer and smaller, shorter bursts. (Bottom:) The behavior of the same three protostars as observed using the proposed PRIMA observing program. Observations taken by PRIMA are displayed in black and observations taken by Herschel are displayed in red. The observing cadence blurs the true behavior of the protostars, as observed in the second panel of this row in which an RV-Short outburst appears to be associated with the brightening of an EX Lup event.