The Timescales of Embedded Star Formation as Observed in STARFORGE

Star formation occurs within dusty molecular clouds that are then disrupted by stellar feedback. However, the timing and physical mechanisms that govern the transition from deeply embedded to exposed stars remain uncertain. Using the STARFORGE simulations, we analyze the evolution of ``embeddedness’’, identifying what drives emergence. We find the transition from embedded to exposed is fast for individual stars, within 1.3 Myr after the star reaches its maximum mass. This rapid transition is dominated by massive stars, which accrete while remaining highly obscured until their feedback eventually balances, then overcomes, the local accretion. For these massive stars, their maximum mass is reached simultaneously with their emergence. Once these stars are revealed, their localized, pre-supernova feedback then impacts the cloud, driving gas clearance. Because massive stars dominate the luminosity, their fast, local evolution dominates the light emergence from the dust. We calculate the dependence of these processes on the mass of the cloud and find that emergence always depends on when massive stars form, which scales with the cloud’s free-fall time. We also measure the evolution of dust and H$α$ luminosities, where for $\sim$2 Myr, these tracers outshine the emerging stellar continuum, reaching their peak when gas and dust remain tightly coupled to the massive stars. These results closely resemble observationally observed lifetimes, tying the observable dust and line emission directly to the same localized processes that drive stellar emergence, evidence that our simulated de-embedding physics is representative of real star-forming regions. Thus, because the initial embedding of the most luminous stars is highly local, the emergence of stars is a faster, earlier, more local event than the overall disruption of the cloud by gas expulsion.

💡 Research Summary

This paper leverages the state‑of‑the‑art STARFORGE simulation suite to quantify how long newly formed stars remain embedded in their natal molecular clouds and what physical processes drive their emergence. The authors analyze two cloud models—a 2 × 10³ M⊙ cloud with a 10 pc radius and a more massive 2 × 10⁴ M⊙ cloud with a 30 pc radius—both simulated with full radiation‑magnetohydrodynamics, chemistry, and all major stellar feedback channels (protostellar jets, radiation pressure, photo‑ionization, photo‑electric heating, line‑driven winds, and supernovae). The simulations resolve individual stars down to ~0.1 M⊙, using sink particles that contain a protostar and an internal gas reservoir to model unresolved accretion.

A novel “embeddedness” metric is introduced, combining line‑of‑sight visual extinction (AV) and the dust infrared luminosity surrounding each star. Stars with AV > 10 mag are classified as embedded, while AV < 2 mag indicates exposure. By tracking the time each star reaches its final mass (t_max) and the time its AV drops sharply (t_emerge), the study finds that the transition from embedded to exposed is rapid: on average 1.3 Myr after t_max. Massive stars (M > 8 M⊙) reach t_max and t_emerge almost simultaneously; their intense UV radiation, radiation pressure, and line‑driven winds quickly overcome local gravity, expelling surrounding gas and dust.

On the cloud‑scale, the emergence of the whole stellar population is tightly linked to the formation of the first massive stars. The authors show that massive‑star formation occurs at roughly half a free‑fall time (t_ff) of the cloud, regardless of cloud mass. Consequently, larger clouds experience proportionally longer absolute emergence times, but the relative timing (as a fraction of t_ff) remains constant. This scaling reproduces observed embedded‑cluster lifetimes of 1–5 Myr across a wide range of galactic environments.

The paper also examines observable tracers. Dust continuum and Hα emission dominate the total luminosity for ~2 Myr before the stellar continuum becomes visible. Their peaks coincide with the period when massive stars are still heavily obscured, confirming that IR‑bright and Hα‑bright phases trace the same localized feedback‑driven clearing process. Once massive stars emerge, both dust and Hα luminosities decline sharply as feedback clears the surrounding gas.

These results bridge the gap between theory and observations: the rapid, locally driven emergence of massive stars explains why infrared and optical tracers suggest different embedded lifetimes, and why JWST and ALMA now resolve small‑scale PDR and PAH structures that correspond to the early, highly obscured phase. The authors conclude that the de‑embedding timescale is fundamentally set by massive‑star formation, which is itself regulated by the cloud’s free‑fall time. Future work should incorporate stellar multiplicity, metallicity variations, and external pressure to explore the diversity of embedded lifetimes in different galactic contexts.