The Timescales of Embedded Star Formation as Observed in STARFORGE

TL;DR

The study tackles how the embedded star-forming phase transitions to exposed stars and what physical processes govern gas dispersal. It leverages high-resolution STARFORGE 3D radiative MHD simulations to track individual stars, their masses and ages, and the associated dust and line emission. A key result is that the embedded-to-exposed transition for individual stars is rapid, about $1.3$ Myr after a star reaches its maximum mass, with massive stars dominating both the accretion phase and the luminosity that drives de-embedding; maximum mass and emergence are concurrent for these stars, and their local feedback clears surrounding gas. Emission tracers peak near $2$ Myr while gas remains bound to the massive stars, and the timing scales with the cloud's free-fall time, linking observable lifetimes to localized physics and aligning with observed embedded lifetimes.

Abstract

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.

Figures (1)

• Figure 1: The simulation time where a sink particle begins to accrete and how long it takes for that particle to reach its maximum mass, which we take to be the zero-point of a star's age. The top panels shows the distribution of stellar formation times, split into two bins, one for stars below 1 M$_\odot$, and one for stars larger than 1 M$_\odot$. The middle panel then shows the average time it takes for the stars in these two bins to achieve their maximum mass. In both bins, but specifically for the larger stars, the accretion time depends on simulation time, which encodes the gas density. The bottom panel then shows each individual star, colored and sized by its final mass, where the y-axis shows the simulation time where each star reaches its maximum mass. The accretion time is then vertical distance above the dashed one-to-one line.