Accretion bursts crystallize silicates in a planet-forming disk

Crystalline silicates form at high temperatures (>900 K) (refs. ^1,2). Their presence in comets^3-6 suggests that high-temperature dust processing occurred in the early Solar System and was subsequently transported outwards to comet-forming regions. However, direct evidence for this crystallization and redistribution in Sun-like protostars has remained unknown. By comparing James Webb Space Telescope mid-infrared spectra of the periodically bursting protostar EC 53 (ref. ⁷), we detect crystalline silicate (forsterite and enstatite) emission features that appear only during the burst. The emergence of these features indicates active crystal formation by thermal annealing in the hot inner disk during the accretion burst. We also detect a nested outflow-a collimated atomic jet enclosed by slower molecular outflows, consistent with magnetohydrodynamic wind models⁸. This configuration provides a mechanism for the outward transport of freshly crystallized silicates⁹. To our knowledge, our results provide the first direct observational evidence of in situ silicate crystallization during episodic accretion bursts in a very young star still embedded in its dense envelope. Although we do not directly detect grains transported to the outer disk, the observed trends are consistent with outward redistribution, indicating that both dust processing and transport occur during the earliest and most dynamic stages of star formation.