2026ApJ...999...56C

We present new two-dimensional radiation hydrodynamic simulations of supernova shock breakout from red supergiants (RSGs) using the CASTRO code. Our progenitors are 20 and 25 M_⊙ solar-metallicity stars evolved from the zero-age main sequence with MESA and exploded in one dimension using FLASH. We consider a range of circumstellar media (CSMs) produced by stellar winds to investigate how preexplosion mass loss affects shock breakout. The multigroup flux-limited diffusion scheme in CASTRO captures the interaction between the explosion shock, its radiation precursor, and the surrounding CSM. We find that strong radiation precursors, generated by radiation leakage behind the shock, can drive fluid instabilities and move the effective photosphere outward before the shock reaches the stellar surface. The resulting breakout emission reaches peak luminosities of ∼10⁴⁴ erg s^–1 with FWHM durations of 1-3 hr, fainter and longer than previous one-dimensional models. The light-curve colors gradually evolve from blue to red after the peak. The 25 M_⊙ model with explosion energy E ∼ 1.69 × 10⁵¹ erg produces an ∼10%-30% higher maximum luminosity than the 20 M_⊙ model with E ∼ 1.09 × 10⁵¹ erg. The dense CSM further extends the breakout rise time by increasing the photon diffusion. These results provide new constraints on RSG atmospheres and mass-loss histories prior to core collapse. Although our selected wavelength ranges fall outside the observational bands of missions such as the Einstein Probe and ULTRASAT, our study still provides valuable insights into the color evolution of shock breakout and can help guide the interpretation of future observations.