2026ApJ...998...68G

Sulfur dioxide (SO₂) ice has been tentatively detected in protostellar envelopes, but its reliability as a solid-state sulfur reservoir remains unclear. We present new measurements of SO₂ ice from 6.8-8.5 μm toward four Class 0 protostars observed with JWST's Mid-Infrared (MIRI) Medium Resolution Spectrometer, as part of the COMs ORigin Investigated by the Next-generation Observatory in Space (CORINOS) program. The sample spans a luminosity range from 1 L_⊙ (B335, IRAS 15398-3359) to 10 L_⊙ (L483, Ser-emb 7). To assess continuum placement uncertainty in absorption spectra, we apply randomized polynomial fits over the restricted region. We fit laboratory spectra from the Leiden Ice Database for Astrochemistry using the open-source Python library Omnifit. We detect the 7.7 μm CH₄ band in all sources and find its column density robust to baseline choice, providing a reference for evaluating the weaker SO₂ feature on its blue shoulder and quantifying baseline-related uncertainty. Three SO₂ laboratory ices were tested: pure SO₂ ice yields 0.3%-1.2% of volatile sulfur may be locked in SO₂ ice (lower and upper limits); CH₃OH:SO₂ ice gives 0.02%-0.18%, but with lower quality fitting. The best-fitting H₂O:SO₂ ice yields 0.2%-0.9%, which we consider the most realistic. These ranges define plausible bounds on SO₂ ice abundances in our sample. We find evidence for SO₂ in Ser-emb 7, L483, and IRAS 15398-3359, but emphasize the noisy spectrum of B335 prevents a definitive detection. Comparing SO₂ ice abundances across the different environments, we assess how conditions influence the role of SO₂ as a potential sulfur reservoir and implications for the longstanding “missing sulfur” problem.