High-resolution Mid-infrared Spectroscopy of SVS13-A with EXES/SOFIA: The Surprisingly High CH 3 OH/H 2 O Ratio in the Planet-forming Zone of a Solar Mass Protostar

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Abstract
Water and methanol are key components of interstellar ices and gas in star- and planet-forming regions, but direct observations of water in low-mass protostars are challenging due to atmospheric absorption. We present high-resolution ( R = 70,500) mid-infrared spectroscopy of the class I protostar SVS13-A with EXES on board SOFIA at 26 μ m, targeting both H 2 O and CH 3 OH absorption lines. Several lines of each species are detected, tracing warm gas with rotational temperatures of ∼140–170 K. Remarkably, the methanol column density is a factor of ∼4 higher than that of water, well above typical interstellar ice ratios (3 OH/H 2 O ratio may reflect selective sublimation due to the distribution of binding energies or ice stratification in the inner envelope. These observations probe the inner regions of the protostar, where planets are expected to form and inherit the chemical composition of their natal environment, providing a direct link between ice sublimation and gas-phase chemistry. Our results represent the first high-spectral-resolution mid-infrared view of both water and methanol toward a low-mass protostar, offering a unique window into the chemical composition of the innermost envelope and planet-forming region and highlighting the diagnostic power of high-resolution mid-infrared spectroscopy to uncover hidden chemical layers and the ice-to-gas transition in embedded protostars.

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