High-resolution mid-IR spectroscopy of SVS 13-A with EXES/SOFIA: The surprisingly high CH$_3$OH/H$_2$O ratio in the planet-forming zone of a solar mass protostar

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 $\sim$140--170 K. Remarkably, the methanol column density is a factor of $\sim$4 higher than that of water, well above typical interstellar ice ratios ($<$10\%). Comparison with previous millimeter observations indicates that absorption and emission probe distinct regions, with the mid-IR lines likely tracing cooler gas along the line of sight. The surprising observed CH$_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.

Figures (5)

• Figure 1: Observed H$_2$O and CH$_3$OH line profiles with the best fitting LTE slab models overplotted in red. The dashed blue line indicates the 1-$\sigma$ flux uncertainty. In the H$_2$O profiles, the nearby atmospheric water absorption causes the flux uncertainty to rise quickly toward lower velocities. The spectra has been binned by 3 pixels.
• Figure 2: Posterior distributions for the slab model fits to water(blue) and methanol(orange). The best fitting parameter values are listed for water, followed by methanol, above each column. The line width of water was fixed to the value determined for methanol to avoid physically unrealistic values. Hence, the posterior likelihood distribution for the FWHM of water is not shown.
• Figure 3: Artistic scheme of the VLA 4A/4B binary system, not in scale. Methanol and water absorption features originate from gas located in front of the dust photosphere at 26 $\mu$m, which is likely driven by VLA 4B (see text). The photosphere size and location is not certain, but the absorption selectively probes only a layer of sublimated ice, rather than the full bulk envelope, in a location where methanol may desorb more efficiently than water.
• Figure 4: (Left) Example of the best fit PSG transmission model and the cubic polynomial used to refine the baseline over the $5_{4,1}-4_{1,4}$ source water line. The data is from the first night's observation on UT 2022-02-24. The source absorption line is visible at v$_{LSR}$= +8.5 km s$^{-1}$. (Right) Example of the linear baseline fit (red) for the $\nu_{12}=2-0$$12_3-13_4$ methanol line. There is no significant atmospheric interference at this wavelength, as shown by the transmission model denoted by the dashed blue line.
• Figure 5: Rotation Diagram of the individually detected CH$_3$OH and H$_2$O lines toward SVS13-A.