![Integrated intensity maps of the 1.7 mm continuum, 183 GHz water line, and three methanol lines in the HD 100546 (top row) and IRS 48 (bottom row) disks. White contours highlight the inner dust ring of HD 100546 and the asymmetric mm-crescent of IRS 48 from their continuum maps, pink contours show 3, 4, and 5σ levels. The white ellipse in the bottom left of each panel shows the beam.-- [astro-ph.EP]](https://i0.wp.com/astrobiology.com/wp-content/uploads/2026/10/An-ALMA.png?fit=1200%2C657&ssl=1)
[astro-ph.EP] Water and methanol are the fundamental constituents of interstellar and cometary ices: understanding their abundances in disks is therefore of high importance to reveal how this ice reservoir evolves across the timeline of star and planet formation.
We present ALMA band 5 line emission observations of water and methanol in the warm Herbig disks of HD 100546 and IRS 48. While the IRS 48 disk is methanol-rich, water vapor is not detected, implying a high methanol-to-water column density ratio >0.2, possibly reflecting a distinct ice composition.
Methanol and water are both detected in the HD 100546 disk, leading to a CH3OH/H2O ratio of ~0.02, consistent with the median of the cometary distribution and with the lower edge of the distribution of Young Stellar Objects. We suggest that the gas-phase methanol-to-water ratio in the HD 100546 disk directly reveals the ice reservoir subsequently sublimated in the warm disk stage.
Moreover, we report a methyl formate-to-methanol ratio in the IRS 48 disk of ~0.2, consistent with the upper edge of the distribution of Young Stellar Objects. The methyl formate-to-methanol ratio in the HD 100546 disk is ~2, much higher than in younger and older sources, possibly reflecting a chemical enhancement of gas-phase methyl formate.
We additionally constrain non-LTE conditions, optical depths, and emitting regions of the methanol lines in both disks. These results offer a new opportunity to probe the chemical trail of molecular complexity in the intermediate disk stage, bridging early and late phases of the star and planet formation history.
Luna Rampinelli, Stefano Facchini, Marta De Simone, Margot Leemker, Alice S. Booth, Myriam Benisty, Pietro Curone, Elizabeth Humphreys, Leonardo Testi
Comments: Accepted for publication in the Astrophysical Journal. 21 pages, 9 figures, 3 tables
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2610.22503 [astro-ph.EP] (or arXiv:2610.22503v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2610.22503
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Submission history
From: Luna Rampinelli
[v1] Thu, 15 Oct 2026 18:01:49 UTC (6,238 KB)
https://arxiv.org/abs/2610.22503
Astrobiology, Astrochemistry,
