Molecular Similarity and Water Diversity in Coeval Binary Disks: JWST/MIRI Observations of Sz 65 and Sz 66

Abstract

We present JWST/MIRI Medium Resolution Spectrometer spectra of the wide-separation (projected separation $=980$ au) binary protoplanetary disks Sz 65 (K7; $0.68M_{\odot }$) and Sz 66 (M3; $0.30M_{\odot }$), reduced using the uniform pipeline of the JWST Disk Infrared Spectral Chemistry Survey. Both disks show rich molecular emission, including H${}_2$O, CO${}_2$, HCN, C${}_2$H${}_2$, and OH. The scaled spectra of the two disks exhibit remarkably similar H${}_2$O, CO${}_2$, and HCN line emission in the 13--18 $\mu$m region, with the only notable difference being stronger C${}_2$H${}_2$ emission in the primary (Sz 65). Beyond 18 $\mu$m, the difference in H${}_2$O line emission between the two disks increases. Both the flux ratios and the slab-model-derived mass ratios of cold to hot H${}_2$O ($\sim$200 K to $\sim$750 K) and warm to hot H${}_2$O ($\sim$450 K to $\sim$750 K) are significantly higher in the secondary (Sz 66). Because binary stars share nearly the same age and metallicity, and as both disks appear compact in millimeter emission ($<30$ au), we suggest that the excess cold H${}_2$O in the secondary is best explained by its unstructured dust disk, in contrast to the primary, which shows gaps at 6 and 20 au. The enhanced cold water in the secondary is consistent with efficient pebble drift across the water snow line and increased H${}_2$O vapor from the sublimation of icy mantles. Our results demonstrate that wide-separation binaries can serve as powerful control samples for isolating the impact of individual disk properties on inner-disk chemistry and evolution.

Published in The Astrophysical Journal; 23 figures, 4 tables

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