Upper limits on CH${}_3$OH in the HD 163296 protoplanetary disk: evidence for a low gas-phase CH${}_3$OH/H${}_2$CO ratio

Abstract

Methanol (CH${}_3$OH) is at the root of organic ice chemistry in protoplanetary disks. However, its weak emission has made detections difficult. To date, gas-phase CH${}_3$OH has been detected in only one Class II disk, TW Hya. We use the Atacama Large Millimeter/submillimeter Array (ALMA) to search for a total of four CH${}_3$OH emission lines in bands 6 and 7 toward the disk around the young Herbig Ae star HD 163296. The disk-averaged column density of methanol and its related species formaldehyde (H${}_2$CO) are estimated assuming optically thin emission in local thermodynamic equilibrium. We compare these results to the gas-phase column densities of the TW Hya disk. No targeted methanol lines were detected individually nor after line stacking. The 3$\sigma$ disk-integrated intensity upper limits are $<51$ mJy km s${}^{-1}$ for the band 6 lines and $<26$ mJy km s${}^{-1}$ for the band 7 lines. The band 7 lines provide the strictest 3$\sigma$ upper limit on disk-averaged column density with $N_{\mathrm{avg}}<5.0\times 10^{11}$ cm${}^{-2}$. The methanol-to-formaldehyde ratio is CH${}_3$OH/H${}_2$CO $<0.24$ in the HD 163296 disk compared to a ratio of $1.27$ in the TW Hya disk. Differences in the stellar irradiation of Herbig disks compared to T Tauri disks likely influence the gaseous methanol and formaldehyde content. Possible reasons for the lower HD 163296 methanol-to-formaldehyde ratio include: a higher than expected gas-phase formation of H${}_2$CO in the HD 163296 disk, uncertainties in the grain surface formation efficiency of CH${}_3$OH and H${}_2$CO, and differences in the disk structure and/or CH${}_3$OH and H${}_2$CO desorption processes that release the molecules from ice mantles back into the gas phase. These results provide observational evidence that the gas-phase chemical complexity found in disks may be strongly influenced by the spectral type of the host star.

Accepted to Astronomy & Astrophysics, 11 pages, 3 figures

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