Upper limits on CH3OH in the HD 163296 protoplanetary disk

Abstract

Context. Methanol (CH3OH) is at the root of organic ice chemistry in protoplanetary disks. Its connection to prebiotic chemistry and its role in the chemical environment of the disk midplane make it an important target for disk chemistry studies. However, its weak emission has made detections difficult. To date, gas-phase CH3OH is detected in only one Class II disk, TW Hya. Aims. We aim to constrain the methanol content of the HD 163296 protoplanetary disk. Methods. We used the Atacama Large Millimeter/submillimeter Array (ALMA) to search for a total of four CH3OH emission lines in bands six and seven toward the disk around the young Herbig Ae star HD 163296. The disk-averaged column density of methanol and its related species formaldehyde (H2CO) were estimated assuming optically thin emission in local thermodynamic equilibrium. We compared these results to the gas-phase column densities of the TW Hya disk. Results. No targeted methanol lines were detected with Keplerian masking in the image plane nor with matched filter analysis in the uv plane individually nor after line stacking. The 3σ disk-integrated intensity upper limits are <51 mJy km s−1 for the band six lines and <26 mJy km s−1 for the band seven lines. The band seven lines provide the strictest 3σ upper limit on disk-averaged column density with Navg < 5.0 × 1011 cm−2. The methanol-to-formaldehyde ratio is CH3OH∕H2CO<0.24 in the HD 163296 disk compared to a ratio of 1.27 in the TW Hya disk. Conclusions. The HD 163296 protoplanetary disk is less abundant in methanol with respect to formaldehyde compared to the disk around TW Hya. Differences in the stellar irradiation in this Herbig Ae disk as compared to that of a disk around a T Tauri star 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 H2CO in the HD 163296 disk, uncertainties in the grain surface formation efficiency of CH3OH and H2CO, and differences in the disk structure and/or CH3OH and H2CO desorption processes that drive the release of 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.