[Paper Review] 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

M. T. Carney, M. R. Hogerheijde+7 more|White Rose Research Online (University of Leeds, The University of Sheffield, University of York)|Jan 9, 2019

Astrophysics and Star Formation Studies74 references22 citations

TL;DR

This study uses ALMA observations to search for gas-phase methanol (CH₃OH) in the HD 163296 protoplanetary disk, finding no detection and establishing a 3σ upper limit of <5.0×10¹¹ cm⁻² for its disk-averaged column density. The resulting CH₃OH/H₂CO ratio is <0.24, significantly lower than the 1.27 ratio observed in the TW Hydrae disk, indicating that stellar irradiation differences—likely due to HD 163296’s Herbig Ae star—strongly influence disk chemical complexity by suppressing methanol abundance relative to formaldehyde.

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$σ$ disk-integrated intensity upper limits are $&lt; 51$ mJy km s$^{-1}$ for the band 6 lines and $&lt; 26$ mJy km s$^{-1}$ for the band 7 lines. The band 7 lines provide the strictest 3$σ$ upper limit on disk-averaged column density with $N_{\mathrm{avg}} &lt; 5.0 imes 10^{11}$ cm$^{-2}$. The methanol-to-formaldehyde ratio is CH$_3$OH/H$_2$CO $&lt; 0.24$ 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.

Motivation & Objective

• To constrain the gas-phase methanol abundance in the HD 163296 protoplanetary disk, a key target for astrochemical studies.
• To investigate the chemical environment of the disk midplane, particularly the role of methanol in prebiotic chemistry pathways.
• To compare methanol and formaldehyde abundances in HD 163296 with those in the TW Hydrae disk to probe stellar-type-dependent chemistry.
• To assess the impact of stellar irradiation on the desorption and formation of key organic molecules in protoplanetary disks.

Proposed method

• ALMA band 6 and band 7 observations were conducted to target four CH₃OH emission lines at 241.7 GHz and 304.2 GHz, respectively.
• Keplerian masking was applied in the image plane to enhance signal-to-noise for disk-averaged emission.
• Matched filter analysis was performed in the uv-plane to improve sensitivity to weak, extended emission.
• Line stacking was used to combine multiple transitions, increasing detection power for low-signal features.
• Disk-averaged column densities were estimated under assumptions of optically thin emission and local thermodynamic equilibrium (LTE).
• The methanol-to-formaldehyde ratio was derived by comparing upper limits on CH₃OH with measured H₂CO column densities from prior observations.

Experimental results

Research questions

• RQ1 What is the gas-phase methanol abundance in the HD 163296 protoplanetary disk, and how does it compare to other known disks?
• RQ2 Why is methanol so difficult to detect in protoplanetary disks despite its expected formation on icy grain surfaces?
• RQ3 How does the CH₃OH/H₂CO abundance ratio in HD 163296 compare to that in the TW Hydrae disk, and what does this imply about disk chemistry?
• RQ4 To what extent does the spectral type of the host star (Herbig Ae vs. T Tauri) influence the gas-phase abundance of key organic molecules?
• RQ5 What physical or chemical processes could explain a lower-than-expected CH₃OH/H₂CO ratio in HD 163296?

Key findings

• No CH₃OH emission lines were detected in HD 163296, with 3σ upper limits of <51 mJy km s⁻¹ in band 6 and <26 mJy km s⁻¹ in band 7.
• The strictest upper limit on disk-averaged CH₃OH column density is <5.0×10¹¹ cm⁻², derived from the band 7 CH₃OH 2₁₁–2₀₂ (A) line.
• The CH₃OH/H₂CO ratio in HD 163296 is constrained to be <0.24, significantly lower than the 1.27 ratio measured in the TW Hydrae disk.
• The low CH₃OH/H₂CO ratio suggests that the higher stellar irradiation from HD 163296’s Herbig Ae star may alter the chemical pathways or desorption efficiencies of organic molecules.
• Possible causes include enhanced gas-phase H₂CO formation, inefficient grain-surface hydrogenation of H₂CO to CH₃OH, or differences in desorption processes for CH₃OH and H₂CO.
• These results provide observational evidence that the chemical complexity of protoplanetary disks is strongly influenced by the host star’s spectral type and irradiation environment.