The TW Hya Rosetta Stone Project II: Spatially resolved emission of formaldehyde hints at low-temperature gas-phase formation

Abstract

Formaldehyde (H${}_2$CO) is an important precursor to organics like methanol (CH${}_3$OH). It is important to understand the conditions that produce H${}_2$CO and prebiotic molecules during star and planet formation. H${}_2$CO possesses both gas-phase and solid-state formation pathways, involving either UV-produced radical precursors or CO ice and cold ($\lesssim 20$ K) dust grains. To understand which pathway dominates, gaseous H${}_2$CO's ortho-to-para ratio (OPR) has been used as a probe, with a value of 3 indicating "warm" conditions and $<3$ linked to cold formation in the solid-state. We present spatially resolved ALMA observations of multiple ortho- and para-H${}_2$CO transitions in the TW Hya protoplanetary disk to test H${}_2$CO formation theories during planet formation. We find disk-averaged rotational temperatures and column densities of $33\pm 2$ K, ($1.1\pm 0.1)\times 10^{12}$ cm${}^{-2}$ and $25\pm 2$ K, $(4.4\pm 0.3)\times 10^{11}$ cm${}^{-2}$ for ortho- and para-H${}_2$CO, respectively, and an OPR of $2.49\pm 0.23$. A radially resolved analysis shows that the observed H${}_2$CO emits mostly at rotational temperatures of 30-40 K, corresponding to a layer with $z/R\ge 0.25$. The OPR is consistent with 3 within 60 au, the extent of the pebble disk, and decreases beyond 60 au to $2.0\pm 0.5$. The latter corresponds to a spin temperature of 12 K, well below the rotational temperature. The combination of relatively uniform emitting conditions, a radial gradient in the OPR, and recent laboratory experiments and theory on OPR ratios after sublimation, lead us to speculate that gas-phase formation is responsible for the observed H${}_2$CO across the TW Hya disk.

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Cited by in corpus (18)

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• Carbon Atom Reactivity with Amorphous Solid Water: H${}_2$O Catalyzed Formation of H${}_2$CO
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• Gas temperature structure across transition disk cavities
• Tracing snowlines and C/O ratio in a planet-hosting disk: ALMA molecular line observations towards the HD169142 disk
• Early Planet Formation in Embedded Disks (eDisk) III: A first high-resolution view of sub-mm continuum and molecular line emission toward the Class 0 protostar L1527 IRS
• An improved rovibrational linelist of formaldehyde, \spec{h212c16o}
• Exploring the link between star and planet formation with Ariel
• A 2-3 mm high-resolution molecular line survey towards the centre of the nearby spiral galaxy NGC 6946
• Investigating the asymmetric chemistry in the disk around the young star HD 142527
• ALMA high-resolution observations unveil planet formation shaping molecular emission in the PDS 70 disk
• ALMA reveals thermal and non-thermal desorption of methanol ice in the HD 100546 protoplanetary disk
• Radial and vertical constraints on the icy origin of H${}_2$CO in the HD 163296 Protoplanetary Disk
• An SMA Survey of Chemistry in Disks around Herbig AeBe Stars
• Cosmic-ray induced sputtering of interstellar formaldehyde ices
• Protoplanetary Disk Chemistry
• First Detection of HC5N in a Class II Disk around TW Hya
• Ortho-Para Chemistry of H2CO in the Protoplanetary Disk TW Hya