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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