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 (20 K) dust grains. To understand which pathway dominates, gaseous H 2 COʼs 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 Atacama Large Millimeter/submillimeter Array 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±2 K, (1.1±0.1)× 10 12 cm −2 and 25±2 K, (4.4±0.3)×10 11 cm −2 for ortho-and para-H 2 CO, respectively, and an OPR of 2.49±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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±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, led us to speculate thatgas-phase formation is responsible for the observed H 2 CO across the TWHya disk.

The chemical inventory of the planet-hosting disk PDS 70

“…Higher temperatures are found for the inner regions, however the snr of the data is low, as reflected by the large inferred uncertainties. Column densities across the 0.5 − 2 interval range between 4 × 10 12 − 4 × 10 13 cm −2 , in line with other protoplanetary disks (e.g., Pegues et al 2020;Terwisscha van Scheltinga et al 2021). A peak optical depth of ∼ 0.1 is derived for the 3 03 -2 20 transition, with the weaker transition showing a factor of ∼ 10 lower optical depth.…”

Gas temperature structure across transition disk cavities

“…The temperature of the (outer) disk midplane is derived from the location of the snowlines of major species such as H 2 O and CO (Mathews et al 2013;Qi et al 2013;van 't Hoff et al 2017van 't Hoff et al , 2018Qi et al 2019;Leemker et al 2021). Additionally rotational diagrams are used to derive the temperature of intermediate layers in disks (e.g., Schwarz et al 2016;Loomis et al 2018;Pegues et al 2020;Terwisscha van Scheltinga et al 2021). In this work, we study the rarely observed J = 6 − 5 transition of 13 CO (E u = 111.1 K) in two transition disks: LkCa15 and HD 169142 with spatially resolved ALMA data.…”

Exploring the link between star and planet formation with Ariel

“…The distribution of H 2 CO suggests that the bulk of the observed H 2 CO in the disc is formed via gas-phase reactions. This is indicated both by the o/p ratios measured in TW Hya (Terwisscha van Scheltinga et al, 2021) and by the vertical distribution of molecular emission in the edge-on disc of IRAS 04302 where H 2 CO mostly arises from an intermediate disc layer, the so called molecular layer (Podio et al, 2020b;van't Hoff et al, 2020). However, for IRAS 04302 H 2 CO emission is detected also in the outer disc midplane, where molecules are expected to be frozen onto the dust grain icy mantles.…”