Browsing by Author "Hogerheijde, Michiel R."

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    Radial and Vertical Constraints on the Icy Origin of H2CO in the HD 163296 Protoplanetary Disk
    (2024) Hernandez-Vera, Claudio; Guzman, Viviana V.; Artur de la Villarmois, Elizabeth; OEberg, Karin I.; Cleeves, L. Ilsedore; Hogerheijde, Michiel R.; Qi, Chunhua; Carpenter, John; Fayolle, Edith C.
    H2CO is a small organic molecule widely detected in protoplanetary disks. As a precursor to grain-surface formation of CH3OH, H2CO is considered an important precursor of O-bearing organic molecules that are locked in ices. Still, since gas-phase reactions can also form H2CO, there remains an open question on the channels by which organics form in disks, and how much the grain versus the gas pathways impact the overall organic reservoir. We present spectrally and spatially resolved Atacama Large Millimeter/submillimeter Array observations of several ortho- and para-H2CO transitions toward the bright protoplanetary disk around the Herbig Ae star HD 163296. We derive column density, excitation temperature, and ortho-to-para ratio (OPR) radial profiles for H2CO, as well as disk-averaged values of N-T similar to 4 x 10(12) cm(-2), T-ex similar to 20 K, and OPR similar to 2.7, respectively. We empirically determine the vertical structure of the emission, finding vertical heights of z/r similar to 0.1. From the profiles, we find a relatively constant OPR similar to 2.7 with radius, but still consistent with 3.0 among the uncertainties, a secondary increase of N-T in the outer disk, and low T-ex values that decrease with disk radius. Our resulting radial, vertical, and OPR constraints suggest an increased UV penetration beyond the dust millimeter edge, consistent with an icy origin but also with cold gas-phase chemistry. This Herbig disk contrasts previous results for the T Tauri disk, TW Hya, which had a larger contribution from cold gas-phase chemistry. More observations of other sources are needed to disentangle the dominant formation pathway of H2CO in protoplanetary disks.
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    The TW Hya Rosetta Stone Project. II. Spatially Resolved Emission of Formaldehyde Hints at Low-temperature Gas-phase Formation
    (2021) Terwisscha van Scheltinga, Jeroen; Hogerheijde, Michiel R.; Cleeves, L. Ilsedore; Loomis, Ryan A.; Walsh, Catherine; Oberg, Karin I.; Bergin, Edwin A.; Bergner, Jennifer B.; Blake, Geoffrey A.; Calahan, Jenny K.; Cazzoletti, Paolo; van Dishoeck, Ewine F.; Guzman, Viviana V.; Huang, Jane; Kama, Mihkel; Qi, Chunhua; Teague, Richard; Wilner, David J.
    Formaldehyde (H2CO) is an important precursor to organics like methanol (CH3OH). It is important to understand the conditions that produce H2CO and prebiotic molecules during star and planet formation. H2CO possesses both gas-phase and solid-state formation pathways, involving either UV-produced radical precursors or CO ice and cold (less than or similar to 20 K) dust grains. To understand which pathway dominates, gaseous H2CO'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 Atacama Large Millimeter/submillimeter Array observations of multiple ortho- and para-H2CO transitions in the TW Hya protoplanetary disk to test H2CO formation theories during planet formation. We find disk-averaged rotational temperatures and column densities of 33 2 K, (1.1 +/- 0.1) x 10(12) cm(-2) and 25 +/- 2 K, (4.4 +/- 0.3) x 10(11) cm(-2) for ortho- and para-H2CO, respectively, and an OPR of 2.49 +/- 0.23. A radially resolved analysis shows that the observed H2CO 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 H2CO across the TW Hya disk.