Radial and Vertical Constraints On The Icy Origin Of H2CO In The HD 163296 Protoplanetary Disk

By Keith Cowing
Status Report
April 10, 2024
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Radial and Vertical Constraints On The Icy Origin Of H2CO In The HD 163296 Protoplanetary Disk
Vertical emission profiles of the brightest H2CO lines detected toward HD 163296. Symbols are colored according to the fitted excitation temperature profile (see middle panel in Figure 6), and each marker represents a different transition, which are labeled on top. For comparison, in the background are shown the temperature structure predicted by empirical observations (Law et al. 2021b, left panel) and by thermo-chemical models (Zhang et al. 2021, right panel). Solid gray lines represent emission surfaces with constant z/r = 0.1, 0.3, and 0.5 values, whereas dashdotted gray lines delineate the CO snow surface traced by the CO freeze-out midplane temperature (∼25 K, Qi et al. 2015). — astro-ph.EP

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 (ALMA) 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 NT∼4×1012 cm−2, Tex∼20 K, and OPR∼2.7, respectively.

We empirically determine the vertical structure of the emission, finding vertical heights of z/r∼0.1. From the profiles, we find a relatively constant OPR∼2.7 with radius, but still consistent with 3.0 among the uncertainties, a secondary increase of NT in the outer disk, and low Tex 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.

Claudio Hernández-Vera, Viviana V. Guzmán, Elizabeth Artur de la Villarmois, Karin I. Öberg, L. Ilsedore Cleeves, Michiel R. Hogerheijde, Chunhua Qi, John Carpenter, Edith C. Fayolle

Comments: 21 pages, 4 tables, 10 figures; Accepted for publication in ApJ
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2404.06133 [astro-ph.EP] (or arXiv:2404.06133v1 [astro-ph.EP] for this version)
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Submission history
From: Claudio Hernández-Vera
[v1] Tue, 9 Apr 2024 08:57:27 UTC (9,009 KB)
Astrobiology, astrochemistry,

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻