Increased H$_2$CO production in the outer disk around HD 163296

M. T. Carney; M. R. Hogerheijde; R. A. Loomis; V. N. Salinas; K. I. Öberg; C. Qi; D. J. Wilner

doi:10.1051/0004-6361/201629342

paper

Increased H $_{2}$ CO production in the outer disk around HD 163296

arXiv:1705.10188 · doi:10.1051/0004-6361/201629342

Abstract

Three formaldehyde lines were observed (H $_{2}$ CO 3 $_{03}$ --2 $_{02}$ , H $_{2}$ CO 3 $_{22}$ --2 $_{21}$ , and H $_{2}$ CO 3 $_{21}$ --2 $_{20}$ ) in the protoplanetary disk around the Herbig Ae star HD 163296 with ALMA at 0.5 arcsecond (60 AU) spatial resolution. H $_{2}$ CO 3 $_{03}$ --2 $_{02}$ was readily detected via imaging, while the weaker H $_{2}$ CO 3 $_{22}$ --2 $_{21}$ and H $_{2}$ CO 3 $_{21}$ --2 $_{20}$ lines required matched filter analysis to detect. H $_{2}$ CO is present throughout most of the gaseous disk, extending out to 550 AU. An apparent 50 AU inner radius of the H $_{2}$ CO emission is likely caused by an optically thick dust continuum. The H $_{2}$ CO radial intensity profile shows a peak at 100 AU and a secondary bump at around 300 AU, suggesting increased production in the outer disk. Different parameterizations of the H $_{2}$ CO abundance were compared to the observed visibilities with $χ^{2}$ minimization, using either a characteristic temperature, a characteristic radius or a radial power law index to describe the H $_{2}$ CO chemistry. Similar models were applied to ALMA Science Verification data of C $^{18}$ O. In all modeling scenarios, fits to the H $_{2}$ CO data show an increased abundance in the outer disk. The overall best-fit H $_{2}$ CO model shows a factor of two enhancement beyond a radius of 270 $\pm$ 20 AU, with an inner abundance of $2 - 5 \times 1 0^{- 12}$ . The H $_{2}$ CO emitting region has a lower limit on the kinetic temperature of $T > 20$ K. The C $^{18}$ O modeling suggests an order of magnitude depletion in the outer disk and an abundance of $4 - 12 \times 1 0^{- 8}$ in the inner disk. The increase in H $_{2}$ CO outer disk emission could be a result of hydrogenation of CO ices on dust grains that are then sublimated via thermal desorption or UV photodesorption, or more efficient gas-phase production beyond about 300 AU if CO is photodisocciated in this region.

Increased H $_{2}$ CO production in the outer disk around HD 163296

Abstract

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Increased H2​CO production in the outer disk around HD 163296

Abstract

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Increased H $_{2}$ CO production in the outer disk around HD 163296