Organic molecules in the protoplanetary disk of DG Tau revealed by ALMA

Abstract

Planets form in protoplanetary disks and inherit their chemical compositions. It is thus crucial to map the distribution and investigate the formation of simple organics, such as formaldehyde and methanol, in protoplanetary disks. We analyze ALMA observations of the nearby disk-jet system around the T Tauri star DG Tau in the o-H${}_2$CO $3_{1,2}-2_{1,1}$ and CH${}_3$OH $3_{-2,2}-4_{-1,4}$ E, $5_{0,5}-4_{0,4}$ A transitions at an unprecedented resolution of $\sim 0.15"$, i.e., $\sim 18$ au at a distance of 121 pc. The H${}_2$CO emission originates from a rotating ring extending from $\sim 40$ au with a peak at $\sim 62$ au, i.e., at the edge of the 1.3mm dust continuum. CH${}_3$OH emission is not detected down to an r.m.s. of 3 mJy/beam in the 0.162 km/s channel. Assuming an ortho-to-para ratio of 1.8-2.8 the ring- and disk-height-averaged H${}_2$CO column density is $\sim 0.3-4\times 10^{14}$ cm${}^{-2}$, while that of CH${}_3$OH is $<0.04-0.7\times 10^{14}$ cm${}^{-2}$. In the inner $40$ au no o-H${}_2$CO emission is detected with an upper limit on its beam-averaged column density of $\sim 0.5-6\times 10^{13}$ cm${}^{-2}$. The H${}_2$CO ring in the disk of DG Tau is located beyond the CO iceline (R${}_{\mathrm{CO}}\sim 30$ au). This suggests that the H${}_2$CO abundance is enhanced in the outer disk due to formation on grain surfaces by the hydrogenation of CO ice. The emission peak at the edge of the mm dust continuum may be due to enhanced desorption of H${}_2$CO in the gas phase caused by increased UV penetration and/or temperature inversion. The CH${}_3$OH/H${}_2$CO abundance ratio is $<1$, in agreement with disk chemistry models. The inner edge of the H${}_2$CO ring coincides with the radius where the polarization of the dust continuum changes orientation, hinting at a tight link between the H${}_2$CO chemistry and the dust properties in the outer disk and at the possible presence of substructures in the dust distribution.

8 pages, 6 figures, accepted for publication on A&A Letters

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