A major asymmetric ice trap in a planet-forming disk

Abstract

Context. Most well-resolved disks observed with the Atacama Large Millimeter/submillimeter Array (ALMA) show signs of dust traps. These dust traps set the chemical composition of the planet-forming material in these disks, as the dust grains with their icy mantles are trapped at specific radii and could deplete the gas and dust at smaller radii of volatiles. Aims. In this work, we analyse the first detection of nitric oxide (NO) in a protoplanetary disk. We aim to constrain the nitrogen chemistry and the gas-phase C/O ratio in the highly asymmetric dust trap in the Oph-IRS 48 disk. Methods. We used ALMA observations of NO, CN, C2H, and related molecules in the Oph-IRS 48 disk. We modeled the effect of the increased dust-to-gas ratio in the dust trap on the physical and chemical structure using a dedicated nitrogen chemistry network in the thermochemical code DALI. Furthermore, we explored how ice sublimation contributes to the observed emission lines. Finally, we used the model to put constraints on the nitrogen-bearing ices. Results. Nitric oxide (NO) is only observed at the location of the dust trap, but CN and C2H are not detected in the Oph-IRS 48 disk. This results in an CN/NO column density ratio of <0.05 and thus a low C/O ratio at the location of the dust trap. Models show that the dust trap cools the disk midplane down to ~30 K, just above the NO sublimation temperature of ~25 K. The main gas-phase formation pathways to NO though OH and NH in the fiducial model predict NO emission that is an order of magnitude lower than what has been observed. The gaseous NO column density can be increased by factors ranging from 2.8 to 10 when the H2O and NH3 gas abundances are significantly boosted by ice sublimation. However, these models are inconsistent with the upper limits on the H2O and OH column densities derived from Herschel PACS observations and the upper limit on CN derived from ALMA observations. As the models require an additional source of NO to explain its detection, the NO seen in the observations is likely the photodissociation product of a larger molecule sublimating from the ices. The non-detection of CN provides a tighter constraint on the disk C/O ratio than the C2H upper limit. Conclusions. We propose that the NO emission in the Oph-IRS 48 disk is closely related to the nitrogen-bearing ices sublimating in the dust trap. The non-detection of CN constrains the C/O ratio both inside and outside the dust trap to be <1 if all nitrogen initially starts as N2 and ≤ 0.6, consistent with the Solar value, if (at least part of) the nitrogen initially starts as N or NH3.

An ALMA Molecular Inventory of Warm Herbig Ae Disks. I. Molecular Rings, Asymmetries, and Complexity in the HD 100546 Disk

“…To search for NO we compared the results of a few different image filters and found H 2 CO to have the highest response and detect three NO lines (two of which are blended) at 5 and 4σ, respectively. There are additional NO transitions covered in these data at 350.689 GHz but they are blended with a CH 3 OH line (as was also seen for the IRS 48 disk by Brunken et al 2022;Leemker et al 2023). 5.…”

“…In HD 100546, NO is the nitrogen-bearing molecule with the highest column density we have detected. NO is not typically targeted in disk observations, but the high NO/CN and overall low CN abundance in the HD 100546 disk may also be related to a lack of volatile depletion (Le Gal et al 2019a;Leemker et al 2023). Across the HD 100546 disk the N(CN)/N(NO) varies from 10 −1 to 1.…”

“…First, SO and CH 3 OH were detected in the HD 100546 disk along with a compact component of H 2 CO, all of which are thought to originate from the warm edge of the millimeter dust cavity (Booth et al 2018(Booth et al , 2021b(Booth et al , 2023a. Observations of IRS 48 then revealed the first detections of SO 2 , NO, and CH 3 OCH 3 in a protoplanetary disk (Booth et al 2021a;van der Marel et al 2021;Brunken et al 2022;Leemker et al 2023). In addition, the measured rotational temperatures of H 2 CO and CH 3 OH in IRS 48 of ≈100-200 K thus indicate the conditions for the sublimation of H 2 O and complex organic molecule (COM) rich-ices (van der Marel et al 2021).…”

An ALMA Molecular Inventory of Warm Herbig Ae Disks. II. Abundant Complex Organics and Volatile Sulphur in the IRS 48 Disk

“…Temmink). This may be due to the low stellar X-ray flux of IRS 48 and/or the presence of gasphase H 2 O (not yet detected in IRS 48 but only inferred; Leemker et al 2023), which effectively destroys HCO + . We do not detect CN in IRS 48, and similar to HD 100546 it has a low CN/HCN ratio when compared to other disks.…”

“…The IRS 48 disk has been well studied with ALMA and hosts the most asymmetric dust trap yet discovered at a distance of 60 au from the central star (van der Marel et al 2013Marel et al , 2021bYang et al 2023). Its gas mass of only 5.5 × 10 −4 M e is much lower than those of other Herbig Ae disks; yet it is very line rich, with detections of the CO isotopologues 12 CO, 13 CO, C 18 O, C 17 O along with SO, SO 2 , 34 SO 2 , NO, H 2 CO, CH 3 OH, CH 3 OCH 3 , and tentatively CH 3 OCHO (van der Marel et al 2013;Booth et al 2021a;van der Marel et al 2021a;Brunken et al 2022;Leemker et al 2023). The significance of the reported nondetections of CS, C 2 H, and CN in IRS 48 was quantified by Booth et al (2021a) and Leemker et al (2023) and indicates a C/O ratio in the disk gas that is significantly less than 1.…”