AB Aur, a Rosetta stone for studies of planet formation

Abstract

Context. AB Aur is a Herbig Ae star that hosts a prototypical transition disk. The disk shows a plethora of features connected with planet formation mechanisms, such as spiral arms, dust cavities, and dust traps. Understanding the physical and chemical characteristics of these features is crucial to advancing our knowledge of the planet formation processes. Aims. We aim to characterize the gaseous disk around the Herbig Ae star AB Aur. A complete spectroscopic study was performed using NOEMA to determine the physical and chemical conditions with high spatial resolution. Methods. We present new NOrthern Extended Millimeter Array (NOEMA) interferometric observations of the continuum and 12CO, 13CO, C18O, H2CO, and SO lines obtained at high resolution. We used the integrated intensity maps and stacked spectra to derive reliable estimates of the disk temperature. By combining our 13CO and C18O observations, we computed the gas-to-dust ratio along the disk. We also derived column density maps for the different species and used them to compute abundance maps. The results of our observations were compared with a set of Nautilus astrochemical models to obtain insight into the disk properties. Results. We detected continuum emission in a ring that extends from 0.6′′ to ~2.0′′, peaking at 0.97′′ and with a strong azimuthal asymmetry. The molecules observed show different spatial distributions, and the peaks of the distributions are not correlated with the binding energy. Using H2CO and SO lines, we derived a mean disk temperature of 39 K. We derived a gas-to-dust ratio that ranges from 10 to 40 along the disk. Abundance with respect to 13CO for SO (~2 × 10−4) is almost one order of magnitude greater than the value derived for H2CO (1.6 × 10−5). The comparison with Nautilus models favors a disk with a low gas-to-dust ratio (40) and prominent sulfur depletion. Conclusions. From a very complete spectroscopic study of the prototypical disk around AB Aur, we derived, for the first time, the gas temperature and the gas-to-dust ratio along the disk, providing information that is essential to constraining hydrodynamical simulations. Moreover, we explored the gas chemistry and, in particular, the sulfur depletion. The derived sulfur depletion is dependent on the assumed C/O ratio. Our data are better explained with C/O ~ 0.7 and S/H = 8 × 10−8.

A major asymmetric ice trap in a planet-forming disk

“…A particular disk to compare to IRS 48 is AB Aur as they both host azimuthal dust traps. SO has been detected in AB Aur and shows a ring-like morphology (Rivière-Marichalar et al 2020). SO emission over the full azimuth could be explained by AB Aur hosting a less azimuthally concentrated dust trap than in IRS 48 as the millimetre-sized grains are detected throughout the whole azimuth of the AB Aur disk (see Fig.…”

“…Disk-specific modelling efforts show that CS/SO ratios of order 100 can be reproduced via gas-phase chemistry where the C/O ratio is super-solar (>1; Dutrey et al 2011;Semenov et al 2018). Even in AB Aur, where SO is detected, an elevated C/O ratio of 1.0 is favoured in the chemical modelling (Rivière-Marichalar et al 2020). Recent observations of the transition disk PDS 70 also report a CS/SO > 100, which is in line with a C/O > 1 (Facchini et al 2021).…”

AB Aur, a Rosetta stone for studies of planet formation

“…The system is located at 162.9 pc from the Sun (Gaia Collaboration 2018), and it is perfectly suited to study the spatial distribution of gas and dust in circumstellar environments in detail. The disk extends out to a radius of 2.3 (∼373 au) in the continuum (Rivière-Marichalar et al 2020). Molecular emission from species such as CS, C 18 O, SO, H 2 CO, and H 2 S is observed with radii of emission peaks ranging from 0.9 to 1.4 .…”

A major asymmetric ice trap in a planet-forming disk: II. prominent SO and SO2 pointing to C/O < 1

“…A particular disk to compare to IRS 48 is AB Aur as they both host azimuthal dust traps. In AB Aur SO has been detected and shows a ring-like morphology (Rivière-Marichalar et al 2020). SO emission over the full azimuth could be explained by AB Aur hosting a less concentrated dust trap than in IRS 48 as the millimeter grains are detected throughout the whole azimuth of the AB Aur disk (see van der Marel et al 2021, (not Paper I) for a comparison of the mm-dust emission from both disks).…”

“…Disk specific modelling efforts show that CS/SO ratios of order 100 can be re-produced via gas-phase chemistry where the C/O ratio is super-solar (>1) (Dutrey et al 2011;Semenov et al 2018). Even in AB Aur, where SO is detected, an elevated C/O ratio of 1.0 is favoured in the chemical modelling (Rivière-Marichalar et al 2020). Recent observations of the transition disk PDS 70 also report a CS/SO >100 which is in line with a C/O>1 (Facchini et al 2021).…”