Characterising the molecular line emission in the asymmetric Oph-IRS 48 dust trap: Temperatures, timescales, and sub-thermal excitation

¹ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
² Center for Astrophysics – Harvard & Smithsonian, 60 Garden St., Cambridge, MA 02138, USA
³ Dipartimento di Fisica, Università degli Studi di Milano, Via Celoria 16, 20133 Milano, Italy
⁴ Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstraße 1, 85748 Garching, Germany
⁵ School of Physics and Astronomy, University of Leeds, Leeds LS2 9JT, UK
⁶ Astronomy Unit, School of Physics and Astronomy, Queen Mary University of London, London E1 4NS, UK
⁷ Department of Astronomy, University of Virginia, Charlottesville, VA 22904, USA
⁸ Department of Earth and Planetary Science, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
⁹ Star and Planet Formation Laboratory, RIKEN Cluster for Pioneering Research, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan

^★ Corresponding author; temmink@strw.leidenuniv.nl

Received: 9 September 2024
Accepted: 18 November 2024

Abstract

Context. The ongoing physical and chemical processes in planet-forming disks set the stage for planet (and comet) formation. The asymmetric disk around the young star Oph-IRS 48 has one of the most well-characterised chemical inventories, showing molecular emission from a wide variety of species at the dust trap: from simple molecules, such as CO, SO, SO₂, and H₂CO, to large complex organics, such as CH₂OH, CH₃OCHO, and CH₃OCH₃. One of the explanations for the asymmetric structure in the disk is dust trapping by a perturbation-induced vortex.

Aims. We aimed to constrain the excitation properties of the molecular species SO₂, CH₃OH, and H₂CO, for which we have used 13, 22, and 7 transitions of each species, respectively. We further characterised the extent of the molecular emission, which differs among molecules, through the determination of important physical and chemical timescales at the location of the dust trap. We also investigated whether the anticyclonic motion of the potential vortex influences the observable temperature structure of the gas.

Methods. Through a pixel-by-pixel rotational diagram analysis, we created maps of the rotational temperatures and column densities of SO₂ and CH₃OH. To determine the temperature structure of H₂CO, we have used line ratios of the various transitions in combination with non-local thermal equilibrium (LTE) RADEX calculations. The timescales for freeze-out, desorption, photodissociation, and turbulent mixing at the location of the dust trap were determined using an existing thermochemical model.

Results. Our rotational diagram analysis yields temperatures of T = 54.8±1.4 K (SO₂) and T = 125.5_−3.5^+3.7 K (CH₃OH) at the emission peak positions of the respective lines. As the SO₂ rotational diagram is well characterised and points towards thermalised emission, the emission must originate from a layer close to the midplane where the gas densities are high enough. The rotational diagram of CH₃OH is, in contrast, dominated by scatter and subsequent non-LTE RADEX calculations suggest that both CH₃OH and H₂CO must be sub-thermally excited higher up in the disk (z/r ~ 0.17–0.25). For H₂CO, the derived line ratios suggest temperatures in the range of T ~ 150-350 K. The SO₂ temperature map hints at a potential radial temperature gradient, whereas that of CH₃OH is nearly uniform and that of H₂CO peaks in the central regions. We, however, do not find any hints of the vortex influencing the temperature structure across the dust trap. The longer turbulent mixing timescale, compared to that of photodissociation, does provide an explanation for the expected vertical emitting heights of the observed molecules. On the other hand, the short photodissociation timescales are able to explain the wider azimuthal molecular extent of SO₂ compared to CH₃OH. The short timescales are, however, not able to explain the wider azimtuhal extent of the H₂CO emission. Instead, it can be explained by a secondary reservoir that is produced through the gas-phase formation routes, which are sustained by the photodissociation products of, for example, CH₃OH and H₂O.

Conclusions. Based on our derived temperatures, we expect SO₂ to originate from deep inside the disk, whereas CO comes from a higher layer and both CH₃OH and H₂CO emit from the highest emitting layer. The sub-thermal excitation of CH₃OH and H2CO suggests that our derived (rotational) temperatures underestimate the kinetic temperature. Given the non-thermal excitation of important species, such as H₂CO and CH₃OH, it is important to use non-LTE approaches when characterising low-mass disks, such as that of IRS 48. Furthermore, for the H₂CO emission to be optically thick, as expected from an earlier derived isotopic ratio, we suggest that the emission must originate from a small radial ‘sliver’ with a width of ~10 au, located at the inner edge of the dust trap.