Minds

Abstract

Context. The Mid-InfraRed Instrument (MIRI) Medium Resolution Spectrometer (MRS) on board the James Webb Space Telescope (JWST) allows us to probe the inner regions of protoplanetary disks, where the elevated temperatures result in an active chemistry and where the gas composition may dictate the composition of planets forming in this region. The disk around the classical T Tauri star Sz 98, which has an unusually large dust disk in the millimetre with a compact core, was observed with the MRS, and we examine its spectrum here. Aims. We aim to explain the observations and put the disk of Sz 98 in context with other disks, with a focus on the H2O emission through both its ro-vibrational and pure rotational emission. Furthermore, we compare our chemical findings with those obtained for the outer disk from Atacama Large Millimeter/submillimeter Array (ALMA) observations. Methods. In order to model the molecular features in the spectrum, the continuum was subtracted and local thermodynamic equilibrium (LTE) slab models were fitted. The spectrum was divided into different wavelength regions corresponding to H2O lines of different excitation conditions, and the slab model fits were performed individually per region. Results. We confidently detect CO, H2O, OH, CO2, and HCN in the emitting layers. Despite the plethora of H2O lines, the isotopo-logue H218O is not detected. Additionally, no other organics, including C2H2, are detected. This indicates that the C/O ratio could be substantially below unity, in contrast with the outer disk. The H2O emission traces a large radial disk surface region, as evidenced by the gradually changing excitation temperatures and emitting radii. Additionally, the OH and CO2 emission is relatively weak. It is likely that H2O is not significantly photodissociated, either due to self-shielding against the stellar irradiation, or UV shielding from small dust particles. While H2O is prominent and OH is relatively weak, the line fluxes in the inner disk of Sz 98 are not outliers compared to other disks. Conclusions. The relative emitting strength of the different identified molecular features points towards UV shielding of H2O in the inner disk of Sz 98, with a thin layer of OH on top. The majority of the organic molecules are either hidden below the dust continuum, or not present. In general, the inferred composition points to a sub-solar C/O ratio (<0.5) in the inner disk, in contrast with the larger than unity C/O ratio in the gas in the outer disk found with ALMA.

MINDS. JWST/MIRI Reveals a Dynamic Gas-rich Inner Disk inside the Cavity of SY Cha

“…Instead, OH is likely excited via collisions or chemical pumping. Similar results were found for the disk around Sz 98 by Gasman et al (2023).…”

“…After continuum subtraction, the MIRI-MRS spectrum of SY Cha is dominated by rovibrational and pure rotational H 2 O vapor emission, which is seen in all subbands (Figures 3 and 4). As with observations of other disks observed with MIRI-MRS, e.g., Kóspál et al (2023), Banzatti et al (2023a), Gasman et al (2023), the lines at longer wavelengths preferentially trace cooler gas (Table 3). In addition to H 2 O, we detect CO, C 2 H 2 , HCN, CO 2 , and OH.…”

“…SY Cha was observed as part of the as part of the Mid-InfraRed Instrument (MIRI) mid-INfrared Disk Survey (MINDS) GTO Program (PID: 1282, PI: T. Henning). Previous results from the MINDS program include the first detection of 13 CO 2 in a protoplanetary disk (Grant et al 2023), rich hydrocarbon chemistry in a disk around a very low-mass star (Tabone et al 2023), abundant water but a substellar C/O ratio in a full protoplanetary disk (Gasman et al 2023), and water in the inner disk of a transition disk known to host planets in its cavity (Perotti et al 2023). An overview of the full MINDS sample is given by Kamp et al (2023) and T. K. Henning et al (2024, in preparation).…”

XUE: Molecular Inventory in the Inner Region of an Extremely Irradiated Protoplanetary Disk

“…For the hotter H 2 O we can exclude temperatures <300 K, but the upper limit on the temperature is poorly constrained. For the cooler H 2 O component the temperature is better constrained to values between ∼450 and ∼700 K. It has been pointed out in the literature that the shorter wavelengths probe H 2 O at higher temperatures, while the longer wavelengths probe the colder regions (Blevins et al 2016;Banzatti et al 2023;Gasman et al 2023). For XUE 1, although the best-fitting values agree with this behavior, when taking into account the confidence intervals, the temperature cascade does not seem to be as extreme, similarly to the case in PDS 70 (Perotti et al 2023).…”

“…The resulting velocity can then be used to provide additional constraints on the emitting surface radius provided that we know the mass of the central star, allowing us to circumvent the degeneracy between the column density and the emitting surface radius. In order to characterize the remaining molecular emission features present in the spectrum of XUE 1, we model them with the LTE model presented in Tabone et al (2023), which has also been used in Grant et al (2023), Perotti et al (2023), and Gasman et al (2023. We included emission from H 2 O (around 7 and 15 μm), 12 CO 2 , 13 CO 2 , OH, C 2 H 2 , and HCN, with molecular data from the HITRAN database (Gordon et al 2022).…”