Issue |
A&A
Volume 689, September 2024
|
|
---|---|---|
Article Number | A231 | |
Number of page(s) | 21 | |
Section | Planets and planetary systems | |
DOI | https://doi.org/10.1051/0004-6361/202450078 | |
Published online | 13 September 2024 |
MINDS. Hydrocarbons detected by JWST/MIRI in the inner disk of Sz28 consistent with a high C/O gas-phase chemistry
1
Kapteyn Astronomical Institute, University of Groningen,
PO Box 800,
9700
AV
Groningen,
The Netherlands
2
Space Research Institute, Austrian Academy of Sciences,
Schmiedlstr. 6,
8042
Graz,
Austria
3
TU Graz, Fakultät für Mathematik, Physik und Geodäsie,
Petersgasse 16
8010
Graz,
Austria
4
Department of Astrophysics/IMAPP, Radboud University,
PO Box 9010,
6500
GL
Nijmegen,
The Netherlands
5
SRON Netherlands Institute for Space Research,
Niels Bohrweg 4,
2333
CA
Leiden,
The Netherlands
6
Leiden Observatory, Leiden University,
2300
RA
Leiden,
The Netherlands
7
Max-Planck Institut für Extraterrestrische Physik (MPE),
Giessenbachstr. 1,
85748,
Garching,
Germany
8
Institute of Astronomy, KU Leuven,
Celestijnenlaan 200D,
3001
Leuven,
Belgium
9
Max-Planck-Institut für Astronomie (MPIA),
Königstuhl 17,
69117
Heidelberg,
Germany
10
Department of Astrophysics, University of Vienna,
Türkenschanzstr. 17,
1180
Vienna,
Austria
11
ETH Zürich, Institute for Particle Physics and Astrophysics,
Wolfgang-Pauli-Str. 27,
8093
Zürich,
Switzerland
12
STAR Institute, Université de Liège,
Allée du Six Août 19c,
4000
Liège,
Belgium
13
Centro de Astrobiología (CAB), CSIC-INTA, ESAC Campus,
Camino Bajo del Castillo s/n,
28692
Villanueva de la Cañada, Madrid,
Spain
14
INAF – Osservatorio Astronomico di Capodimonte,
Salita Moiariello 16,
80131
Napoli,
Italy
15
Dublin Institute for Advanced Studies,
31 Fitzwilliam Place,
Dublin
D02 XF86,
Ireland
16
Université Paris-Saclay, CNRS, Institut d’Astrophysique Spatiale,
91405
Orsay,
France
Received:
22
March
2023
Accepted:
18
July
2024
Context. With the advent of JWST, we are acquiring unprecedented insights into the physical and chemical structure of the inner regions of planet-forming disks where terrestrial planet formation occurs. Very low-mass stars (VLMSs) are known to have a high occurrence of the terrestrial planets orbiting them. Exploring the chemical composition of the gas in these inner disk regions can help us better understand the connection between planet-forming disks and planets.
Aims. The MIRI mid-Infrared Disk Survey (MINDS) project is a large JWST guaranteed time program whose aim is to characterise the chemistry and physical state of planet-forming and debris disks. We used the JWST-MIRI/MRS spectrum to investigate the gas and dust composition of the planet-forming disk around the VLMS Sz28 (M5.5, 0.12 M⊙).
Methods. We used the dust-fitting tool DuCK to determine the dust continuum and to place constraints on the dust composition and grain sizes. We used 0D slab models to identify and fit the molecular spectral features, which yielded estimates on the temperature, column density, and emitting area. To test our understanding of the chemistry in the disks around VLMSs, we employed the thermochemical disk model PRODIMO and investigated the reservoirs of the detected hydrocarbons. We explored how the C/O ratio affects the inner disk chemistry.
Results. JWST reveals a plethora of hydrocarbons, including CH3, CH4, C2H2, 13CCH2, C2H6, C3H4, C4H2 and C6H6 which suggests a disk with a gaseous C/O > 1. Additionally, we detect CO2, 13CO2, HCN, and HC3N. H2O and OH are absent from the spectrum. We do not detect polycyclic aromatic hydrocarbons. Photospheric stellar absorption lines of H2O and CO are identified. Notably, our radiation thermo-chemical disk models are able to produce these detected hydrocarbons in the surface layers of the disk when C/O > 1. The presence of C, C+, H, and H2 is crucial for the formation of hydrocarbons in the surface layers, and a C/O ratio larger than 1 ensures the surplus of C needed to drive this chemistry. Based on this, we predict a list of additional hydrocarbons that should also be detectable. Both amorphous and crystalline silicates (enstatite and forsterite) are present in the disk and we find grain sizes of 2 and 5 μm.
Conclusions. The disk around Sz28 is rich in hydrocarbons, and its inner regions have a high gaseous C/O ratio. In contrast, it is the first VLMS disk in the MINDS sample to show both distinctive dust features and a rich hydrocarbon chemistry. The presence of large grains indicates dust growth and evolution. Thermo-chemical disk models that employ an extended hydrocarbon chemical network together with C/O >1 are able to explain the hydrocarbon species detected in the spectrum.
Key words: astrochemistry / line: identification / protoplanetary disks / brown dwarfs / stars: low-mass / infrared: planetary systems
© The Authors 2024
Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
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