MINDS. Hydrocarbons detected by JWST/MIRI in the inner disk of Sz28 consistent with a high C/O gas-phase chemistry

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<SUB>⊙</SUB>). 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 P<SUB>RO</SUB>D<SUB>I</SUB>M<SUB>O</SUB> 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 CH<SUB>3</SUB>, CH<SUB>4</SUB>, C<SUB>2</SUB>H<SUB>2</SUB>, <SUP>13</SUP>CCH<SUB>2</SUB>, C<SUB>2</SUB>H<SUB>6</SUB>, C<SUB>3</SUB>H<SUB>4</SUB>, C<SUB>4</SUB>H<SUB>2</SUB> and C<SUB>6</SUB>H<SUB>6</SUB> which suggests a disk with a gaseous C/O > 1. Additionally, we detect CO<SUB>2</SUB>, <SUP>13</SUP>CO<SUB>2</SUB>, HCN, and HC<SUB>3</SUB>N. H<SUB>2</SUB>O and OH are absent from the spectrum. We do not detect polycyclic aromatic hydrocarbons. Photospheric stellar absorption lines of H<SUB>2</SUB>O 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<SUP>+</SUP>, H, and H<SUB>2</SUB> 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.