Hydrocarbon chemistry in the inner regions of planet-forming disks

Abstract

Context. The analysis of the mid-infrared spectra helps understanding the composition of the gas in the inner, dense and warm terrestrial planet forming region of disks around young stars. ALMA has detected hydrocarbons in the outer regions of the planet forming disk and Spitzer detected C2H2 in the inner regions. JWST-MIRI provides high spectral resolution observations of C2H2 and a suite of more complex hydrocarbons are now reported. Interpreting the fluxes observed in the spectra is challenging and radiation thermo-chemical codes are needed to properly take into account the disk structure, radiative transfer, chemistry and thermal balance. Various disk physical parameters like the gas-to-dust ratio, dust evolution including radial drift, dust growth and settling can affect the fluxes observed in the mid-IR. Still, thermo-chemical disk models were not always successful in matching all observed molecular emission bands simultaneously. Aims. The goal of this project is two-fold. Firstly, we analyse the warm carbon chemistry in the inner regions of the disk, namely within 10 au, to find pathways forming C2H2 potentially missing from the existing chemical networks. Secondly, we analyse the effect of the new chemistry on the line fluxes of acetylene. Methods. We used the radiative thermo-chemical disk code called PRODIMO to expand the hydrocarbon chemistry that occurs in a typical standard T Tauri disks. We used the UMIST and the KIDA rate databases for collecting reactions for the species. We included a number of three-body and thermal decomposition reactions from the STAND2020 network. We also included isotopomers for the species that were present in the databases. The chemistry was then analysed in the regions that produce observable features in the mid-infrared spectra. We studied the effect of expanding the hydrocarbon chemistry on the mid-infrared spectra. Results. Acetylene is formed via two pathways in the surface layers of disks: neutral-neutral and ion-neutral. They proceed via the hydrogenation of C or C+, respectively. Thus, the abundances of C, C+, H and H2 affect the formation of C2H2. Therefore, also the formation of H2 indirectly affects the abundance of acetylene. Chemisorbed H is more efficient in forming H2 compared to physisorbed H at warm temperatures and hence increases the abundance of C2H2. Conclusions. We provide a new extended warm chemical network that considers up to eight carbon atom long species, while also taking into account different isotopomers and can form the building blocks of PAHs: C6H6. For a standard T Tauri disk with a canonical value of gas-to-dust mass, the line fluxes increase only by a factor of less than 2. JWST is now detecting hydrocarbons such as methane, acetylene, and C4H2 in disks with a high C/O ratio. Hence, this new extended warm hydrocarbon network will aid in interpreting the observed mid-infrared fluxes.