Hydrocarbon chemistry in inner regions of planet forming disk

Jayatee Kanwar

Mid-infrared spectra probe the composition of the gas in the inner, dense and warm regions of the disk where terrestrial planets form. ALMA has detected hydrocarbons like c-C3H2, C2H, etc in the outer regions of the planet-forming disk. With the high spectral resolution and sensitivity of JWST we are able to detect now many hydrocarbons besides the C2H2 detected by Spitzer.

We present here an extensive analysis of the warm carbon chemistry in the inner regions of the disk, i.e. within 10au to identify pathways forming C2H2 that are potentially missing from the existing chemical networks in literature. Our new extended warm chemical network considers 8 carbon atom long species, different isotopomers and can build the simplest form of a cyclic hydrocarbon, C6H6, a starting point for PAH formation. In this new network, acetylene is formed via two pathways in the disk surface layers: neutral-neutral and ion-neutral chemistry. The neutral-neutral pathway proceeds via the hydrogenation of C and the ion neutral pathway proceeds via addition of H to C+. Thus, the abundances of all species, C, C+, H and H2 affect the formation of C2H2. We explore the C/O ratio to study the impact on the chemistry. We also study the implications of the new chemical network on the mid-IR line fluxes of acetylene and compare our chemical modeling results to the multitude of hydrocarbons observed in GTO JWST spectra of disks around very low mass stars (MINDS, PI: Th. Henning). The goal is to understand if the chemistry in the disks proceeds by breaking down carbonaceous dust/PAHs or if they are formed from atoms and smaller molecules (bottom up).

[Poster PDF File]