Exocometary gas inventories at the epoch of volatile delivery
Abstract
The presence of exocometary gas in young (10-100 Myr) debris disks presents a unique opportunity to probe the composition of exocomets during the late stages of terrestrial planet formation. This is the evolutionary stage when ice-rich impacts are proposed to change the volatile environment of terrestrial planets, setting the stage for prebiotic chemistry. In these young exocometary belts, high concentrations of debris result in frequent collisions and release of molecular gas, which rapidly photodissociates to produce atomic gas. Atomic inventories can only be comprehensively probed at UV wavelengths with HST, in absorption against the stellar background, within systems that are viewed edge-on. We propose a UV chemical survey of gas released in two newly discovered, 15-16 Myr old exocometary belts around A stars HD110058 and HD131488. These are ideal targets as they are edge-on, rich in molecular gas based on CO emission detected by ALMA, and present circumstellar gas absorption at optical wavelengths. Exploiting the ALMA-HST synergy will allow us to put better constraints on the composition of these belts than ALMA observations alone can provide. We will use COS and STIS to measure abundance ratios such as C/O, C/N and C/Fe and further model the elemental composition of exocometary belts. These ratios will be compared to exoplanets, young disks and Solar System comets, providing a missing link in the study of planet formation and physical-chemical evolution. Finally, splitting our observations into several visits will enable us to look for star-grazing exocomets, providing evidence that inward scattering, and potentially volatile delivery, is ongoing in these systems.
- Publication:
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HST Proposal
- Pub Date:
- June 2019
- Bibcode:
- 2019hst..prop15916M