Periodic Reporting for period 1 - OCEANS (Origin and Composition of Exocomets Around Nearby Stars)
Reporting period: 2021-09-01 to 2023-08-31
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. This action's goal is to use Exocometary Science as a unique tool for probing the composition of planetary systems in the crucial, last period of terrestrial planet formation. In particular, the action aimed to expand current observational approaches, focused on observations of carbon monoxide (CO) gas at millimetre wavelengths, to ultraviolet (UV) and (IR) wavelengths, in order to access atomic and yet unseen molecular species, including water, to probe the entire chemical variety of exocomets. To probe these yet unseen crucial species, the action aimed to exploit new and upcoming ground and space observatories. Additionally, the action aimed to study the origin of exocomet compositions - so far consistent with Solar System comets - and their link to a potentially common belt formation location in young protoplanetary disks. This was to be achieved through the REASONS survey dataset, an observational population study determining the location of tens of belts of exocomets.By achieving these goals, the action aimed to prepare for compositional inventories of exocomets, allowing us to put our Solar System comets into the broader context of extrasolar planetary systems, exoplanets and young disks, and providing a missing link in the study of planet formation and physical-chemical evolution.
Due to a change of institution ultimately driven by family reasons, the action had to be terminated after only 4 of the 24 months originally foreseen. This implies that only a small part of the planned work could be carried out.
Due to a change of institution ultimately driven by family reasons, the action had to be terminated after only 4 of the 24 months originally foreseen. This implies that only a small part of the planned work could be carried out.
The research carried out over the 4 months focused mostly on two areas. The first was analysis, interpretation and write-up of the REASONS survey, a millimetre-wavelength imaging survey of ~65 belts of exocomets, the largest resolved population study to date. The multi-dimensional REASONS dataset (containing belt and host star properties) was explored to search for trends for selected parameters of interest, which were compared to and interpreted in the context of theoretical models. A particularly significant preliminary result is that exocometary belts lose mass over time, in a way that is location-dependent (smaller belts evolve faster) and consistent with simple collisional evolution models where solid mass decreases as the inverse of time. As small belts closer to their host stars evolve faster, they become undetectable at younger ages; this implies that our observations of older stars are crucially biased toward larger belts. Therefore, the observed distribution of exocometary belts is not simply the same as the distribution of belt formation locations; population synthesis modelling including the effects of collisional evolution and observational bias (as originally envisioned for WP2 of this action) will be needed to derive formation locations from the observed population.
The second was analysis and modelling of HST data on two exocometary belts viewed edge-on. Narrow circumstellar lines were identified using atomic and molecular line lists, and a Python package was developed to simultaneously fit multiple lines to derive physical properties of interest. After testing on published archival data the model was fit to carbon lines, with preliminary results revealing large amounts of atomic carbon, potentially sufficient to shield CO from photodissociation and solving the mysteriously high CO gas masses observed in these systems.
The second was analysis and modelling of HST data on two exocometary belts viewed edge-on. Narrow circumstellar lines were identified using atomic and molecular line lists, and a Python package was developed to simultaneously fit multiple lines to derive physical properties of interest. After testing on published archival data the model was fit to carbon lines, with preliminary results revealing large amounts of atomic carbon, potentially sufficient to shield CO from photodissociation and solving the mysteriously high CO gas masses observed in these systems.
The project has begun revealing the potential of analysing a large population of resolved planetesimal belts, as possible for the first time through the REASONS survey results. This enables accurate collisional evolution modelling, and future, detailed multi-dimensional modelling linking belt observed locations to their original formation location. This will uniquely shed light on the composition exocomets are formed with.
At the same time, the purpose-built model to fit circumstellar absorption lines from exocometary gas will enable continuous progress on determining temperatures and abundances of exocometary species beyond carbon, such as carbon monoxide, as well as other volatile and refractory species - eventually producing comprehensive exocometary compositional inventories for our two targets and for later observations of other planetary systems hosting exocometary gas.
At the same time, the purpose-built model to fit circumstellar absorption lines from exocometary gas will enable continuous progress on determining temperatures and abundances of exocometary species beyond carbon, such as carbon monoxide, as well as other volatile and refractory species - eventually producing comprehensive exocometary compositional inventories for our two targets and for later observations of other planetary systems hosting exocometary gas.