Periodic Reporting for period 1 - ICEDRAGON (Modelling of dust formation and chemistry in AGB outflows and disks)
Reporting period: 2021-09-01 to 2023-08-31
In their twilight years, solar-like stars in the asymptotic giant branch (AGB) phase enrich the interstellar medium (ISM), the space between the star,s with fresh material (gas and dust) for new stars and planets. AGB stars lose their outer layers to the ISM through a stellar outflow or wind, forming an extended circumstellar envelope (CSE). The wind is thought to be dust-driven, with dust grains forming close to the star. State-of-the-art observations have revealed the composition of the inner CSE, allowing the first identification of gas-phase seeds for dust grains, and the presence of disks around AGB stars. Despite major knowledge gains over the past three decades, it is still not fully understood how dust forms, grows, and drives the stellar wind, limiting our understanding of both stellar evolution and the chemical enrichment of the ISM. Moreover, the structure and chemistry of AGB disks is unknown; if similar to protoplanetary disks, second generation planet formation around old stars may be possible therein.
Solving these puzzles requires new and sophisticated models that connect dust formation with chemistry and couple gas and dust chemistry throughout the wind and in the disk. With this project, we have quantified the organic refractory coating of interstellar dust grains delivered by AGB outflows to the ISM. This was done by merging previous model developments into the most comprehensive chemical model of an AGB outflow so far: the model can account for the effects of a clumpy outflow and the presence of a companion star and includes an extensive dust-gas chemical network. We find that the organic refractory coating of dust is significantly affected by these factors, potentially leading to a surface coverage of several monolayers. This will affect the sticking/coagulation properties of this dust, as well as its efficiency in absorbing and reflecting stellar light. These theoretical results guide future observations to test their predictions.
We also developed the first chemical model of an AGB disk. To do so, we retrieved the density and temperature profile of the disk from archival observations. This new physical model was necessary as the results of a model available in the literature could not be reproduced. It also allowed for the fellow to be trained in working with interferometric observations and retrieval using three-dimensional radiative transfer models, expanding her skillset. In an upcoming publication, the unique chemistry of the AGB disk will be discussed and compared to that of PPDs. Both the physical and chemical model will be made publicly available.
At the University of Leeds, the fellow was able to continue her collaboration with theoretical and experimental chemists. This led to a currently ongoing project where dust clustering reactions are included in the chemical network, the initial step in developing the first chemical model that is applicable to the entire AGB outflow, from stellar surface to where the outflow merges with the ISM.
Solving these puzzles requires new and sophisticated models that connect dust formation with chemistry and couple gas and dust chemistry throughout the wind and in the disk. With this project, we have quantified the organic refractory coating of interstellar dust grains delivered by AGB outflows to the ISM. This was done by merging previous model developments into the most comprehensive chemical model of an AGB outflow so far: the model can account for the effects of a clumpy outflow and the presence of a companion star and includes an extensive dust-gas chemical network. We find that the organic refractory coating of dust is significantly affected by these factors, potentially leading to a surface coverage of several monolayers. This will affect the sticking/coagulation properties of this dust, as well as its efficiency in absorbing and reflecting stellar light. These theoretical results guide future observations to test their predictions.
We also developed the first chemical model of an AGB disk. To do so, we retrieved the density and temperature profile of the disk from archival observations. This new physical model was necessary as the results of a model available in the literature could not be reproduced. It also allowed for the fellow to be trained in working with interferometric observations and retrieval using three-dimensional radiative transfer models, expanding her skillset. In an upcoming publication, the unique chemistry of the AGB disk will be discussed and compared to that of PPDs. Both the physical and chemical model will be made publicly available.
At the University of Leeds, the fellow was able to continue her collaboration with theoretical and experimental chemists. This led to a currently ongoing project where dust clustering reactions are included in the chemical network, the initial step in developing the first chemical model that is applicable to the entire AGB outflow, from stellar surface to where the outflow merges with the ISM.
Spherically symmetric outflows are observed to be the exception rather than the rule. The classic gas-phase only, spherically symmetric chemical kinetics model is not appropriate for the majority of observed outflows. In the previous years, we have included several physical and chemical advancements step-by-step: a porous density distribution, dust–gas chemistry, and internal UV photons originating from a close-by stellar companion. Now, we combine these layers of complexity into the most chemically and physically advanced chemical kinetics model of AGB outflows to date. By varying over all model parameters, we obtain a holistic view of the outflow's composition and how it (inter)depends on the different complexities. This also impacts the organic refractory coating of interstellar dust grains delivered by AGB outflows.
The results were presented as a contributed paper at a Faraday Discussion. A stellar companion was found to have the largest influence, especially when combined with a porous outflow. We compile sets of gas-phase molecules that trace the importance of dust–gas chemistry and allow us to infer the presence of a companion and porosity of the outflow. This shows that our new chemical model can be used to infer physical and chemical properties of specific outflows.
A specific example of a spherically asymmetric outflow is the dusty disk around the AGB star L2 Puppis. The density and temperature structure of the disk were retrieved from ALMA observations in 2017. However, we were not able to reproduce its results. We therefore retrieved a new density and temperature structure from the same archival ALMA data. This was done by using well-documented parametrisations for protoplanetary disks and the new 3D radiative transfer code Magritte. While this led to a significant delay to the project, the fellow was taught how to work with ALMA data and how to use the novel radiative transfer code. Acquiring these skills went over and above the initial plan; they are great assets to her future academic career. The new physical model was then used as input to the host’s disk chemical model. The preliminary results show interesting chemical regions within the disk, including unexpected carbon-rich molecules in the midplane close to the star. Furthermore, the chemical composition of the disk appears to be a tool to determine the disk’s age, which has not been done so far. The results will be disseminated at upcoming astrochemistry and evolved stars conferences.
While the chemical model includes dust-gas chemistry, dust formation itself is not included. Thanks to the fellowship, the fellow was able to strengthen her collaboration with Prof Plane at the University of Leeds. A nucleation network will be included in the new, comprehensive chemical model in October 2023. During the fellowship, the fellow’s chemical models have contributed to the interpretation of observations in two papers, leading to second and third authorship.
The results were presented as a contributed paper at a Faraday Discussion. A stellar companion was found to have the largest influence, especially when combined with a porous outflow. We compile sets of gas-phase molecules that trace the importance of dust–gas chemistry and allow us to infer the presence of a companion and porosity of the outflow. This shows that our new chemical model can be used to infer physical and chemical properties of specific outflows.
A specific example of a spherically asymmetric outflow is the dusty disk around the AGB star L2 Puppis. The density and temperature structure of the disk were retrieved from ALMA observations in 2017. However, we were not able to reproduce its results. We therefore retrieved a new density and temperature structure from the same archival ALMA data. This was done by using well-documented parametrisations for protoplanetary disks and the new 3D radiative transfer code Magritte. While this led to a significant delay to the project, the fellow was taught how to work with ALMA data and how to use the novel radiative transfer code. Acquiring these skills went over and above the initial plan; they are great assets to her future academic career. The new physical model was then used as input to the host’s disk chemical model. The preliminary results show interesting chemical regions within the disk, including unexpected carbon-rich molecules in the midplane close to the star. Furthermore, the chemical composition of the disk appears to be a tool to determine the disk’s age, which has not been done so far. The results will be disseminated at upcoming astrochemistry and evolved stars conferences.
While the chemical model includes dust-gas chemistry, dust formation itself is not included. Thanks to the fellowship, the fellow was able to strengthen her collaboration with Prof Plane at the University of Leeds. A nucleation network will be included in the new, comprehensive chemical model in October 2023. During the fellowship, the fellow’s chemical models have contributed to the interpretation of observations in two papers, leading to second and third authorship.
The most comprehensive chemical model of an AGB outflow so far was developed during the fellowship. The model is valid from the dust-interaction zone onwards and includes the effects of a clumpy outflow and the presence of a companion star as well as dust-gas chemistry. The results of this model have an impact beyond the AGB phase: the refractory organic coverage of AGB dust grains has an impact on the evolution of dust in the ISM. In her new position as Oort Fellow at Leiden Observatory, the fellow will follow up on these results, applying for JWST time and collaborating with specialists in the astrochemistry of the ISM to test the model’s predictions.
The first chemical model of an AGB disk will showcase the impact deviations from spherical symmetry have on the composition of the outflow. This will be essential to both interpret current observations and acquire new observations. Furthermore, it paves the way for the investigation of different deviations such as spirals. These will impact not only our understanding of AGB outflows and their chemistry, but also feeds into the ISM.
The first chemical model of an AGB disk will showcase the impact deviations from spherical symmetry have on the composition of the outflow. This will be essential to both interpret current observations and acquire new observations. Furthermore, it paves the way for the investigation of different deviations such as spirals. These will impact not only our understanding of AGB outflows and their chemistry, but also feeds into the ISM.