Periodic Reporting for period 1 - DipolarSound (Internal structure of red-giant stars through the sound of dipole oscillation modes)
Período documentado: 2021-10-01 hasta 2023-03-31
In this project we address the issue that if we 'listen' to red-giant stars (stars that are further evolved than the Sun) they can sound differently, while they look similar at the suface. This indicates that the internal structures of these stars are different. The origin of these differences is not known. Therefore the aim of the DipolarSound proposal is to unravel the physical conditions and physical processes at play in red-giant stars using mixed dipole oscillation modes and to understand the underlying physical origin of the different oscillation spectra observed in red-giant stars.
This project has importance for society in acquiring fundamental knowledge, to investigate the future fate of our Sun (and potentially of Earth), understand the building blocks of the Milky Way and perform research in conditions that cannot be reproduced on Earth.
This project has importance for society in acquiring fundamental knowledge, to investigate the future fate of our Sun (and potentially of Earth), understand the building blocks of the Milky Way and perform research in conditions that cannot be reproduced on Earth.
Since the start of the project, I have build a team of the proposed 4 PhD students and 2 postdocs. One PhD student (Quentin Coppee) started at the beginning of the project (October 2021) and the first postdoc (Dr. Anthony Noll) started in February 2022. Towards the end of last year three more PhD students (Francisca Espinoza, Beatriz Bordadagua, Jeong Yun Choi) and a second postdoc (Dr. Felix Ahlborn) joined the group. The following projects have started:
suppressed dipole mode stars: Quentin is analysing stars with suppressed dipole modes. The first step he is taking is to see if the radial modes show differences compared to stars that don't have suppressed dipole modes. Preliminary results show that there is potentially a significant difference in the linewidth of the modes. As this is a small effect, we are very careful in creating different samples of stars to make sure that we compare like for like and can come to a solid result. This work is ongoing. At the same time, we have a student (Jonas Müller, paid by HITS and collaborating on the project), who is performing stellar models with different configurations of magnetic fields in the core. Fossiel fields in the core, are proposed as one of the origins of the suppressed dipole mode stars. Jonas already found that some configurations of the magnetic field would not impact the radial modes while others would.
messy stars: with a MSc student (Teresa Braun) we have tried to identify and characterise messy stars. Together with a long term collaborator (Prof. Elsworth) we found a subclass of messy stars where the period spacing is much smaller than expected from canonical evolution. We also tried to model these stars, where we assumed a mass-loss event just before the onset of the helium core burning in these stars. A paper announcing this set of stars and a second paper containing the modelling have been submitted for publication (Elsworth, Braun & Hekker, MNRAS submitted & Hekker, Elsworth, Braun, Basu, MNRAS submitted). Jeong Yun Choi is now continuing this project for the messy stars that do not belong to the sample described in the publications.
clean stars: we are making a lot of progress on the TACO code. We have had a computer scientist involved last summer and we are now currently testing the code. Francisca Espinoza is heavily working with TACO to prepare a catalogue of frequencies for the about 20 000 red giant stars that have been observed with Kepler. We want to publish this together with the code.
On rotation we have currently two people. Dr. Felix Ahlborn is working on a project to investigate how accurate the best fit model needs to be to perform rotational inversions and obtain reliable results. This project shows that models with different parameters will be able to provide solid results on rotation inversions. Last year, we already published a paper (Ahlborn et al. 2022) on a new objective function for inversions, such that it is now also possible to obtain envelope rotation rates. Up to this work, it was only possible to obtain an upper limit. In terms of angular momentum (AM) transport, Beatriz Bordadagua is implementing the AM transport via mixed modes in models. This scenario was presented first by Belkacem et al. in 2015, but never implemented. Beatriz is now doing that. The plan is that she will also implement other AM transport mechanisms and investigate which mechanism is dominating at what stage of evolution, and if these mechanisms can transport enough AM to explain the observations.
Inversions of stellar structures are currently on the way by Lynn Buchele (another PhD student paid by HITS, contributing to the ERC). They are currently finalising inversions for low-mass main-sequence stars and moving on to more massive main-sequence stars and more evolved stars. Finally, Dr. Anthony Noll is working on a project to investigate the impact of reaction rates and different types of convective mixing and overshoot to investigate what physics is needed to match the asymptotic period spacings as observed for red-clump stars with the results from models. Higher reaction rates in the models would indeed increase the period spacing, although not by a sufficient amount to match the observations. Combined with semi-convection the match is improving.
suppressed dipole mode stars: Quentin is analysing stars with suppressed dipole modes. The first step he is taking is to see if the radial modes show differences compared to stars that don't have suppressed dipole modes. Preliminary results show that there is potentially a significant difference in the linewidth of the modes. As this is a small effect, we are very careful in creating different samples of stars to make sure that we compare like for like and can come to a solid result. This work is ongoing. At the same time, we have a student (Jonas Müller, paid by HITS and collaborating on the project), who is performing stellar models with different configurations of magnetic fields in the core. Fossiel fields in the core, are proposed as one of the origins of the suppressed dipole mode stars. Jonas already found that some configurations of the magnetic field would not impact the radial modes while others would.
messy stars: with a MSc student (Teresa Braun) we have tried to identify and characterise messy stars. Together with a long term collaborator (Prof. Elsworth) we found a subclass of messy stars where the period spacing is much smaller than expected from canonical evolution. We also tried to model these stars, where we assumed a mass-loss event just before the onset of the helium core burning in these stars. A paper announcing this set of stars and a second paper containing the modelling have been submitted for publication (Elsworth, Braun & Hekker, MNRAS submitted & Hekker, Elsworth, Braun, Basu, MNRAS submitted). Jeong Yun Choi is now continuing this project for the messy stars that do not belong to the sample described in the publications.
clean stars: we are making a lot of progress on the TACO code. We have had a computer scientist involved last summer and we are now currently testing the code. Francisca Espinoza is heavily working with TACO to prepare a catalogue of frequencies for the about 20 000 red giant stars that have been observed with Kepler. We want to publish this together with the code.
On rotation we have currently two people. Dr. Felix Ahlborn is working on a project to investigate how accurate the best fit model needs to be to perform rotational inversions and obtain reliable results. This project shows that models with different parameters will be able to provide solid results on rotation inversions. Last year, we already published a paper (Ahlborn et al. 2022) on a new objective function for inversions, such that it is now also possible to obtain envelope rotation rates. Up to this work, it was only possible to obtain an upper limit. In terms of angular momentum (AM) transport, Beatriz Bordadagua is implementing the AM transport via mixed modes in models. This scenario was presented first by Belkacem et al. in 2015, but never implemented. Beatriz is now doing that. The plan is that she will also implement other AM transport mechanisms and investigate which mechanism is dominating at what stage of evolution, and if these mechanisms can transport enough AM to explain the observations.
Inversions of stellar structures are currently on the way by Lynn Buchele (another PhD student paid by HITS, contributing to the ERC). They are currently finalising inversions for low-mass main-sequence stars and moving on to more massive main-sequence stars and more evolved stars. Finally, Dr. Anthony Noll is working on a project to investigate the impact of reaction rates and different types of convective mixing and overshoot to investigate what physics is needed to match the asymptotic period spacings as observed for red-clump stars with the results from models. Higher reaction rates in the models would indeed increase the period spacing, although not by a sufficient amount to match the observations. Combined with semi-convection the match is improving.
We made progress beyond the state-of-the-art on several fronts:
- the new objective function for rotational inversions on the red-giant branch is the first to be able to measure envelope rotation rates, which is critical to constrain the angular momentum transport
- we are obtaining structure inversions for many more stars than ever before.
- we are incorporating radial mode characteristics of the suppressed dipole mode stars in the analysis, which has not been done so far. Together with the modeling of different magnetic fields in the core this will provide a new perspective of these stars.
- the discovery of the stars with smaller than expected period spacings and the modeling thereof
Before the end of the project we do expect:
- inversions of more massive main-sequence stars and more evolved stars to verify the physics in our models
- we expect TACO to be publicly available as well as TACO results, this will be an unprecedented source of data
- we expect to have constraints and scenarios on the mechanisms that cause the suppressed dipole mode stars and messy stars.
- we expect to obtain both envelope and core rotation rates for a significant set of red giant branch stars
- we expect to have more insights in the AM transport taking place in these stars
- we aim to have a procedure for detailed modeling of red giants.
The overall aim is to understand and improve the physics of stars and improve inputs of our stellar models.
- the new objective function for rotational inversions on the red-giant branch is the first to be able to measure envelope rotation rates, which is critical to constrain the angular momentum transport
- we are obtaining structure inversions for many more stars than ever before.
- we are incorporating radial mode characteristics of the suppressed dipole mode stars in the analysis, which has not been done so far. Together with the modeling of different magnetic fields in the core this will provide a new perspective of these stars.
- the discovery of the stars with smaller than expected period spacings and the modeling thereof
Before the end of the project we do expect:
- inversions of more massive main-sequence stars and more evolved stars to verify the physics in our models
- we expect TACO to be publicly available as well as TACO results, this will be an unprecedented source of data
- we expect to have constraints and scenarios on the mechanisms that cause the suppressed dipole mode stars and messy stars.
- we expect to obtain both envelope and core rotation rates for a significant set of red giant branch stars
- we expect to have more insights in the AM transport taking place in these stars
- we aim to have a procedure for detailed modeling of red giants.
The overall aim is to understand and improve the physics of stars and improve inputs of our stellar models.