Periodic Reporting for period 4 - ICYBOB (Initial Conditions of YMCs, Birth of OB associations and long term evolution of stellar clusters)
Reporting period: 2023-12-01 to 2024-11-30
Stars dominate the visible matter in the Universe. Most of the stars we see are older stars, which are grouped into loose associations or clusters, or indeed are individual stars like our Sun. However we do not yet have a good understanding of the transition from molecular clouds, the birth sites of stars, to OB associations (groups of stars containing massive O or B stars that likely disperse) or older clusters. We also cannot explain the existence of very dense young massive clusters (YMCs), the most massive of which may be the progenitors of modern day globular clusters. The goal of this proposal is to perform numerical simulations on different scales, and of different stages of a cluster’s life, from the formation of YMCs, the formation and evolution of OB associations, to the evolution of clusters and associations in galaxies. Over the past couple of decades, significant progress has been made on understanding the formation and evolution of molecular clouds in a galactic context. The next challenge is to follow cluster evolution. With Gaia set to transform stellar astronomy in our Galaxy, our work will provide a fundamental theoretical counterpart. Key questions we will address include i) how does gas disperse from new clusters and what happens to that gas, ii) how do YMCs form, iii) how do Giant Molecular Clouds (GMCs) evolve into OB associations, and iv) how long can clusters survive for as they orbit a galaxy and what causes their destruction. Our overall objectives are to establish the initial conditions for YMCs and OB associations, examine the role of magnetic fields, study the formation and evolution of YMCs and OB associations in different galactic environments including the dispersal of gas, and see how the evolution of clusters and OB associations would appear to an observer, and whether the simulated clusters resemble real clusters.
Understanding how stars form is one of the fundamental questions in astrophysics, and of great interest to society. Our work and movies are often used in public talks and talks to schools, encouraging the wider public to be interested in science, and for younger people to take up physics and STEM subjects. Our work also broadly ties in with questions such as how did our Sun form, was the Sun in a cluster or association, and if so, what was the impact of radiation from massive stars on the early evolution of the solar system.
Understanding how stars form is one of the fundamental questions in astrophysics, and of great interest to society. Our work and movies are often used in public talks and talks to schools, encouraging the wider public to be interested in science, and for younger people to take up physics and STEM subjects. Our work also broadly ties in with questions such as how did our Sun form, was the Sun in a cluster or association, and if so, what was the impact of radiation from massive stars on the early evolution of the solar system.
We have investigated cluster formation both in a galactic context and from colliding clouds. We have shown that the density and velocity flows of the gas (dependent on galactic conditions) determine the types of cluster which form. High densities and highly converging flows promote the formation of young massive clusters (Dobbs et al. 2020, Liow et al. 2020). In galaxy scale models, we find that often high density regions also correspond to converging flows, and we find such conditions where there are strong spiral arms. Both Rieder et al. 2022 and Dobbs et al. 2022a highlight that massive clusters in these simulations form by a combination of ongoing star formation, and mergers between smaller clusters.
We modelled the main feedback processes, photoionisation (Bending et al. 2020), supernovae (Bending et al., 2022., Dobbs et al. 2022b, Herrington et al. 2022) and winds (Ali et al. 2022). Photoionisation has the greatest impact on the gas and star formation, and can disperse gas from clusters on Myr timescales (Dobbs et al. 2022). In Dobbs et al. 2022b we show some of the first calculations to produce OB associations (unbound groups of stars). Photoionisation is essential to produce OB associations, and leads to ongoing star formation over several Myr. We produce one association with similar properties (mass, age, size) to Orion. Photoionisation is also required to reproduce the observed cluster radii and masses. We also considered how cluster formation depends on galactic environment by modelling cluster formation in galactic subregions containing a bar, spiral arms and inter-arm regions. We found massive clusters most likely to form in the bar, and inner spiral arms, and OB associations in inter arm and outer arm regions (Ali et al. 2023).
In Dobbs & Wurster 2021, we showed from simulations of colliding flows that the effect of the magnetic field is strongly dependent on its orientation. In later work applying magnetic fields in galactic subregions, where flows are not necessarily continuous, we found that strong magnetic fields can reduce star formation by about 40% (Herrington et al. 2024). We also reproduced the Crutcher relation (magnetic field strength versus density), though we found that for stronger field strengths, the field was too flat with density.
We also produced synthetic observations of the clusters we simulated, including investigating comparing 2D and 3D measurements (Buckner et al. 2022). Whilst we found similar qualitative conclusions with 2D versus 3D, quantitative conclusions including ascertaining cluster substructure, were much less accurate. We also placed our simulated clusters at different distances in the galaxy and calculated their properties using synthetic observations, again finding that identification of substructure within clusters difficult to measure, but also that accurate qualitative observations can be made even with a surprising level of incompleteness (Buckner et al. 2024).
We worked on a method for following individual star particles in larger scale simulations (i.e. at scales above where the stellar initial mass function can be resolved) (Rieder et al. 2022, Liow et al. 2022), in the AMUSE code (and Bending in sphNG). The calculations in Rietder et al. 2022 were able to model stars down to 1 solar mass in a galactic subregion.
We modelled the main feedback processes, photoionisation (Bending et al. 2020), supernovae (Bending et al., 2022., Dobbs et al. 2022b, Herrington et al. 2022) and winds (Ali et al. 2022). Photoionisation has the greatest impact on the gas and star formation, and can disperse gas from clusters on Myr timescales (Dobbs et al. 2022). In Dobbs et al. 2022b we show some of the first calculations to produce OB associations (unbound groups of stars). Photoionisation is essential to produce OB associations, and leads to ongoing star formation over several Myr. We produce one association with similar properties (mass, age, size) to Orion. Photoionisation is also required to reproduce the observed cluster radii and masses. We also considered how cluster formation depends on galactic environment by modelling cluster formation in galactic subregions containing a bar, spiral arms and inter-arm regions. We found massive clusters most likely to form in the bar, and inner spiral arms, and OB associations in inter arm and outer arm regions (Ali et al. 2023).
In Dobbs & Wurster 2021, we showed from simulations of colliding flows that the effect of the magnetic field is strongly dependent on its orientation. In later work applying magnetic fields in galactic subregions, where flows are not necessarily continuous, we found that strong magnetic fields can reduce star formation by about 40% (Herrington et al. 2024). We also reproduced the Crutcher relation (magnetic field strength versus density), though we found that for stronger field strengths, the field was too flat with density.
We also produced synthetic observations of the clusters we simulated, including investigating comparing 2D and 3D measurements (Buckner et al. 2022). Whilst we found similar qualitative conclusions with 2D versus 3D, quantitative conclusions including ascertaining cluster substructure, were much less accurate. We also placed our simulated clusters at different distances in the galaxy and calculated their properties using synthetic observations, again finding that identification of substructure within clusters difficult to measure, but also that accurate qualitative observations can be made even with a surprising level of incompleteness (Buckner et al. 2024).
We worked on a method for following individual star particles in larger scale simulations (i.e. at scales above where the stellar initial mass function can be resolved) (Rieder et al. 2022, Liow et al. 2022), in the AMUSE code (and Bending in sphNG). The calculations in Rietder et al. 2022 were able to model stars down to 1 solar mass in a galactic subregion.
We have followed cluster formation in a galactic context, rather than within individual molecular clouds, resolving the properties of the clusters over time and in different galactic regions (see Ali et al. 2023). We confirmed that massive young clusters are able to form rapidly in the high density, converging flow regions present in spiral arms. We confirmed, through studying supernovae and winds (Bending et al. 2022, Ali et al. 2022), that photoionisation is the main feedback process affecting cluster evolution. Ali et al. 2022 and Bending 2020 follow feedback processes of ionisation and winds on spiral arm scales using methods which are typically used for individual molecular cloud simulations rather than larger scale simulations. In Rieder et al. 2022 we follow cluster formation using individual star particles (i.e. every star is represented by an individual particle), a resolution usually confined to much smaller scale simulations.
In Dobbs et al. 2022b, we produced one of the first calculations to show the formation of an OB association in a galactic context. The OB association formed also displayed properties and morphology remarkably similar to the well known Orion OB association.
In Herrington et al. 2024 we showed simulations of galactic subregions including magnetic fields, again among the first of these types of calculations to include magnetic fields. We reproduced the Crutcher relation in molecular clouds, which is a fundamental observed relation for magnetic field strength versus density, but which has not been well studied in numerical simulations.
In Dobbs et al. 2022b, we produced one of the first calculations to show the formation of an OB association in a galactic context. The OB association formed also displayed properties and morphology remarkably similar to the well known Orion OB association.
In Herrington et al. 2024 we showed simulations of galactic subregions including magnetic fields, again among the first of these types of calculations to include magnetic fields. We reproduced the Crutcher relation in molecular clouds, which is a fundamental observed relation for magnetic field strength versus density, but which has not been well studied in numerical simulations.