SCIENTIFIC RATIONALE
Little is known about stars in the mass range between those that end their lives as white dwarfs and those that die in spectacular supernova explosions. The uncertainty stems from the fact that stars in this transition mass range from about ~7 to 11 solar masses are both difficult to model theoretically and there are few observational clues as to their evolutionary history. Following the ignition of carbon in the core under conditions of partial degeneracy, the stars then continue to evolve through the asymptotic giant branch (AGB) phase and are known as super-AGB stars. Super-AGB stars have oxygen-neon degenerate cores as opposed to the carbon-oxygen cores of their lower mass counterparts. The final fate of single super-AGB stars depends on the rate of mass loss from the surface: if the core can grow big enough to reach the Chandrasekhar mass then it will explode as an electron capture supernova.
At least three different types of supernova are connected with the limits of this mass range: 1) their upper limit, Mup, is the lowest mass of single core collapse supernova progenitors (SN Type II); 2) their lower limit, Mco, represents the upper limit for the formation of C-O white dwarfs, whose subsequent evolution through mass transfer in a binary system may lead to thermonuclear supernova (SN Type Ia); and 3) single stars in the super-AGB upper mass range may be the site of electron- capture supernovae, while accretion onto their remnants in close binaries may result in accretion induced collapses. Over the past few years, theoretical models have made significant steps towards understanding the evolution and fate of stars in the transition mass range but crucial uncertainties remain. The problem starts on the main sequence, with numerical and physical choices about the treatment of convective overshoot and rotation determining the size of the C-O core at the beginning of the AGB, and hence the mass limits (Mup and Mco). The description of the AGB phase of evolution is itself uncertain, stemming from uncertainties in modelling convection, mass loss and rotation. The theoretical modelling of stars that experience off-centre carbon ignition and carbon flames is highly uncertain, and this uncertainty is amplified by the fact that the thermonuclear rates of carbon burning are not well determined. The binary mass transfer mechanisms that lead to Type Ia supernovae, delayed binary SNII and accretion induced collapse are a significant source of uncertainty, a problem exasperated by the difficulties in modelling the common envelope phase of evolution. Finally, the mechanisms (e.g., the URCA process and mixing) that lead to electron-capture supernova are also highly uncertain. Observations of bright, evolved stars in young star clusters (e.g., especially those in the Magellanic Clouds) will greatly improve our knowledge on the AGB-supernova mass transition.
AGB stars and supernovae are the main actors in the story of gas and dust production in galaxies. For this reason, knowledge of their stellar yields and the uncertainties that affect them are essential for many key topics in modern astrophysics. These topics include the abundance gradients in galaxies such as our own Milky Way, the role of super-AGB stars in the origin and evolution of globular clusters, and the interpretation of the extinction curves of stellar populations of various complexities and at different redshifts. Massive AGB and Super-AGB stars are currently one of the favoured sites for producing the multiple populations in Galactic globular clusters but uncertainties in the modelling hinder firm conclusions. Super-AGB stars may also be one of the few sites of the intermediate-neutron capture process in the Galaxy: the i-process, as well as being able to synthesize elements by the slow-neutron capture process. Few models of the i-process in super-AGB stars currently exist but observational evidence (e.g., abundances of metal-poor stars) here will be the key tool to guide models. One of the proposed sites of the rapid neutron capture process are electron- capture supernovae. For this reason, understanding the frequency of electron-capture supernovae in the Galaxy (e.g., how many super-AGB stars go through this evolutionary channel) and their nucleosynthesis output is vital if we are to understand the origin of half of all elements heavier than iron.
The scope of the meeting is to gather scientists active in these fields to stimulate open discussions on these many questions and topics. This conference is particularly timely because we now have models of super-AGB stars and EC-SN, as well as observations of bright evolved stars in galaxies that could be either super-AGB stars or light-curves from possible EC supernova. Furthermore, abundances of metal-poor stars have recently suggested heavy element nucleosynthesis beyond the standard s and r-processes and only now are the first predictions becoming available for comparison to observations. The conference will be structured into four sessions, focused on the physics of AGB and super-AGB stars, on the evolution into electron- capture and other types of supernovae, on the role of AGB stars, super-AGB stars and supernova (Types II and Type Ia) on the chemical evolution in galaxies and star clusters, on binary evolution and population synthesis tools required to infer rates of the different classes of supernovae. The observational consequences of the theoretical predictions for super-AGB stars will be discussed in a dedicated session. Potential massive and/or super-AGB stars have been identified in Local Group galaxies and efforts are underway to investigate their properties in detail. Spectroscopic programmes are looking for the distinct abundances patterns that have been predicted. Equally important is long-term monitoring from the ground and from Spitzer; this is providing insight into the pulsation characteristics and hence into mass-loss expectations. Within our own Galaxy resolved mapping with ALMA is providing the detailed history of mass- loss from individual stars. Mass-loss is critical to the evolution of these massive AGB stars and will probably be the most important factor in determining which ones become supernovae. Although models of AGB evolution have improved dramatically over recent decades they have yet to provide really useful predictions on pulsation and mass-loss, so we anticipate providing a forum where observations will confront theory and result in joint programmes for the upcoming space and ground-based facilities.
Little is known about stars in the mass range between those that end their lives as white dwarfs and those that die in spectacular supernova explosions. The uncertainty stems from the fact that stars in this transition mass range from about ~7 to 11 solar masses are both difficult to model theoretically and there are few observational clues as to their evolutionary history. Following the ignition of carbon in the core under conditions of partial degeneracy, the stars then continue to evolve through the asymptotic giant branch (AGB) phase and are known as super-AGB stars. Super-AGB stars have oxygen-neon degenerate cores as opposed to the carbon-oxygen cores of their lower mass counterparts. The final fate of single super-AGB stars depends on the rate of mass loss from the surface: if the core can grow big enough to reach the Chandrasekhar mass then it will explode as an electron capture supernova.
At least three different types of supernova are connected with the limits of this mass range: 1) their upper limit, Mup, is the lowest mass of single core collapse supernova progenitors (SN Type II); 2) their lower limit, Mco, represents the upper limit for the formation of C-O white dwarfs, whose subsequent evolution through mass transfer in a binary system may lead to thermonuclear supernova (SN Type Ia); and 3) single stars in the super-AGB upper mass range may be the site of electron- capture supernovae, while accretion onto their remnants in close binaries may result in accretion induced collapses. Over the past few years, theoretical models have made significant steps towards understanding the evolution and fate of stars in the transition mass range but crucial uncertainties remain. The problem starts on the main sequence, with numerical and physical choices about the treatment of convective overshoot and rotation determining the size of the C-O core at the beginning of the AGB, and hence the mass limits (Mup and Mco). The description of the AGB phase of evolution is itself uncertain, stemming from uncertainties in modelling convection, mass loss and rotation. The theoretical modelling of stars that experience off-centre carbon ignition and carbon flames is highly uncertain, and this uncertainty is amplified by the fact that the thermonuclear rates of carbon burning are not well determined. The binary mass transfer mechanisms that lead to Type Ia supernovae, delayed binary SNII and accretion induced collapse are a significant source of uncertainty, a problem exasperated by the difficulties in modelling the common envelope phase of evolution. Finally, the mechanisms (e.g., the URCA process and mixing) that lead to electron-capture supernova are also highly uncertain. Observations of bright, evolved stars in young star clusters (e.g., especially those in the Magellanic Clouds) will greatly improve our knowledge on the AGB-supernova mass transition.
AGB stars and supernovae are the main actors in the story of gas and dust production in galaxies. For this reason, knowledge of their stellar yields and the uncertainties that affect them are essential for many key topics in modern astrophysics. These topics include the abundance gradients in galaxies such as our own Milky Way, the role of super-AGB stars in the origin and evolution of globular clusters, and the interpretation of the extinction curves of stellar populations of various complexities and at different redshifts. Massive AGB and Super-AGB stars are currently one of the favoured sites for producing the multiple populations in Galactic globular clusters but uncertainties in the modelling hinder firm conclusions. Super-AGB stars may also be one of the few sites of the intermediate-neutron capture process in the Galaxy: the i-process, as well as being able to synthesize elements by the slow-neutron capture process. Few models of the i-process in super-AGB stars currently exist but observational evidence (e.g., abundances of metal-poor stars) here will be the key tool to guide models. One of the proposed sites of the rapid neutron capture process are electron- capture supernovae. For this reason, understanding the frequency of electron-capture supernovae in the Galaxy (e.g., how many super-AGB stars go through this evolutionary channel) and their nucleosynthesis output is vital if we are to understand the origin of half of all elements heavier than iron.
The scope of the meeting is to gather scientists active in these fields to stimulate open discussions on these many questions and topics. This conference is particularly timely because we now have models of super-AGB stars and EC-SN, as well as observations of bright evolved stars in galaxies that could be either super-AGB stars or light-curves from possible EC supernova. Furthermore, abundances of metal-poor stars have recently suggested heavy element nucleosynthesis beyond the standard s and r-processes and only now are the first predictions becoming available for comparison to observations. The conference will be structured into four sessions, focused on the physics of AGB and super-AGB stars, on the evolution into electron- capture and other types of supernovae, on the role of AGB stars, super-AGB stars and supernova (Types II and Type Ia) on the chemical evolution in galaxies and star clusters, on binary evolution and population synthesis tools required to infer rates of the different classes of supernovae. The observational consequences of the theoretical predictions for super-AGB stars will be discussed in a dedicated session. Potential massive and/or super-AGB stars have been identified in Local Group galaxies and efforts are underway to investigate their properties in detail. Spectroscopic programmes are looking for the distinct abundances patterns that have been predicted. Equally important is long-term monitoring from the ground and from Spitzer; this is providing insight into the pulsation characteristics and hence into mass-loss expectations. Within our own Galaxy resolved mapping with ALMA is providing the detailed history of mass- loss from individual stars. Mass-loss is critical to the evolution of these massive AGB stars and will probably be the most important factor in determining which ones become supernovae. Although models of AGB evolution have improved dramatically over recent decades they have yet to provide really useful predictions on pulsation and mass-loss, so we anticipate providing a forum where observations will confront theory and result in joint programmes for the upcoming space and ground-based facilities.