Final Report Summary - SAHA-NUC (Stellar Astrophysics, Helioseismology, Asteroseismology and Nucleosynthesis)
Observational stellar astronomy has experienced a very rapid development in the last decade, thanks to ground-based large-scale stellar surveys (e.g. APOGEE, GAIA-ESO) but also due to asteroseismology. In particular, CoROT and, most notably, Kepler are revolutionizing studies of stellar structure and physics and Galactic structure. The combination of asteroseismic techniques, that allow determination of stellar masses, ages and distances, with spectroscopic information, makes it possible to characterize galactic stellar populations in unprecedented ways.
This project is aimed at advancing the theoretical framework needed to exploit the wealth of observational data available. By advancing modeling of stellar structure, evolution and nucleosynthesis, the expected outcomes of the project include: accurate modeling of solar and stellar structure, development of asteroseismic probes of stellar interior properties, application of stellar models to asteroseismic and/or spectrophotometric data to determine stellar parameters (mass, age, distance) and interior structure. Specific aspects of the project are given below, where most important results are also presented.
** Solar structure. The solar abundance problem, i.e. the discrepancy between solar models constructed with the newest solar surface abundances and the solar structure inferred from helioseismic data. The interaction of the early Sun with the protoplanetary disk, its impact on present-day observables, i.e. helioseismic and solar neutrino data, and potential signatures of the formation of terrestrial planets in the solar surface composition. We have developed solar models that go beyond the paradigm of the standard solar model and consider the interaction of the young Sun with the protoplanetary disk by allowing matter chemically processed in the disk to be accreted by the Sun, changing its surface and interior chemical profiles. Using current helioseismic and solar neutrino data we have shown it is possible to put constraints of the accretion history of the Sun. Also, we have shown the enormous scientific potential that a measurement of solar CNO neutrinos has for solar and stellar. We have continued to use solar models as a test bench for physical processes in stars.
** Stellar structure and asteroseismology. Stellar oscillations detected in solar-like stars can be used to probe stellar interior properties as well as to determine fundamental stellar parameters (age, mass, radius) otherwise hardly accesible. The theoretical aspect of this part of the project consists in two parts: 1) the development and validation of asteroseismic probes, e.g. specific combination of oscillation frequencies, sensitive to stellar properties to determine, for example, the extension of convective regions in stars; 2) calculation of new grids of state-of-the-art stellar evolutionary models (more than 2 million models) and new Monte Carlo and Bayesian methods to determine stellar parameters from asteroseismic and/or spectrophotometric data. The observational aspects consist of: 1) applying the theoretical tools to actual asteroseismic data from the Kepler mission to determine stellar properties and contrast results against models; 2) combining Kepler data with spectrophotometric data (e.g. from APOGEE) determine fundamental stellar parameters, including distance, of stars with solar-like oscillations.
A number of results have emerged directly or indirectly from this work. It has been demonstrated that stellar distances derived from asteroseismology are accurate and precise and compare favorable to current astrometric distances. Indeed, asteroseismic distances can be precisely obtained for red giant stars up to 6 kiloparsecs. These results show the potential of asteroseismology as a tool for studies of Galactic stellar populations and structure. Within the framework of the Kepler Asteroseismic Scientific Consortium (KASC), these have been applied to a large number (~500) of dwarf stars observed by Kepler upon which the first Kepler Catalogue is based. Similar work is now undergoing in collaboration with APOGEE, combining spectroscopic and asteroseismic data to produce a catalogue of thousands of giant stars with accurate mass, abundances, kinematics and distance determination.
Oscillation patterns in red giant branch (RGB) stars allow us to discriminate between (RGB) stars on the red clump and the red bump. Based on Kepler data for red giant stars in the open clusters NGC 6791 and NGC 6819 we have been able to put constraints on the total amount of mass lost by stars during the red giant branch evolution. This is the first time such a feat is accomplished, and will help guide models of stellar mass loss, a poorly understood phenomenon, particularly in the case of super-solar metallicity stars such is the case of NGC 6791. Strikingly, results show that effectively a very small amount of mass is lost during the RGB phase and also that there seems not to be a strong dependence on the stellar metallicity.
The statistical Bayesian method for determining stellar parameters and distances has also been used in combination with three-dimensional (3D), non-local thermodynamic equilibrium (non-LTE) spectroscopic analysis of stars in the RAVE experiment to show that a traditional 1D LTE determination of stellar surface temperature, gravity and metallicity, leads to an overestimation of stellar distances of up to factors of two. In this context, we have put forward the possibility that the structure of the Galactic Halo, claimed by some to be formed by two components rotating in different directions, might be the result of wrong determination of stellar distances.
** More asteroseismology. During the period of this project, we have discovered a new class of pulsating stars. The binary J0247-25, discovered by WASP, is formed by a main sequence star and the core of a red giant stripped of its envelope. This remnant core has a very low mass (0.18 solar masses) and is the progenitor of Extremely Low Mass White Dwarf stars (ELMWD). We have found the pre-ELMWD shows p-mode pulsations and that at least one of the observed modes is non-radial, which makes it possible to use the oscillation spectrum to constrain stellar properties. We have shown that binary evolution models are able to correctly predict the mass of the pre-ELMWD and its pulsation spectrum.
** Evolved low- and intermediate-mass stars. About half of the chemical elements beyond the iron peak are created in the late evolutionary phases of stars ending their lives as white dwarfs. From a phenomenological point of view, the occurrence of the slow neutron capture process (s-process) in low and intermediate mass stars is reasonably well understood. Extremely Metal Poor stars (EMPs), the relics of the first generation of stars in the Milky Way, however, defy common wisdom by showing seemingly odd element abundance patterns that are not explained by canonical nucleosynthesis models.
Here we propose to study alternative scenarios for the origin of the abundance patterns observed in the most metal poor stars in the galaxy focusing, in particular, in events involving a phenomenon known as hydrogen (or proton) induced flash, known to take place preferentially in EMPs. Specifically, the main interest has been set on low mass stars that undergo a helium core flash which induces, at the very low metalicities characteristic of two most metal poor EMPs known to date (HE1327-2326 & HE0107-5240), mixing of protons into the helium burning core. This event can explain, at least qualitatively, the large overabundances of CNO elements observed in those stars and, according to our results, has the potential of producing s-process elements in comparable amounts to those detected in these and other EMPs.
This project is aimed at advancing the theoretical framework needed to exploit the wealth of observational data available. By advancing modeling of stellar structure, evolution and nucleosynthesis, the expected outcomes of the project include: accurate modeling of solar and stellar structure, development of asteroseismic probes of stellar interior properties, application of stellar models to asteroseismic and/or spectrophotometric data to determine stellar parameters (mass, age, distance) and interior structure. Specific aspects of the project are given below, where most important results are also presented.
** Solar structure. The solar abundance problem, i.e. the discrepancy between solar models constructed with the newest solar surface abundances and the solar structure inferred from helioseismic data. The interaction of the early Sun with the protoplanetary disk, its impact on present-day observables, i.e. helioseismic and solar neutrino data, and potential signatures of the formation of terrestrial planets in the solar surface composition. We have developed solar models that go beyond the paradigm of the standard solar model and consider the interaction of the young Sun with the protoplanetary disk by allowing matter chemically processed in the disk to be accreted by the Sun, changing its surface and interior chemical profiles. Using current helioseismic and solar neutrino data we have shown it is possible to put constraints of the accretion history of the Sun. Also, we have shown the enormous scientific potential that a measurement of solar CNO neutrinos has for solar and stellar. We have continued to use solar models as a test bench for physical processes in stars.
** Stellar structure and asteroseismology. Stellar oscillations detected in solar-like stars can be used to probe stellar interior properties as well as to determine fundamental stellar parameters (age, mass, radius) otherwise hardly accesible. The theoretical aspect of this part of the project consists in two parts: 1) the development and validation of asteroseismic probes, e.g. specific combination of oscillation frequencies, sensitive to stellar properties to determine, for example, the extension of convective regions in stars; 2) calculation of new grids of state-of-the-art stellar evolutionary models (more than 2 million models) and new Monte Carlo and Bayesian methods to determine stellar parameters from asteroseismic and/or spectrophotometric data. The observational aspects consist of: 1) applying the theoretical tools to actual asteroseismic data from the Kepler mission to determine stellar properties and contrast results against models; 2) combining Kepler data with spectrophotometric data (e.g. from APOGEE) determine fundamental stellar parameters, including distance, of stars with solar-like oscillations.
A number of results have emerged directly or indirectly from this work. It has been demonstrated that stellar distances derived from asteroseismology are accurate and precise and compare favorable to current astrometric distances. Indeed, asteroseismic distances can be precisely obtained for red giant stars up to 6 kiloparsecs. These results show the potential of asteroseismology as a tool for studies of Galactic stellar populations and structure. Within the framework of the Kepler Asteroseismic Scientific Consortium (KASC), these have been applied to a large number (~500) of dwarf stars observed by Kepler upon which the first Kepler Catalogue is based. Similar work is now undergoing in collaboration with APOGEE, combining spectroscopic and asteroseismic data to produce a catalogue of thousands of giant stars with accurate mass, abundances, kinematics and distance determination.
Oscillation patterns in red giant branch (RGB) stars allow us to discriminate between (RGB) stars on the red clump and the red bump. Based on Kepler data for red giant stars in the open clusters NGC 6791 and NGC 6819 we have been able to put constraints on the total amount of mass lost by stars during the red giant branch evolution. This is the first time such a feat is accomplished, and will help guide models of stellar mass loss, a poorly understood phenomenon, particularly in the case of super-solar metallicity stars such is the case of NGC 6791. Strikingly, results show that effectively a very small amount of mass is lost during the RGB phase and also that there seems not to be a strong dependence on the stellar metallicity.
The statistical Bayesian method for determining stellar parameters and distances has also been used in combination with three-dimensional (3D), non-local thermodynamic equilibrium (non-LTE) spectroscopic analysis of stars in the RAVE experiment to show that a traditional 1D LTE determination of stellar surface temperature, gravity and metallicity, leads to an overestimation of stellar distances of up to factors of two. In this context, we have put forward the possibility that the structure of the Galactic Halo, claimed by some to be formed by two components rotating in different directions, might be the result of wrong determination of stellar distances.
** More asteroseismology. During the period of this project, we have discovered a new class of pulsating stars. The binary J0247-25, discovered by WASP, is formed by a main sequence star and the core of a red giant stripped of its envelope. This remnant core has a very low mass (0.18 solar masses) and is the progenitor of Extremely Low Mass White Dwarf stars (ELMWD). We have found the pre-ELMWD shows p-mode pulsations and that at least one of the observed modes is non-radial, which makes it possible to use the oscillation spectrum to constrain stellar properties. We have shown that binary evolution models are able to correctly predict the mass of the pre-ELMWD and its pulsation spectrum.
** Evolved low- and intermediate-mass stars. About half of the chemical elements beyond the iron peak are created in the late evolutionary phases of stars ending their lives as white dwarfs. From a phenomenological point of view, the occurrence of the slow neutron capture process (s-process) in low and intermediate mass stars is reasonably well understood. Extremely Metal Poor stars (EMPs), the relics of the first generation of stars in the Milky Way, however, defy common wisdom by showing seemingly odd element abundance patterns that are not explained by canonical nucleosynthesis models.
Here we propose to study alternative scenarios for the origin of the abundance patterns observed in the most metal poor stars in the galaxy focusing, in particular, in events involving a phenomenon known as hydrogen (or proton) induced flash, known to take place preferentially in EMPs. Specifically, the main interest has been set on low mass stars that undergo a helium core flash which induces, at the very low metalicities characteristic of two most metal poor EMPs known to date (HE1327-2326 & HE0107-5240), mixing of protons into the helium burning core. This event can explain, at least qualitatively, the large overabundances of CNO elements observed in those stars and, according to our results, has the potential of producing s-process elements in comparable amounts to those detected in these and other EMPs.