Final Report Summary - PROSPERITY (Probing Stellar Physics and Testing Stellar Evolution through Asteroseismology)
PROSPERITY stands for "Probing stellar physics and testing stellar evolution through asteroseismology". With the method of asteroseismology, astrophysicists are able to exploit the properties of stellar oscillation modes to probe the interior physics of the stars, which is inaccessible otherwise. Prior to PROSPERITY, such probing was essentially done from acoustic modes that penetrate the outer envelope of the stars, because these are the easiest to observe. Because the stellar material gets denser and denser owards the interior, it becomes harder for the oscillations to penetrate it. However, some oscillations, so-called gravity modes, manage to travel all the way to the centre of the star and these are by far the most interesting ones as they allow to deduce the properties of the stellar cores and that is where the life of the star is ruled. The research in PROSPERITY is largely based on such gravity modes, whose signal is hard to detect in data taken at Earth-based observatories. We were in the perfect era to discover and exploit such gravity modes with the ERC grant, as two space missions capable of detecting them were in operation (CoRoT launched end 2006 and Kepler launched in March 2009).
One highlight of the research was the discovery of a chemically inhomogeneous zone surrounding the core region, where hydrogen is fused into helium, in a star with a mass of about 8 times the one of the Sun. In order to explain the observed properties of its gravity modes, a level of mixing of the stellar material in that zone which was hitherto unknown to exist had to be included. This mixing affects the future life of such stars and implies they will grow older than previously thought. In a next step, we need to understand if this previously unknown mixing is star-dependent or not.
Another highlight concerns the discovery of mixed modes in red giant stars. Mixed modes probe both the core region and the outer layers simultaneously. Prior to PROSPERITY, the existence of mixed modes was anticipated in young stars, but we detected them in red giants, which are large elderly stars as the Sun will be in about 5 billion years. It turns out that these mixed modes allow to distinguish the evolutionary stage of the red giants. Some of them produce their energy from fusing hydrogen into helium in a zone surrounding their helium core, but others already passed that stage and also fuse helium into carbon in their core. Prior to the Kepler mission, it was
not possible to distinguish between these two phases of life, while the mixed modes have led to the opportunity to do so. Probably the most important discovery of PROSPERITY came from more than two years of uninterrupted Kepler data of red giants, leading to the detection of rotational splitting of mixed dipole modes. This revealed that the cores of red giant stars rotate only ten times faster than their envelopes, while models used prior to our discovery predicted this ratio to be more than 100. Clearly, much stronger coupling between the core and envelope of evolved stars occurs than anticipated, so that the theory of stellar evolution must be revised on this front.
During our project, we anticipated for surprises, given that the CoRoT and Kepler missions opened up a new window in time-domain astrophysics. This led to the discovery of beaming binaries. These are systems consisting of two stars approaching the end of their lives moving so close to each other in a joint orbit, that tidal forces deform them. This deformation translates into a spectacular binary light curve and allows a detailed interpretation of their evolutionary status. For one such newly discovered beaming binary from Kepler data, we managed to deduce that one star is a burned-out white dwarf while the other one has a nuclear reactor transforming helium into carbon in a zone surrounding the carbon/oxygen core, which is a very particular and relatively short phase in the life of such a binary. This is an important constraint for the improvement of models describing how such an object will end its life.
One highlight of the research was the discovery of a chemically inhomogeneous zone surrounding the core region, where hydrogen is fused into helium, in a star with a mass of about 8 times the one of the Sun. In order to explain the observed properties of its gravity modes, a level of mixing of the stellar material in that zone which was hitherto unknown to exist had to be included. This mixing affects the future life of such stars and implies they will grow older than previously thought. In a next step, we need to understand if this previously unknown mixing is star-dependent or not.
Another highlight concerns the discovery of mixed modes in red giant stars. Mixed modes probe both the core region and the outer layers simultaneously. Prior to PROSPERITY, the existence of mixed modes was anticipated in young stars, but we detected them in red giants, which are large elderly stars as the Sun will be in about 5 billion years. It turns out that these mixed modes allow to distinguish the evolutionary stage of the red giants. Some of them produce their energy from fusing hydrogen into helium in a zone surrounding their helium core, but others already passed that stage and also fuse helium into carbon in their core. Prior to the Kepler mission, it was
not possible to distinguish between these two phases of life, while the mixed modes have led to the opportunity to do so. Probably the most important discovery of PROSPERITY came from more than two years of uninterrupted Kepler data of red giants, leading to the detection of rotational splitting of mixed dipole modes. This revealed that the cores of red giant stars rotate only ten times faster than their envelopes, while models used prior to our discovery predicted this ratio to be more than 100. Clearly, much stronger coupling between the core and envelope of evolved stars occurs than anticipated, so that the theory of stellar evolution must be revised on this front.
During our project, we anticipated for surprises, given that the CoRoT and Kepler missions opened up a new window in time-domain astrophysics. This led to the discovery of beaming binaries. These are systems consisting of two stars approaching the end of their lives moving so close to each other in a joint orbit, that tidal forces deform them. This deformation translates into a spectacular binary light curve and allows a detailed interpretation of their evolutionary status. For one such newly discovered beaming binary from Kepler data, we managed to deduce that one star is a burned-out white dwarf while the other one has a nuclear reactor transforming helium into carbon in a zone surrounding the carbon/oxygen core, which is a very particular and relatively short phase in the life of such a binary. This is an important constraint for the improvement of models describing how such an object will end its life.