Markov chain Monte Carlo inversions of the internal rotation of Kepler subgiants

Context. The measurement of the internal rotation of post-main-sequence stars using data from space-based photometry missions has demonstrated the need for an efficient angular momentum transport in stellar interiors. No clear solution has emerged so far, and it remains a challenge for stellar modellers to explain the observed trends. Aims. We constrained the shape of the internal rotation profile of six Kepler subgiants that were studied in details in 2014 and also the properties of the missing angular momentum transport process that acts in stellar interiors from Markov chain Monte Carlo (MCMC) inversions of the internal rotation. Methods. We applied a new MCMC inversion technique to existing Kepler subgiant targets and tested various shapes of the internal rotation profile of the six subgiants that were observed in 2014. We also constrained the limitations on the number of free parameters that can be used in the MCMC inversion, showing the limitations in the amount of information in the seismic data. Results. First, we show that large-scale fossil magnetic fields are not able to explain the internal rotation of subgiants, similarly to what was determined from detailed studies of Kepler red giants. We are also able to constrain the location of the transition in the internal rotation profile for the most evolved stars in the available set of subgiants. We find that some of them exhibit a transition that is located close to the border of the helium core, but one object exhibit a transition located much higher in radius. Conclusions. We conclude that various processes might be at play that would explain our observations, but a consistent detailed modelling of all available subgiants is required to reveal the physical nature of the angular momentum process, in particular, for the least evolved objects. In addition, it is paramount to increase the number of stars for which these inferences are possible (e.g. with the future PLATO mission) because they play a key role in validating candidates for the transport process.