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Context: The observations of solar-like oscillations in evolved stars havebrought important constraints on their internal rotation rates. To correctlyreproduce these data, an efficient transport mechanism is needed in addition tomeridional circulation and shear instability. Aims: We study the efficiency ofthe transport of angular momentum during the subgiant phase. Results: Theprecise asteroseismic measurements of both core and surface rotation ratesavailable for the six Kepler targets enable a precise determination of theefficiency of the transport of angular momentum needed for each of thesesubgiants. These results are found to be insensitive to all the uncertaintiesrelated to the modelling of rotational effects before the post-main sequencephase. An interesting exception in this context is the case of young subgiants(typical values of log(g) close to 4), because their rotational properties aresensitive to the degree of radial differential rotation on the main sequence.These young subgiants constitute therefore perfect targets to constrain thetransport of angular momentum on the main sequence from asteroseismicobservations of evolved stars. As for red giants, we find that the efficiencyof the additional transport process increases with the mass of the star duringthe subgiant phase. However, the efficiency of this undetermined mechanismdecreases with evolution during the subgiant phase, contrary to what is foundfor red giants. Consequently, a transport process with an efficiency thatincreases with the degree of radial differential rotation cannot account forthe core rotation rates of subgiants, while it correctly reproduces therotation rates of red giant stars. This suggests that the physical nature ofthe additional mechanism needed for the internal transport of angular momentummay be different in subgiant and red giant stars.

Asteroseismology of evolved stars to constrain the internal transport of angular momentum. I. Efficiency of transport during the subgiant phase