Observations of surface magnetic fields are now within reach for many stellar
types thanks to the development of Zeeman-Doppler Imaging. These observations
are extremely useful for constraining rotational evolution models of stars, as
well as for characterizing the generation of magnetic field. We recently
demonstrated that the impact of coronal magnetic field topology on the
rotational braking of a star can be parametrized with a scalar parameter: the
open magnetic flux. However, without running costly numerical simulations of
the stellar wind, reconstructing the coronal structure of the large scale
magnetic field is not trivial. An alternative -broadly used in solar physics-
is to extrapolate the surface magnetic field assuming a potential field in the
corona, to describe the opening of the field lines by the magnetized wind. This
technique relies on the definition of a so-called source surface radius, which
is often fixed to the canonical value of 2.5Rsun. However this value likely
varies from star to star. To resolve this issue, we use our extended set of
2.5D wind simulations published in 2015, to provide a criteria for the opening
of field lines as well as a simple tool to assess the source surface radius and
the open magnetic flux. This allows us to derive the magnetic torque applied to
the star by the wind from any spectropolarimetric observation. We conclude by
discussing some estimations of spin-down time scales made using our technique,
and compare them to observational requirements.Comment: Accepted for publication in the Astrophysical Journa
