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The diameter and evolutionary state of Procyon A

We report the angular diameter measurement obtained with the VINCI/VLTI instrument on the nearby star Procyon A (alpha CMi A, F5IV-V), at a relative precision of +/- 0.9%. We obtain a uniform disk angular diameter in the K band of UD = 5.376 +/- 0.047 mas and a limb darkened value of LD = 5.448 +/- 0.053 mas. Together with the Hipparcos parallax, this gives a linear diameter of 2.048 +/- 0.025 D_sun. We use this result in combination with spectroscopic, photometric and asteroseismic constraints to model this star with the CESAM code. One set of modeling parameters that reproduces the observations within their error bars are an age of 2314 Myr, an initial helium mass fraction Y_i = 0.301 and an initial mass ratio of heavy elements to hydrogen Z/X_i = 0.0314. We also computed the adiabatic oscillation spectrum of our model of Procyon A, giving a mean large frequency separation of Delta nu_0 = 54.7 microHz. This value is in agreement with the seismic observations by Martic et al. (1999, 2001). The interferometric diameter and the asteroseismic large frequency spacing together suggest a mass closer to 1.4 M_sun than to 1.5 M_sun. We conclude that Procyon is currently ending its life on the main sequence, as its luminosity class indicates.