  The general context of this paper is the study of magnetic fields in the
pre-main sequence intermediate mass Herbig Ae/Be stars. Magnetic fields are
likely to play an important role in pre-main sequence evolution at these
masses, in particular in controlling the gains and losses of stellar angular
momentum. The particular aim of this paper is to announce the detection of a
structured magnetic field in the Herbig Ae star HD 190073, and to discuss
various scenarii for the geometry of the star, its environment and its magnetic
field. We have used the ESPaDOnS spectropolarimeter at CFHT in 2005 and 2006 to
obtain high-resolution and signal-to-noise circular polarization spectra which
demonstrate unambiguously the presence of a magnetic field in the photosphere
of this star. Nine circular polarization spectra were obtained, each one
showing a clear Zeeman signature. This signature is suggestive of a magnetic
field structured on large scales. The signature, which corresponds to a
longitudinal magnetic field of 74+- 10 G, does not vary detectably on a
one-year timeframe, indicating either an azimuthally symmetric field, a zero
inclination angle between the rotation axis and the line of sight, or a very
long rotation period. The optical spectrum of HD 190073 exhibits a large number
of emission lines. We discuss the formation of these emission lines in the
framework of a model involving a turbulent heated region at the base of the
stellar wind, possibly powered by magnetic accretion. This magnetic detection
brings an important element for our understanding of stellar magnetism at
intermediate masses.
