  Context. The mechanism of disk formation around fast-rotating Be stars is not
well understood. In particular, it is not clear which mechanisms operate, in
addition to fast rotation, to produce the observed variable ejection of matter.
The star Achernar is a privileged laboratory to probe these additional
mechanisms because it is close, presents B-Be phase variations on timescales
ranging from 6 yr to 15 yr, a companion star was discovered around it, and
probably presents a polar wind or jet. Aims. Despite all these previous
studies, the disk around Achernar was never directly imaged. Therefore we seek
to produce an image of the photosphere and close environment of the star.
Methods. We used infrared long-baseline interferometry with the PIONIER/VLTI
instrument to produce reconstructed images of the photosphere and close
environment of the star over four years of observations. To study the disk
formation, we compared the observations and reconstructed images to previously
computed models of both the stellar photosphere alone (normal B phase) and the
star presenting a circumstellar disk (Be phase). Results. The observations
taken in 2011 and 2012, during the quiescent phase of Achernar, do not exhibit
a disk at the detection limit of the instrument. In 2014, on the other hand, a
disk was already formed and our reconstructed image reveals an extended H-band
continuum excess flux. Our results from interferometric imaging are also
supported by several H-alpha line profiles showing that Achernar started an
emission-line phase sometime in the beginning of 2013. The analysis of our
reconstructed images shows that the 2014 near-IR flux extends to 1.7 - 2.3
equatorial radii. Our model-independent size estimation of the H-band continuum
contribution is compatible with the presence of a circumstellar disk, which is
in good agreement with predictions from Be-disk models.
