Variability and parsec-scale radio structure of candidate compact
  symmetric objects
  We report results on multi-epoch Very Large Array (VLA) and pc-scale Very
Long Baseline Array (VLBA) observations of candidate compact symmetric objects
(CSOs) from the faint sample of high frequency peakers. New VLBA observations
could resolve the radio structure in about 42 per cent of the observed sources,
showing double components that may be either mini-lobes or core-jet structures.
Almost all the sources monitored by the VLA show some variability on time scale
of a decade, and only 1 source does not show any significant variation. In 17
sources the flux density changes randomly as it is expected in blazars, and in
4 sources the spectrum becomes flat in the last observing epoch, confirming
that samples selected in the GHz regime are highly contaminated by beamed
objects. In 16 objects, the pc-scale and variability properties are consistent
with a young radio source in adiabatic expansion, with a steady decrease of the
flux density in the optically-thin part of the spectrum, and a flux density
increase in the optically-thick part. For these sources we estimate dynamical
ages between a few tens to a few hundreds years. The corresponding expansion
velocity is generally between 0.1c and 0.7c, similar to values found in CSOs
with different approaches. The fast evolution that we observe in some CSO
candidates suggests that not all the objects would become classical
Fanaroff-Riley radio sources.
