The Position/Structure Stability of Four ICRF2 Sources
  Four close radio sources in the International Celestial Reference Frame
(ICRF) catalog were observed using phase referencing with the VLBA at 43, 23
and 8.6 GHz, and with VERA at 23 GHz over a one year period. The goal was to
determine the stability of the radio cores, and to assess structure effects
associated with positions in the ICRF. Although the four sources were compact
at 8.6 GHz, the VLBA images at 43 GHz with 0.3-mas resolution showed that all
were composed of several components. A component in each source was identified
as the radio core using some or all of the following emission properties:
compactness, spectral index, location at the end of the extended emission
region, and stationary in the sky. Over the observing period, the relative
positions between the four radio cores were constant to 0.02 mas---the phase
referencing positional accuracy obtained at 23 and 43 GHz among the
sources---suggesting that once a radio core is identified, it remains
stationary in the sky to this accuracy. Other radio components in two of the
four sources had detectable motion in the radio jet direction. Comparison of
the 23 and 43 GHz VLBA images with the VLBA 8.6 GHz images and the ICRF
positions suggests that some ICRF positions are dominated by a moving jet
component; hence, they can be displaced up to 0.5 mas from the radio core, and
may also reflect the motion of the jet component. Future astrometric efforts to
determine a more accurate quasar reference frame at 23 and 43 GHz and from the
VLBI2010 project are discussed, and supporting VLBA or EVN observations of ICRF
sources at 43 GHz are recommended in order to determine the internal structure
of the sources. A future collaboration between the radio (ICRF) and the optical
frame of Gaia is discussed.
