  Substructures - dark lanes and tails - of umbral dots (UDs) were predicted by
numerical simulations of magnetoconvection. We analyse a 6 h 23 min time series
of broadband images of a large umbra in the active region NOAA 10634, acquired
with the 1-m Swedish Solar Telescope, in the wavelength band around 602 nm. A
43 min part of this series was reconstructed with the MFBD method, reaching a
spatial resolution of 0.14". We measure brightness, size, lifetime, and
horizontal velocities of various umbral structures. Most (90 %) of UDs and
bright point-like features in faint LBs split and merge, and their median
lifetimes are 3.5 or 5.7 min, depending on whether the split or merge event is
considered as the end of their life. Both UDs and features in faint LBs that do
not split or merge are clearly smaller (0.15") than the average size (0.17") of
all features. Horizontal motions of umbral bright small-scale features are
directed either into the umbra or along faint LBs with mean horizontal
velocities of 0.34 km/s. Features faster than 0.4 km/s appear mostly at the
periphery of the umbra. The intensity of dark lanes, measured in four bright
central UDs (CUDs), is by a factor 0.8 lower than the peak intensity of CUDs.
The width of dark lanes is probably less than the resolution limit 0.14". The
characteristic time of substructure changes of UDs is ~4 min. We observe narrow
(0.14") bright and dark filaments connected with PUDs. Usually one dark and two
bright filaments form a 0.4" wide tail attached to one PUD, resembling a short
dark-cored penumbral filament. Our results indicate the similarity between PUDs
and PGs located at the tips of bright penumbral filaments. The features seen in
numerical MHD simulations are consistent with our observations of dark lanes in
CUDs and tails attached to PUDs.
