  We report molecular line observations, made with ASTE and SEST, and dust
continuum observations at 0.87 mm, made with APEX, towards the cold dust core
G305.136+0.068. The molecular observations show that the core is isolated and
roughly circularly symmetric and imply that it has a mass of
$1.1\times10^3~M_\odot$. A simultaneous model fitting of the spectra observed
in four transitions of CS, using a non-LTE radiative transfer code, indicates
that the core is centrally condensed, with the density decreasing with radius
as $r^{-1.8}$, and that the turbulent velocity increases towards the center.
The dust observations also indicate that the core is highly centrally condensed
and that the average column density is 1.1 g cm$^{-2}$, value slightly above
the theoretical threshold required for the formation of high mass stars. A fit
to the spectral energy distribution of the emission from the core indicates a
dust temperature of $17\pm2$ K, confirming that the core is cold. Spitzer
images show that the core is seen in silhouette from 3.6 to 24.0 $\mu$m and
that is surrounded by an envelope of emission, presumably tracing an externally
excited photo-dissociated region. We found two embedded sources within a region
of 20" centered at the peak of the core, one of which is young, has a
luminosity of $66~L_\odot$ and is accreting mass with a high accretion rate, of
$\sim1\times10^{-4}~M_\odot$ yr$^{-1}$. We suggest that this object corresponds
to the seed of a high mass protostar still in the process of formation. The
present observations support the hypothesis that G305.136+0.068 is a massive
and dense cold core in an early stage of evolution, in which the formation of a
high mass star has just started.
