  Context: Stars form in the cold dense cores of interstellar molecular clouds
and the detailed knowledge of the spectrum of masses of such cores is clearly a
key for the understanding of the origin of the IMF. To date, observations have
presented somewhat contradictory evidence relating to this issue. Aims: In this
paper we propose to derive the mass function of a complete sample of dense
molecular cores in a single cloud employing a robust method that uses uses
extinction of background starlight to measure core masses and enables the
reliable extension of such measurements to lower masses than previously
possible. Methods: We use a map of near-infrared extinction in the nearby Pipe
dark cloud to identify the population of dense cores in the cloud and measure
their masses. Results: We identify 159 dense cores and construct the mass
function for this population. We present the first robust evidence for a
departure from a single power-law form in the mass function of a population of
cores and find that this mass function is surprisingly similar in shape to the
stellar IMF but scaled to a higher mass by a factor of about 3. This suggests
that the distribution of stellar birth masses (IMF) is the direct product of
the dense core mass function and a uniform star formation efficiency of
30%+/-10%, and that the stellar IMF may already be fixed during or before the
earliest stages of core evolution. These results are consistent with previous
dust continuum studies which suggested that the IMF directly originates from
the core mass function. The typical density of ~10^4/cm^3 measured for the
dense cores in this cloud suggests that the mass scale that characterizes the
dense core mass function may be the result of a simple process of thermal
(Jeans) fragmentation.
