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Search for starless clumps in the ATLASGAL survey

In this study, we present an unbiased sample of the earliest stages of massive star formation across 20 square-degree of the sky. Within the region 10deg < l < 20deg and |b| < 1deg, we search the ATLASGAL survey at 870 micron for dense gas condensations. These clumps are carefully examined for indications of ongoing star formation using YSOs from the GLIMPSE source catalog as well as sources in the 24 micron MIPSGAL images, to search for starless clumps. We calculate the column densities as well as the kinematic distances and masses for sources where the v_lsr is known from spectroscopic observations. Within the given region, we identify 210 starless clumps with peak column densities > 1 x 10e23 cm^(-2). In particular, we identify potential starless clumps on the other side of the Galaxy. The sizes of the clumps range between 0.1 pc and 3 pc with masses between a few tens of solar masses up to several ten thousands of solar masses. Most of them may form massive stars, but in the 20 square-degree we only find 14 regions massive enough to form stars more massive than 20 solar masses and 3 regions with the potential to form stars more massive than 40 40 solar masses. The slope of the high-mass tail of the clump mass function for clumps on the near side of the Galaxy is 2.2 and, therefore, Salpeter-like. We estimate the lifetime of the most massive starless clumps to be 60000 yr. The sample offers a uniform selection of starless clumps. In the large area surveyed, we only find a few potential precursors of stars in the excess of 40 solar masses. It appears that the lifetime of these clumps is somewhat shorter than their free-fall times, although both values agree within the errors. In addition, these are ideal objects for detailed studies and follow-up observations.