Search for starless clumps in the ATLASGAL survey

Context. Understanding massive star formation requires comprehensive knowledge about the initial conditions of this process. The cradles of massive stars are believed to be located in dense and massive molecular clumps. Aims: In this study, we present an unbiased sample of the earliest stages of massive star formation across 20 deg2 of the sky. Methods: Within the region 10° < l < 20° and |b| < 1°, we search the ATLASGAL survey at 870 μm 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 μm MIPSGAL images, to search for starless clumps. We calculate the column densities as well as the kinematic distances and masses for sources where the vlsr is known from spectroscopic observations. Results: Within the given region, we identify 210 starless clumps with peak column densities >1 × 1023 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 M⊙ up to several ten thousands of M⊙. Most of them may form massive stars, but in the 20 deg2 area we only find 14 regions massive enough to form stars more massive than 20 M⊙ and 3 regions with the potential to form stars more massive than 40 M⊙. The slope of the high-mass tail of the clump mass function for clumps on the near side of the Galaxy is α = 2.2 and, therefore, Salpeter-like. We estimate the lifetime of the most massive starless clumps to be (6 ± 5) × 104 yr. Conclusions: 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 M⊙. 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.