The initial conditions of stellar protocluster formation – II. A catalogue of starless and protostellar clumps embedded in IRDCs in the Galactic longitude range 15° ≤ l ≤ 55°

Abstract

We present a catalogue of starless and protostellar clumps associated with infrared dark clouds (IRDCs) in a 40° wide region of the inner Galactic plane (|b| ≤ 1°). We have extracted the far-infrared (FIR) counterparts of 3493 IRDCs with known distance in the Galactic longitude range 15° ≤ l ≤ 55° and searched for the young clumps using Herschel infrared Galactic plane survey, the survey of the Galactic plane carried out with the Herschel satellite. Each clump is identified as a compact source detected at 160, 250 and 350 μm. The clumps have been classified as protostellar or starless, based on their emission (or lack of emission) at 70 μm. We identify 1723 clumps, 1056 (61 per cent) of which are protostellar and 667 (39 per cent) starless. These clumps are found within 764 different IRDCs, 375 (49 per cent) of which are only associated with protostellar clumps, 178 (23 per cent) only with starless clumps, and 211 (28 per cent) with both categories of clumps. The clumps have a median mass of ∼250 M⊙ and range up to >104 M⊙ in mass and up to 105 L⊙ in luminosity. The mass–radius distribution shows that almost 30 per cent of the starless clumps identified in this survey could form high-mass stars; however these massive clumps are confined in only ≃4 per cent of the IRDCs. Assuming a minimum mass surface density threshold for the formation of high-mass stars, the comparison of the numbers of massive starless clumps and those already containing embedded sources suggests an upper limit lifetime for the starless phase of ∼105 yr for clumps with a mass M > 500 M⊙.