The evolution of temperature and density structures of OB cluster-forming molecular clumps

Abstract

Context. OB star clusters originate from parsec-scale massive molecular clumps, while individual stars may form in ≲0.1 pc scale dense cores. The thermal properties of the clump gas are key factors governing the fragmentation process, and are closely affected by gas dynamics and feedback of forming stars. Aims. We aim to understand the evolution of temperature and density structures on the intermediate-scale (≲0.1–1 pc) extended gas of massive clumps. This gas mass reservoir is critical for the formation of OB clusters, due to their extended inflow activities and intense thermal feedback during and after formation. Methods. We performed ~0.1 pc resolution observations of multiple molecular line tracers (e.g., CH3CCH, H2CS, CH3CN, CH3OH) that cover a wide range of excitation conditions, toward a sample of eight massive clumps. The sample covers different stages of evolution, and includes infrared-weak clumps and sources that are already hosting an HII region, spanning a wide luminosity-to-mass ratio (L∕M) range from ~1 to ~100 (L⊙/M⊙). Based on various radiative transfer models, we constrain the gas temperature and density structures and establish an evolutionary picture, aided by a spatially dependent virial analysis and abundance ratios of multiple species. Results. We determine temperature profiles varying in the range 30–200 K over a continuous scale, from the center of the clumps out to 0.3–0.4 pc radii. The clumps’ radial gas density profiles, described by radial power laws with slopes between −0.6 and ~−1.5, are steeper for more evolved sources, as suggested by results based on dust continuum, representing the bulk of the gas (~104 cm−3), and on CH3OH lines probing the dense gas (≳106–108 cm−3) regime. The density contrast between the dense gas and the bulk gas increases with evolution, and may be indicative of spatially and temporally varying star formation efficiencies. The radial profiles of the virial parameter show a global variation toward a sub-virial state as the clump evolves. The linewidths probed by multiple tracers decline with increasing radius around the central core region and increase in the outer envelope, with a slope shallower than the case of the supersonic turbulence (σv ∝ r0.5) and the subsonic Kolmogorov scaling (σv ∝ r0.33). In the context of evolutionary indicators for massive clumps, we also find that the abundance ratios of [CCH]/[CH3OH] and [CH3CN]/[CH3OH] show correlations with clump L∕M.