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

Y Lin,F Wyrowski,K M Menten,T Csengeri,S Leurini,H B Liu,A F Izquierdo

doi:10.1051/0004-6361/202142023

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.

Highlights

Massive star-forming clumps are progenitors of OB clusters (Williams et al 2000; Motte et al 2018)
Outline of the modeling and analysis procedure In this work, we aim to provide a description of the gas density and temperature of massive clumps by performing radiative transfer calculations of molecular lines and the multi-wavelength dust continuum
Massive clumps have average molecular hydrogen densities of typically ∼104 cm−3 (Csengeri et al 2014; Urquhart et al 2018); the collisional partner participating in the excitation and de-excitation of molecular lines considered in this paper is primarily hydrogen gas

Summary

Introduction

Massive star-forming clumps are progenitors of OB clusters (Williams et al 2000; Motte et al 2018). They typically have masses of 1000 M over a spatial scale of ∼1 pc. Fragmentation and accretion processes of OB star clusters are strongly influenced by the evolution of the kinematics, density, and temperature structure of parsec-scale gas clumps (Girichidis et al 2011; Lee & Hennebelle 2019; Padoan et al 2020), and vice versa (Krumholz et al 2012; Offner et al 2009; Hennebelle et al 2020). The most massive stars is deterministic, favoring particular physical environments instead of randomly occurring in molecular clouds (Kroupa et al 2013). This, together with dominant feedback caused by massive stars, may determine the evolutionary track of massive clumps. Observational evidence can be collected by sampling a wide range of clumps at different evolutionary stages

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Conclusion