ATLASGAL-selected massive clumps in the inner Galaxy^★

VI. Kinetic temperature and spatial density measured with formaldehyde

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
e-mail: xdtang@mpifr-bonn.mpg.de
² Xinjiang Astronomical Observatory, Chinese Academy of Sciences, 830011 Urumqi, PR China
³ Key Laboratory of Radio Astronomy, Chinese Academy of Sciences, 830011 Urumqi, PR China
⁴ Astronomy Department, King Abdulaziz University, PO Box 80203, 21589 Jeddah, Saudi Arabia
⁵ INAF–Istituto di Radioastronomia & Italian ALMA Regional Centre, Via P. Gobetti 101, 40129 Bologna, Italy
⁶ INAF–Osservatorio Astronomico di Cagliari, Via della Scienza 5, 09047 Selargius (CA), Italy
⁷ School of Physical Sciences, University of Kent, Ingram Building, Canterbury, Kent CT2 7NH, UK
⁸ School of Astronomy and Space Science, Nanjing University, 210093 Nanjing, PR China

Received: 25 October 2017
Accepted: 27 November 2017

Abstract

Context. Formaldehyde (H₂CO) is a reliable tracer to accurately measure the physical parameters of dense gas in star-forming regions.

Aim. We aim to determine directly the kinetic temperature and spatial density with formaldehyde for the ~100 brightest ATLASGAL-selected clumps (the TOP100 sample) at 870 μm representing various evolutionary stages of high-mass star formation.

Methods. Ten transitions (J = 3–2 and 4–3) of ortho- and para-H₂CO near 211, 218, 225, and 291 GHz were observed with the Atacama Pathfinder EXperiment (APEX) 12 m telescope.

Results. Using non-LTE models with RADEX, we derived the gas kinetic temperature and spatial density with the measured para-H₂CO 3₂₁–2₂₀/3₀₃–2₀₂, 4₂₂–3₂₁/4₀₄–3₀₃, and 4₀₄–3₀₃/3₀₃–2₀₂ ratios. The gas kinetic temperatures derived from the para-H₂CO 3₂₁–2₂₀/3₀₃–2₀₂ and 4₂₂–3₂₁/4₀₄–3₀₃ line ratios are high, ranging from 43 to >300 K with an unweighted average of 91 ± 4 K. Deduced T_kin values from the J = 3–2 and 4–3 transitions are similar. Spatial densities of the gas derived from the para-H₂CO 4₀₄–3₀₃/3₀₃–2₀₂ line ratios yield 0.6–8.3 × 10⁶ cm⁻³ with an unweighted average of 1.5 (±0.1) × 10⁶ cm⁻³. A comparison of kinetic temperatures derived from para-H₂CO, NH₃, and dust emission indicates that para-H₂CO traces a distinctly higher temperature than the NH₃ (2, 2)/(1, 1) transitions and the dust, tracing heated gas more directly associated with the star formation process. The H₂CO line widths are found to be correlated with bolometric luminosity and increase with the evolutionary stage of the clumps, which suggests that higher luminosities tend to be associated with a more turbulent molecular medium. It seems that the spatial densities measured with H₂CO do not vary significantly with the evolutionary stage of the clumps. However, averaged gas kinetic temperatures derived from H₂CO increase with time through the evolution of the clumps. The high temperature of the gas traced by H₂CO may be mainly caused by radiation from embedded young massive stars and the interaction of outflows with the ambient medium. For L_bol/M_clump ≳ 10 L_⊙/M_⊙, we find a rough correlation between gas kinetic temperature and this ratio, which is indicative of the evolutionary stage of the individual clumps. The strong relationship between H₂CO line luminosities and clump masses is apparently linear during the late evolutionary stages of the clumps, indicating that L_{H_2CO} does reliably trace the mass of warm dense molecular gas. In our massive clumps H₂CO line luminosities are approximately linearly correlated with bolometric luminosities over about four orders of magnitude in L_bol, which suggests that the mass of dense molecular gas traced by the H₂CO line luminosity is well correlated with star formation.