The feedback of an HC HII region on its parental molecular core

The case of core A1 in the star-forming region G24.78+0.08

¹ INAF-Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
e-mail: mosca@arcetri.astro.it
² I. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany
³ Max Planck Institut for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
⁴ SRON Netherlands Institute for Space Research, Landleven 12, 9747 AD Groningen, The Netherlands
⁵ Kapteyn Astronomical Institute, University of Groningen, 9700 AV Groningen, The Netherlands
⁶ Max Planck Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁷ Jodrell Bank Centre for Astrophysics, The University of Manchester, Alan Turing Building, Manchester M13 9PL, UK
⁸ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
⁹ Department of Astrophysics/IMAPP, Radboud University, PO Box 9010, 6500 GL Nijmegen, The Netherlands
¹⁰ School of Physics and Astronomy, University of Leeds, West Yorkshire, Leeds LS2 9JT, UK
¹¹ UK Astronomy Technology Centre, Royal Observatory Edinburgh, Blackford Hill, Edinburgh EH9 3HJ, UK
¹² Institute of Astronomy and Astrophysics, University of Tübingen, Auf der Morgenstelle 10, 72076 Tübingen, Germany
¹³ Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal
¹⁴ Centre for Astrophysics, University of Hertfordshire, Hatfield AL10 9AB, UK
¹⁵ Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Str. 1, 85748 Garching, Germany
¹⁶ Indian Institute of Space Science and Technology, Thiruvananthapuram 695547 Kerala, India

Received: 22 January 2018
Accepted: 21 March 2018

Abstract

Context. G24.78+0.08 is a well known high-mass star-forming region, where several molecular cores harboring OB young stellar objects are found inside a clump of size ≈1 pc. This article focuses on the most prominent of these cores, A1, where an intense hypercompact (HC) HII region has been discovered by previous observations.

Aims. Our aim is to determine the physical conditions and the kinematics of core A1, and study the interaction of the HII region with the parental molecular core.

Methods. We combine ALMA 1.4 mm high-angular resolution (≈0.′′2) observations of continuum and line emission with multi-epoch Very Long Baseline Interferometry data of water 22 GHz and methanol 6.7 GHz masers. These observations allow us to study the gas kinematics on linear scales from 10 to 10⁴ au, and to accurately map the physical conditions of the gas over core A1.

Results. The 1.4 mm continuum is dominated by free-free emission from the intense HC HII region (size ≈1000 au) observed to the North of core A1 (region A1N). Analyzing the H30α line, we reveal a fast bipolar flow in the ionized gas, covering a range of LSR velocities (VLSR) of ≈60 km s−1. The amplitude of the VLSR gradient, 22 km s−1 mpc⁻¹, is one of the highest so far observed towards HC HII regions. Water and methanol masers are distributed around the HC HII region in A1N, and the maser three-dimensional (3D) velocities clearly indicate that the ionized gas is expanding at high speed (≥200 km s−1) into the surrounding molecular gas. The temperature distribution (in the range 100–400 K) over core A1, traced with molecular (CH₃OCHO, ¹³CH₃CN, ¹³CH₃OH, and CH₃CH₂CN) transitions with level energy in the range 30 K ≤ E_u/k ≤ 300 K, reflects the distribution of shocks produced by the fast-expansion of the ionized gas of the HII region. The high-energy (550 K ≤ E_u/k ≤ 800 K) transitions of vibrationally excited CH₃CN are likely radiatively pumped, and their rotational temperature can significantly differ from the kinetic temperature of the gas. Over core A1, the VLSR maps from both the 1.4 mm molecular lines and the 6.7 GHz methanol masers consistently show a VLSR gradient (amplitude ≈0.3 km s−1 mpc⁻¹) directed approximately S–N. Rather than gravitationally supported rotation of a massive toroid, we interpret this velocity gradient as a relatively slow expansion of core A1.