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

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 ≤ Eu/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 ≤ Eu/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.

Discovery of a Photoionized Bipolar Outflow toward the Massive Protostar G45.47+0.05

“…Along the outflow direction, the centroid velocity linearly increases with distance at velocities V lsr +33 km s −1 or V lsr +88 km s −1 (marked by the red triangles). The fitted velocity gradients are 4.9 ± 0.19 km s −1 mas −1 (0.58 ± 0.02 km s −1 au −1 ) for the blue-shifted emissions and 2.6 ± 0.09 km s −1 mas −1 (0.31 ± 0.01 km s −1 au −1 ) for the red-shifted emissions, which are among the highest seen in ionized flows around forming massive stars (e.g., Moscadelli et al 2018). Note that the two high-velocity Gaussian components in the H30α spectrum (Figure 2c) have central velocities of V lsr = +25 and +80 km s −1 , which are close to the velocities where the centroid pattern changes from rotation to outflow.…”

Chasing discs around O-type (proto)stars

“…We originally targeted G17.64 along with five other luminous O-type (proto)stars 1 with ALMA and found mixed evidence for discs around the various targets using the typical CH 3 CN tracer (Cesaroni et al 2017). Our detailed investigations to-date of two of these six sources, G31.41+0.31 and G24.78+0.08 (Beltrán et al 2018;Moscadelli et al 2018), found clear velocity gradients in the CH 3 CN 12-11 line transitions. In G31.41+0.31, the gradient is indicative of rotational spin-up and infall towards the two central cores, whereas for G24.78+0.08, it is due to wind and outflow feedback from the O-star at the centre of an HCHII region.…”

Exploring the formation pathways of formamide

“…There are several sources associated with this star-forming region but we focus on the hot molecular cores A1 and A2. G24 A1 contains a hypercompact HII region (∼1000 au) which has been determined to be expanding through methanol, water maser, and recombination line observations (Beltrán et al 2007;Moscadelli et al 2018). G345.49+1.47 (hereafter G345, also known as IRAS 16562-3959), shown in the bottom panel of Figure 1, is located at a distance of 2.4 kpc with a bolometric luminosity of 1.5 ×10 5 L .…”

“…For full details on observations and continuum subtraction see Cesaroni et al (2017). The continuum intensity for subsources showing H30α emission were corrected for free-free emission with direct measurements for G17 from Maud et al (2018), G24 A1 from Moscadelli et al (2018), and calculated for G345 using the spectral index fit from Guzmán et al (2016).…”