The ALMA-QUARKS survey: Hot Molecular Cores are a long-standing phenomenon in the evolution of massive protostars

TL;DR

We address how hot molecular cores persist and evolve during high-mass star formation by leveraging the QUARKS ALMA survey to resolve Hot Molecular Fragments (HMFs) within protoclusters. We identify 125 HMFs across 43 fields via CH3CN (12-11) and classify them by association with CO outflows and 1.3 mm continuum, revealing four categories including externally heated shell-like HMFs near HC/UC H II regions. Non-LTE modeling shows HMFs have temperatures from ~95 to ~798 K (median ~209 K), and many HMFs remain associated with hot cores across evolutionary stages, implying HMCs can be long-lived. The study argues that sequential high-mass star formation within protoclusters is driven more by turbulent/thermal fragmentation than by feedback-triggered collect-and-collapse, reshaping the traditional HMC evolutionary scenario and informing feedback processes in young protoclusters.

Abstract

We present an analysis of the QUARKS survey sample, focusing on protoclusters where Hot Molecular Cores (HMCs, traced by CH3CN(12--11)) and UC HII regions (traced by H30α/H40α) coexist. Using the high-resolution, high-sensitivity 1.3 mm data from the QUARKS survey, we identify 125 Hot Molecular Fragments (HMFs), which represent the substructures of HMCs at higher resolution. From line integrated intensity maps of CH3CN(12--11) and H30α, we resolve the spatial distribution of HMFs and UC HII regions. By combining with observations of CO outflows and 1.3 mm continuum, we classify HMFs into four types: HMFs associated with jet-like outflow, with wide-angle outflow, with non-detectable outflow, and shell-like HMFs near UC HII regions. This diversity possibly indicates that the hot core could be polymorphic and long-standing phenomenon in the evolution of massive protostars. The separation between HMFs and H30α/H40αemission suggests that sequential high-mass star formation within young protoclusters is not likely related to feedback mechanisms.

Figures (13)

• Figure 1: Images of the hydrogen recombination lines emssion and the outflow for four exemplar sources. The background shows the three-color image composed by the red-shifted $^{12}$CO (2--1) (red), H30$\alpha$ integrated line emission (green) and the blue-shifted $^{12}$CO (2--1) (blue). The gray contours and orange contours represent the 1.3 mm continuum and the CH$_3$CN (12$_3$--11$_3$) integrated line emission, respectively. H30$\alpha$ integrated velocity range is [Vlsr-40 km s$\rm ^{-1}$, Vlsr+40 km s$\rm ^{-1}$], where Vlsr is the central velocity (Table \ref{['tab:hii']} column (9)). For CH$_3$CN (12$_3$--11$_3$), the integrated velocity ranges are different for different fields. The contour levels were plotted from 3 $\sigma$ to the peak intensity of the field, with 8 logarithmically spaced contours between these values. White markers mark the positions of different type of HMFs: triangle (jet-like outflow), pentagon (wide-angle outflow), star (no/weak outflow) and cross (shell-like shape). The synthesized beams are shown in the lower left corner, and the scale bar is indicated in the lower right corner of each image.
• Figure 2: Example of astrodendro results at different scales. The left, middle, and right panels show data from ACA quarks2, ACA+TM2 quarks3, and ACA+TM1+TM2 LXC2024, respectively. The background shows the 1.3 mm continuum. The red contours represent the integrated intensity of CH$_3$CN (12–11), with contour levels ranging from 3 $\sigma$ to 0.95 $\times$ the peak value, evenly spaced into five levels. The green ellipses outline the structures identified by astrodendro. The synthesized beams (left panel: $\sim$5; middle panel: $\sim$ 1; right panel: $\sim$ 0.3) are shown in the lower left corner, and the scale bar is indicated in the lower right corner of each panel.
• Figure 3: The spectra of all four windows at Band 6 at the CH$_3$CN (12-11) emission map peaks (the positions of the white markers of Figure \ref{['fig:example']}) for different kinds of HMFs. Top-left panels: I17016-4124-1; top-right panels: I16351-4722-1; bottom-left panels: I16562-3959-2; bottom-right panels: 13471-6120-1. The positions of $^{12}$CO (2--1), CH$_3$CN (12-11), C$_2$H$_5$CN (v=0), CH$_3$CHO (v=0,1&2) and H30$\alpha$ are marked with red shadows.
• Figure 4: An example result from simultaneously fitting the multi-transitions of CH$_3$CN (12-11) pixel by pixel on I13471-6120 field using spectuner. Upper panels: rotational temperature (left) and maps of column density (right). Lower panels: maps of $V_\mathrm{LSR}$ (left, the system velocity has already been deducted) and line width (right). The red contours are the CH$_3$CN (12$_3$--11$_3$) integrated line emission, and the levels are from 3 $\sigma$ to the peak intensity of the field, with 8 logarithmically spaced contours between these values.
• Figure 5: Kinetic temperature of HMFs versus H30$\alpha$$\int I \mathrm{d} V$ of the nearest HC/UC H ii regions. Left panel: shell-like HMFs (orange pentagons). Right panel: candidate internally heated HMFs with outflows (pink pentagons) and no/weak outflow (green pentagons). A linear regression is applied to shell-like and candidate internally heated HMFs. The corresponding relations $T_\mathrm{kin}\propto\,\int I \mathrm{d} V^{~0.154}$ and $T_\mathrm{kin}\propto\,\int I \mathrm{d} V^{-0.024}$ are shown with red and blue solid lines. The red and blue shadows show 3$\sigma$ uncertainties of fitting parameters.