Insights on Gas Distribution and Dynamics in Massive Proto-cluster G358.46$-$0.39: Possible Multiplicity in G358.46$-$0.39 MM1a

TL;DR

The paper analyzes ALMA Band 7 data to map C$^{17}$O, SiO, HC$^{3}$N, and SO$^{2}$ and to quantify $^{12}$CO and SiO outflows in G358.46-0.39, aiming to understand the protostellar content and gas kinematics. It finds filamentary and dumbbell C$^{17}$O structures likely shaped by the nearby HII region MM2, a broad SiO outflow suggesting an unresolved YSO in MM1a, and compact HC$^{3}$N/SO$^{2}$ emission with velocity gradients consistent with a rotating disk/envelope around MM1a, indicating multiplicity. Outflow energetics imply powerful, relatively short-lived flows with dynamical timescales of $t_d \,\sim\,10^{3}$ yr and high inferred mass accretion rates around $\dot{M}_{outflow} \sim 10^{-3}$ M$_\odot$ yr$^{-1}$, supporting at least two driving sources in MM1a. The results emphasize the role of multiplicity in massive star formation and demonstrate the value of multi-tracer ALMA analyses for disentangling complex kinematics in proto-clusters.

Abstract

This work explored the spatial distribution of C$^{17}$O, SiO, HC$_{3}$N and SO$_{2}$ molecules, as well as the energetics of outflows in G358.46$-$0.39 proto-cluster using ALMA band 7 archival data, with the aim of providing an improved understanding of its protostellar nature, gas kinematics and dynamics. G358.46$-$0.39 is previously known to consist of 4 dust continuum cores (MM1a, MM1b, MM1c and MM2). The integrated intensity map of C$^{17}$O reveals filamentary and dumbbell-shaped structures that are probably compressed gases from the expansion of the HII region MM2. The SiO emission reveals spatially overlapped blue and red outflow lobes, likely driven by an unresolved young stellar object (YSO) in MM1a. The spatial distribution of HC$_{3}$N and SO$_{2}$ molecules in MM1a shows a compact morphology, with no detectable HC$_{3}$N and SO$_{2}$ emissions in the other cores. The SO$_{2}$ emission reveals a clear velocity gradient in MM1a, as well as large velocity dispersion ($\sim$ 3\,\kms) within the inner core of MM1a, which are consistent with rotating structures. We estimated the mass, momentum and energy outflow rate, as well as other outflow parameters. The SiO outflow exhibits a different morphology compared to the $^{12}$CO outflow morphology previously observed in MM1a. The SiO and $^{12}$CO outflows are probably associated with disks of separate cores with one face-on and the other edge-on, pointing to multiplicity of YSOs in MM1a. The properties of MM1a indicate that it is a massive protostar that is actively accreting and undergoing star formation.

Figures (8)

• Figure 1: $Top$: Integrated intensity map of C$^{17}$O emission (Background). $Bottom$: Integrated intensity map of C$^{17}$O (magenta contours with levels = 0.18, 0.23, 0.26, 0.32, 0.37, 0.44, 0.56, 0.71, 0.83 Jy beam$^{-1}$ kms$^{-1}$) and ALMA dust continuum image 2023MNRAS.520.4747U superimposed on the Spitzer three-colour composite image 2003PASP..115..953B. Green and magenta crosses show the peak position of the dust continuum cores. Cyan stars, triangles and circle represent the peak position of the 6.7 GHz CH$_{3}$OH 2010MNRAS.404.1029C, 22 GHz H$_2$O 2014MNRAS.442.2240W and 95 GHz CH$_{3}$OH 2017ApJS..231...20Y masers, respectively. White ellipse at the left bottom corner represents the ALMA synthesized beam.
• Figure 2: C$^{17}$O emission channel maps. Gray contours, green crosses, cyan stars, triangles and circle are the same as defined in Fig. \ref{['fig:c17o_m0']}.
• Figure 3: C$^{17}$O spectra of: (a) MM1a (b) MM1b (c) MM1c and (d) MM2. Vertical dotted line indicates the systemic velocity of the source.
• Figure 4: SiO line (a) integrated intensity map (background) and (b) spectrum. Gray contours, green crosses, cyan stars, triangles, circle and white ellipse are the same as defined in Fig. \ref{['fig:c17o_m0']}. Vertical dotted line is the same as in Fig. \ref{['fig:C17O_line']}.
• Figure 5: $Left$: The blue-shifted and red-shifted SiO emission superimposed on the ALMA dust continuum image (background) as blue and red contours, respectively. $Right$: Zoomed-in of the selected region. Cyan and magenta contours indicate the direction of the bipolar outflow traced by $^{12}$CO in MM1a 2023MNRAS.520.4747U. Green cross, cyan star and triangle are the same as defined in Fig. \ref{['fig:c17o_m0']}. Black ellipse at the left bottom corner represents the ALMA synthesized beam.