Probing Star-Forming Properties via ALMA Observations of Massive Protocluster IRAS 15596-5301

TL;DR

This study investigates whether mild filament–filament collisions can trigger massive star formation in IRAS 15596-5301 using ALMA 870 μm and 3 mm data. It identifies 34 dense cores, including 3 hot molecular cores with 22 detected species, and reveals two velocity components corresponding to blue- and red-shifted filaments that overlap and are connected by a bridge-like PV structure. The authors infer a non-head-on collision at the filament edges with a relative velocity difference of Δv ≈ 7 km s$^{-1}$ and a localized enhancement of star-formation activity evidenced by methanol masers and extended shock-tracer emission, alongside a depressed H13CO+/HCO+ ratio in the overlap. A K-means clustering analysis shows the collision zone hosts more evolved, chemically rich cores, suggesting the collision triggers or accelerates massive star formation. These results illuminate how mild filament interactions can shape core fragmentation and star-forming efficiency in massive protoclusters.

Abstract

To deepen our understanding of star-forming properties, we studied a massive protocluster IRAS 15596-5301 using ALMA 870 um and 3 mm data. High-resolution 870 um data reveal 34 dense cores, including 3 hot molecular cores, with subsequent line surveys detecting 22 molecular species toward them. Two velocity components (I15596-red/I15596-blue) were found in the averaged H13CO+(1-0) spectrum, and two filaments were identified from velocity-resolved integrated intensity maps. A spatial overlap between the two filaments was observed, and this overlapping region exhibits a distinct bridge-shaped feature in the position-velocity diagram constructed along the entire filamentary structures. Combined with the reduced H13CO+/HCO+ ratio in the overlapping region and the three-dimensional position-position-velocity cube data, we conclude that a non-head-on collision occurs between the edges of the two filamentary structures in IRAS 15596-5301. Cluster analysis demonstrates that clusters located in the collision region host more evolved chemical rich dense cores than their counterparts in other regions. Our results thus indicate that star formation in I15596 is triggered or accelerated by a mild non-head-on collision between two filaments.

Figures (8)

• Figure 1: Panel (a): continuum emission at 3 mm wavelenth of I15596 in color scale and with white contours, overlaid with the 870$\mu$m continuum emission map with the black contours. The white contour levels are [3, 6, 9, 10, 15, 20, 30, 70, 90%] of peak intensity (13.1 mJy beam$^{-1}$). The black contours levels are [6$\sigma$, 12$\sigma$, 24$\sigma$, 48$\sigma$, 96$\sigma$] ($\sigma_{rms}$ = 0.5 mJy beam$^{-1}$). Panel (b): a zoomed-in view of distribution of identified dense cores in 870 $\mu$m map. The blue ellipses with numbers denote identified dense cores and their names. The beam sizes of 870 $\mu$m is shown on the lower left corner with red ellipse, respectively. A 0.1 pc scale bar is shown at the bottom right corner of each panel.
• Figure 2: Panels (a), (b) and (c): The line survey results of SPW 29, LTE modeling overlaid with observed spectra of SPW 29 at 870$\mu$m band extracted from the continuum peak position of cores 14, 19 and 20, respectively. The black stepped lines are the observed spectra, while the red curve displays the synthetic spectrum generated by MADCUBA using the optimal-fit parameters. The core names are labeled in the upper left corners. The line names are marked on the plot.
• Figure 3: Panels (a), (b) and (c): The line survey results of SPW 31, LTE modeling overlaid with observed spectra of SPW 31 at 870 $\mu$m band extracted from the continuum peak position of cores 14, 19 and 20, respectively. The black stepped lines are the observed spectra, while the red curve displays the synthetic spectrum generated by MADCUBA using the optimal-fit parameters. The core names are labeled in the upper left corners. The line names are marked on the plot.
• Figure 4: Average spectrum of H$^{13}$CO$^{+}$ (1-0) for the entire surveyed region. The solid black stepped line is the observed H$^{13}$CO$^{+}$ (1-0) spectrum, while the colored dashed profiles represent different Gaussian components. The orange horizontal dashed line denotes the 3$\sigma$$_{\rm ave}$ ($\sigma$$_{\rm ave}$=0.0003 Jy beam$^{-1}$) threshold. Gaussian components with amplitudes below this threshold are excluded from subsequent analysis. The blue vertical dashed line indicates the boundary between two prominent blue-shifted [--78.6, --73.8] km s$^{-1}$ and red-shifted [--73.8, --68.5] km s$^{-1}$ velocity components.
• Figure 5: Panel (a): The blue and red contours represent integrated instensity map of blue-shifted (I15596-blue with a velocity range of --78.6 to --73.8 km s$^{-1}$) and red-shifted (I15596-red with a velocity range of --73.8 to --68.5 km s$^{-1}$) component, respectively. The filamentary skeletons identified from H$^{13}$CO$^{+}$(1-0) integrated intensity maps I15596-red and I15596-blue are marked as a red and blue solid bold lines, respectively. The colored background is the H$^{13}$CO$^{+}$/HCO$^{+}$ integrated intensity ratio map, within the velocity range of [--73.5, --72.5] km s$^{-1}$. The dashed black open circle indicate the colliding region. White star denotes bright $Spitzer$ 8 $\mu$m emission region. The synthesized beam of 3 mm data is shown in a blue ellipse on the lower left corner, and the 0.1 pc scale bar is presented at the bottom right corner. Panel (b): PV diagram of H$^{13}$CO$^{+}$ is generated along the entire filamentary structures. The I15596-red and I15596-blue components are marked by a red and blue dashed boxes, respectively. The black dashed circle denote the bridge-shaped structures.