Linear filament and nested cluster evolution tomography (LANCET) I. Capture the evolution of dense gas in 14-parsec filament G316.8

Abstract

A dynamic view of mass assembly is essential for understanding the formation of massive stars and clusters. Interpreting evolutionary diagnostics from Galactic-wide surveys, however, requires careful control of distance and environmental variations. The G316.8 filament provides an ideal laboratory: a 14-pc nearly linear structure composed of three contiguous subregions with comparable molecular gas reservoirs (~10,000 $M_\odot$ each) but spanning a clear evolutionary sequence from an infrared dark cloud (young) through a massive young stellar object (intermediate) to an HII region (evolved). As part of the Linear filament and nested cluster evolution tomography (LANCET) project, we mapped the full filament with the Atacama Compact Array at 1.3 mm, achieving 0.08 pc resolution over 17.1 pc$^2$. Combined with Herschel and APEX/ArTéMiS data, we derived high-resolution temperature and column-density maps. We quantify structural evolution using dense-fragment statistics, column-density PDFs, and $Δ$-variance analysis. From young to evolved regions, the maximum fragment mass increases from 8 to 490 $M_\odot$, while the dense-gas mass fraction ($>0.5$ g cm$^{-2}$) rises from 0.4% to 9.6%. The N-PDF develops a secondary power-law tail and the $Δ$-variance slope becomes progressively shallower, indicating ongoing assembly of dense sub-parsec structures. Our further ALMA 12m continuum and spectral line data will extend this dynamic scenarios down to 800 AU scale.

Figures (10)

• Figure 1: Background color map composites of Spitzer 3.6 and 5.8 $\mu$m and Herschel 70 $\mu$m images. The overlaid white contours show the high-resolution H$_2$ column density map obtained from Sect. \ref{['result:sed']} with levels of 3.0, 5.0, 8.0, 12.6, and $20.0 \times 10^{22}$ cm$^{-2}$. The blue, orange, and red boxes outline the young, intermediate, and evolved parts, respectively. Massive clumps embedded in each part are shown with color ellipses. The yellow contours show MeerKAT 1.28 GHz continuum emission at levels of 100, 180, and 260 mJy beam$^{-1}$ (8). The orange cross shows the G316.763$-$0.011 maser spot for OH and H$_2$O, while the red cross shows the G316.812$-$0.057 maser spot for OH, CH$_3$OH, and H$_2$O.
• Figure 2: ACA 7m continuum emission at 1.3 mm overlaid on the composite Spitzer 3.6 $\mu$m and 5.8 $\mu$m emission and Herschel 70 $\mu$m emission. The 1.3 mm map is shown with filled contours at levels of 10, 20, 40, 80, 160, and 320 mJy beam$^{-1}$. The mosaicked field of view is outlined by primary beam response (yellow contour at 0.2). The white contours define the dense region at levels of $3.0$ and $5.0 \times 10^{22}$ cm$^{-2}$.
• Figure 3: Comparison between low-resolution and multi-resolution maps of dust temperature (upper) and H$_2$ column density (lower). The white and yellow polygons indicate the fields of view of the APEX and our ACA 7m observations, respectively. For the temperature image, the resolution is 182 inside and 363 outside the white closed polygon. For the column density image, the resolution outside the yellow closed polygon is the same as the temperature map but as high as $6\arcsec$ within the yellow closed polygon.
• Figure 4: High-resolution column density images zoomed-in toward the three subregions: young (left), intermediate (middle), and evolved (right). The $6\arcsec$ resolution is shown in the bottom left corner. The white contours show MeerKAT 1.28 GHz continuum emission at levels of 100, 180, and 260 mJy beam$^{-1}$.
• Figure 5: Result of source extraction by astrodendro. Left: Dendrogram plot highlighting the leaf structures and their locations. Right: ID numbers and region masks (red contours) overlaid on the 1.3 mm continuum map.