ALMA-IMF. XXI.: N$_2$H$^+$ kinematics in the G012.80 protocluster: Evidence for filament rotation and evolution

TL;DR

This paper dissectes the dense-gas kinematics of the G012 protocluster by specializing in N$_2$H$^+$ (1-0) while leveraging DCN, H41$\alpha$, C$^{18}$O, and SiO tracers to connect gas motions with core formation. It identifies two dominant filaments, R1 and R2, that exhibit contrasting dynamical states: R1 shows a perpendicular velocity gradient suggestive of rotation, while R2 displays compact velocity structure and an ongoing, efficient core-formation phase. The authors derive line-mass profiles for both filaments, revealing high-density states with gravity dominating rotational effects in R1 and a more gravity-driven collapse in R2, resulting in markedly different SFRs and SFEs within the same region. They further quantify N$_2$H$^+$ abundances and core velocities, demonstrating that R2 is more evolved and actively forming stars, whereas R1 remains younger and more rotation-dominated. Overall, G012 presents filaments at different evolutionary stages, offering a rare, in-situ view of filament rotation, accretion, and star-formation history in a single protocluster, with implications for high-mass star formation in clustered environments.

Abstract

(abridged) We aim to characterize kinematic processes in the G012.80 protocluster. We principally focus on the N$_2$H$^+$(1$-$0) emission to trace the dense and cold gas. Additionally, we use lines such as DCN(3$-$2), H41$α$, C$^{18}$O(1$-$0), and SiO(5$-$4), as well as continuum maps. We perform a N$_2$H$^+$ hyperfine spectral line fitting to analyze multiple velocity components and spectral parameters. We estimate velocity gradients, column densities, and line-mass profiles for the two main filaments in G012, named R1 and R2. Line-mass profiles follow $λ$($ω$) = 5660 M$_{\odot}$ pc$^{-1}$($ω$/pc)$^{0.30}$ (R1) and $λ$($ω$) = 6943 M$_{\odot}$ pc$^{-1}$($ω$/pc)$^{0.20}$ (R2), which are much larger than those of typical low-mass filaments. R1 and R2 show disparate position-velocity (PV) features. R1 exhibits a transverse velocity gradient of 10.4 kms$^{-1} $pc$^{-1}$ and few dense cores. This gradient is interpreted with a simple rotation toy model, combined with line-mass profile, and corresponds to a rotational timescale of 0.1 Myr. In contrast, R2 exhibits compact velocity structures ($Δ$V < 2 kms$^{-1}$), likely due to collapse, as evidenced by the presence of a comparatively large number of massive cores and protostellar outflows. R2 is forming prestellar and protostellar cores at a rate of 55.3 M$_{\odot}$ Myr$^{-1}$, with an efficiency similar to the Orion Integral Shaped Filament (ISF). The R1 filament, in contrast, lacks protostellar cores and only contains a few prestellar cores, resulting in an estimated SFR of 4.2 M$_{\odot}$ Myr$^{-1}$, more than an order of magnitude below that of R2. Combining these lines of evidence, we suggest that R1 is younger and still rotating, while R2 has evolved to collapse with a higher SFR. G012 thus hosts filaments at different evolutionary stages.

Figures (17)

• Figure 1: Multi-wavelength view and molecular gas distribution of the G012 protocluster. Left hand side (l.h.s): Spitzer RGB composite figure of the G012 protocluster at 8 $\mu$m (red), 4.5 $\mu$m (green) and 3.6 $\mu$m (blue). The two cyan contours trace the N$_2$H$^+$ integrated intensity emission at 25 and 100 K $\rm km\,s^{-1}$, respectively. Right hand side (r.h.s): N$_2$H$^+$ integrated intensity map with contours at 25 and 100 K $\rm km\,s^{-1}$ corresponding to a $\mathrm{S/N} > 12$, shown in black (same levels as in the left panel). Blue boxes highlight the two main filamentary structures, R1 and R2, with estimated lengths of 0.56 pc for both. We show the average spectra of both regions in the insets, the black curve represents the data within the black boxes. Red markers represent ionizing regions (X markers), radio sources (+ markers), and infrared sources (dot markers) detected in the region by haschick1983. The black ellipse in the bottom-right corner represents the beam size of the N$_2$H$^+$ data. The integrated intensity map shows a "disrupted" morphology, with filamentary structures around the protocluster's center.
• Figure 2: N$_2$H$^+$(1$-$0) integrated intensity (upper panels), velocity centroid (middle panels), and velocity dispersion (bottom panels) of the FVC (left panels) and SVC (right panels). Black contours trace the N$_2$H$^+$ integrated intensity emission at 25 and 100 K $\rm km\,s^{-1}$, respectively. Red boxes in the upper-left panel display the spatial location of the R1 and R2 filaments. The black ellipse at the bottom-right corner represents the beam size of the N$_2$H$^+$ data.
• Figure 3: Position-position and position-velocity diagrams of the N$_2$H$^+$ observations. Upper-left: G012 integrated intensity of FVC (blue shades) and SVC (green shades). We display the DCN cores from DCN catalog with black "+" symbols. The armante2024 core catalog is divided into four categories: cores detected only with N$_2$H$^+$ (red "$\times$" symbols), cores detected only with DCN data (red circles), cores detected with N$_2$H$^+$ and DCN (red triangles), and cores neither detected in both N$_2$H$^+$ nor DCN (black triangles). Symbol sizes are proportional to the estimated core mass. The black ellipse in the bottom right corner represents the beam size of N$_2$H$^+$ data. Upper-right: PV diagram along the y-axis; the velocity axis is subtracted from the N$_2$H$^+$ systemic $V_{\mathrm{LSR}}$ of the protocluster (35.5 $\rm km\,s^{-1}$). The black arrow illustrates a slope of 5 pc ($\rm km\,s^{-1}$)$^{-1}$, which corresponds to a timescale of $\sim$ 0.2 Myr. That is, the timescales that approximately correspond to some of the extended structures in this PV diagram. At the top of R1 we observe a wrapped (or "double helix") type velocity field with spreads of more than $\sim$ 3 $\rm km\,s^{-1}$. Meanwhile, R2 appears very compact in velocity along its extent, with small-scale spatial "wiggles". Bottom-left: PV diagram in the perpendicular direction compared to the upper-right panel. Here we observe the emergence, albeit somewhat hidden in the overall velocity field of an approximately uniform and extended gradient in R1 apparent near $\Delta$X $\sim$ 0.60 to 0.75 pc, $\Delta$V $\sim$ -2 to 0 $\rm km\,s^{-1}$. Meanwhile R2 appears as the compact "blob" of cores (triangles and x-symbols) on the r.h.s. of the panel, characterized by the absence of an obvious gradient in position and velocity.
• Figure 4: Zoomed PV diagrams of the main filaments R1 (l.h.s.) and R2 (r.h.s.), enclosed in the black boxes of Fig. \ref{['fig:pv_sub']}, using the same core markers and color scheme. R1 presents a wrapping "double-helix" signature that is most obvious toward the top of the diagram and which is dominated by the FVC velocities (blue color scale, as in Fig. \ref{['fig:pv_sub']}). R2 exhibits comparatively compact velocity variations ($\Delta V_{\mathrm{max}}~\sim~1.5$ km s$^{-1}$) along the filament, and contains a high number of massive cores (1 M$_{\odot}$ - 3 M$_{\odot}$).
• Figure 5: Relative abundance map in G012. The black contour highlights the mask of $\frac{\tau}{e(\tau)}$ > 2 applied to the N$_2$H$^+$ column density (see Sec. \ref{['sec:xfact']}); most of the pixels removed by this mask do not affect the main filaments significantly. The areas lacking data (top of R1 and bottom of R2) are due to the H$_2$ column density map lack of coverage in the filaments. We find a representative relative abundance value of 0.93 $\times$ 10$^{-10}$. The black circle in the bottom-left corner represents the beam size of the H$_2$ map.