Hierarchical Structure and Self-gravity in the Rosette Molecular Cloud

Abstract

We analyze the hierarchical structure in the Rosette Molecular Cloud (RMC) using $^{13}$CO J=1-0 data from the Milky Way Imaging Scroll Painting (MWISP) survey with a non-binary Dendrogram algorithm that allows multiple branches to emerge from parent structures. A total of 588 substructures are identified, including 458 leaves and 130 branches. The physical parameters of the substructures, including peak brightness temperature ($T_{\rm peak}$), brightness temperature difference ($T_{\rm diff}$), radius ($R$), mass ($M$), velocity dispersion ($σ_v$), and surface density ($Σ$), are characterized. The $T_{\rm peak}$ and $T_{\rm diff}$ distributions follow exponential functions with characteristic values above $5σ_{\rm RMS}$. The statistical properties and scaling relations, i.e., $σ_v$-$R$, $M$-$R$, and $σ_v$-$RΣ$ relations are in general consistent with those from traditional segmentation methods. The mass and radius follow power-law distributions with exponents of 2.2-2.5, with slightly flatter slopes for substructures inside the HII region. The velocity dispersion scales weakly with radius ($σ_v \propto R^{0.45\pm 0.03}$, $r = 0.58$), but shows a tighter correlation with the product of surface density and size ($σ_v \propto (ΣR)^{0.29\pm 0.01}$, $r = 0.73$). Self-gravitating substructures are found across scales from $\sim$0.2 to 10 pc, and nearly all structures with peak brightness above 4 K are gravitationally bound ($α_{\rm vir} < 2$). The fraction of bound structures increases with mass, size, and surface density, supporting the scenario of global hierarchical collapse (GHC) for the evolution of molecular clouds, in which molecular clouds and their substructures are undergoing multiscale collapse.

Figures (15)

• Figure 1: RGB map of the $^{13}$CO emission of the RMC. Blue, green, and red colors correspond to the integrated intensity of the $^{13}$CO emission in the velocity range of [$-$2, 5.5], [5.5, 13], and [13, 20.5] $\rm km\ s^{-1}$, respectively. The boundary of the Rosette Nebula given by Quireza2006 is overlaid with a white circle.
• Figure 2: Examples of tree diagrams identified by (a) the original binary Dendrogram algorithm and (b) our modified non-binary Dendrogram algorithm.
• Figure 3: (a) Integrated intensity map of $^{13}$CO emission of the RMC within $-$2 to 20.5 km s$^{-1}$ overlaid with the projected boundaries of hierarchical trunks. The boundaries are colored the same way in panel (b). (b) Tree diagram of the $\rm ^{13}CO$ emission of the RMC generated using the modified non-binary Dendrogram algorithm. The $x$ axis indicates the identifiers of each structure, which have no physical meaning, while the $y$ axis represents the brightness temperature spans of the structures. Hierarchical trunks are highlighted with different colors, while monadic trunks are marked in black.
• Figure 4: Histograms of (a) $T_{\text{peak}}$ ($h$) and (b) $T_{\text{diff}}$ ($l$) of the identified substructures. The green, red, and blue histograms correspond to all structures in the RMC, structures inside the Rosette nebula, and those outside the nebula, respectively.
• Figure 5: Histograms of the (a) radius, (b) mass, (c) velocity dispersion, and (d) virial parameter of the identified substructures. Colors in this figure have the same meanings as in Figure \ref{['fig4']}. The vertical grey dashed line in panel (d) presents $\alpha_{\text{vir}} = 2$.