The Stellar "Snake"-III: Co-evolution of Stars and Molecular Clouds Unveiled by Gaia, MWISP, and LAMOST

Abstract

By combining multi-band data from Gaia DR3, MWISP CO, and LAMOST DR11 LSR/MSR, we investigate the co-evolution of stars and their parent molecular cloud in a snake-like stellar structure, named Snake III. Based on 5-D phase-space selection, we identified 5683 member stars (median age 7.6 Myr) across approximately $300 \times 500 \times 175$ pc$^3$ volume, along with 12 embedded open clusters. Then we use BEEP distances combined with $^{12}$CO velocities to clearly identify the molecular clouds associated with the stellar complex in spatial and kinematics. The molecular cloud density increases with Galactic longitude, with older open clusters forming in cavities near higher-density regions (except ASCC 125), while young field stars currently form preferentially in present-day high-density environments, indicating that cloud density regulates the star-formation sequence. $^{12}$CO excitation temperature, centroid velocity, velocity dispersion and H$α$ emission reveal that early feedback first compresses cloud edges to trigger new stars, then sweeps and disperses the parent clouds. The extremely young cluster (ASCC 125, 4.4 Myr) lies near the densest region yet is surrounded by a shell with bidirectional density-velocity perturbations, consistent with a delayed-triggering scenario under the combined influence of UBC 178 stellar-wind feedback and a suspected supernova blast. Our results naturally demonstrate that snake-like stellar structures are filamentary relics of hierarchical star formation within giant molecular clouds. They provide direct observational evidence that cloud density and early feedback jointly modulate the progression of star formation, offering a clear and young laboratory for studying star-cloud co-evolution.

Figures (14)

• Figure 1: Spatial distribution of the member stars of Snake III. The color bar encodes distance. Black circles mark the centers and angular sizes of the open clusters. The black arrow in the top-right corner indicates the median tangential velocity of the entire sample, while the remaining arrows show the median tangential velocities of the individual cluster samples, with their lengths proportional to the velocity magnitudes. All velocities are given with respect to the LSR.
• Figure 2: Spatial distribution of Snake III member stars (gray dots) and cluster member stars (shown as dots in various colors) in Cartesian coordinates (X, Y, Z). The coordinates of the Sun are (X, Y, Z) = (0.0, 0.0, 0.0) kpc
• Figure 3: Tangential velocity in ($l, b$) distribution of Snake III member stars (gray dots) and cluster member stars (shown as dots in various colors). All velocities are given with respect to the LSR.
• Figure 4: Distribution of the observed $V_r$s for all member candidates along Galactic longitude $l$. The $V_r$s are obtained from the multiple surveys (color-coded as specified in the legend). The histograms of $V_r$s are displayed in the right sub-panel. The median values are marked with dashed lines. The color-coding is the same as the main panel.
• Figure 5: log Age distribution histograms of the Snake I member stars (upper panel) and Snake III member stars (lower panel) fitting by Sagitta. Contour lines of different colors delineate intervals of PMS probability (green for pms$<$ 0.01, red for pms$>$ 0.1, and blue for pms$>$ 0.8), and the median log Age and number of stars calculated for each interval is marked in the corresponding color.