S2D2: Small-scale Significant substructure DBSCAN Detection II. Tracing episodes and gradients of star formation activity

Abstract

We provide the community with a homogeneous catalogue of small, significant substructures (henceforth NESTs) extracted from the spatial distribution of Young Stellar Objects (YSOs) in a large, consistent sample of star-forming regions. The catalog allows us to explore the relevance of small scale spatial substructure and discuss the interpretation of NESTs as tracers of star formation activity and remnants of the star formation process. We apply our procedure to consistent catalogues of YSOs to obtain NESTs in a sample of star-forming regions. We apply a photometric classification scheme to obtain the evolutionary stage of YSOs and statistically explore the distribution of class 0/I objects as a proxy of recent star formation activity. The region sample is diverse (in distance, size, structure, and global evolutionary stage), and we consequently find different structural properties and star formation histories. Most NESTs in regions with high recent star formation activity show even higher levels of activity. Moreover, the proportion of NESTs with higher activity than the region average increases with the global level of activity of the region. In approximately half of the regions we also find significant spans in the evolutionary stages of the NESTs, consistent with gradients and episodes of star formation. The combination of NESTs with a statistical exploration of the star formation history within each region provides robust and powerful insights into the star formation process. Our results support the role of NESTs as pristine remnants of star formation in highly active regions,stressing the role of fragmentation. The combination of small structures with large scale spatio-evolutionary patterns suggests hyerarchical, prolonged, dynamic, and complex star formation scenarios.

Figures (13)

• Figure 1: Spatial distribution and relative risk maps for NGC2264. Left: Spatial distribution of objects in NGC2264. Dots represent each source in the complete sample, coloured according to the evolutionary stage classification described in section \ref{['estimEvol']} (green for Class0I, red for Class II, black for Class III, and empty for unclassified and non stellar). Right: Four panel composite showcasing the associated densities and relative risk maps.Top left: KDE estimate corresponding to the classified subsample in the left pannel. Top right: Relative risk maps of Class 0/I objects. Bottom panels: Relative risk maps of Class II and Class III objects.
• Figure 2: Relative density of NESTs compared to that of the region $\rho_{rel}$ vs. $Q$ structural parameter. The marginal distributions are shown as density profiles on their respective axes. The hatched zones indicate values of $Q$ corresponding unequivocally to substructured ($Q\lessapprox 0.7$) and concentrated ($Q\gtrapprox 0.87$) distributions.
• Figure 3: Stacked bar diagram representing the fraction of stellar objects of each evolutionary state in each (not flagged) region, ordered by Class 0/I fraction. The colour of the bars represents the regime attributed to each region: purple for HRA regions, green for IRA regions and yellow for LRA regions, as explained in the main text. The intensity of each bar represents the different classes of objects in each region, with darker shades representing less evolved objects.
• Figure 4: Bar plots display the average fraction of C0/I objects in each region, with dotted black lines indicating the limits for categorizing NESTs as high-activity or low activity, that is $\bar{f}_{C0/I} \pm \sigma_{{f}_{C0/I}}$. The colour of the bars represents the regime of each region, and black dots represent the values of $f_{C0/I}$ in each NEST, along with its corresponding standard deviation, in decreasing order. Regions within each regime are ordered by $f_{NEST, max}$, the maximum value of $f_{C0/I}$ in NESTs. The value $f_{C0/I}=0.1$ has been marked in all panels for comparison.
• Figure 5: Comparison of the structures from Kuhnetal14 and Getmanetal18. Top left: Histogram of the size of NESTs (red) and the structures from Kuhnetal14 and Getmanetal18 in kAU. Top Right: Histogram of relative average density and the structures from Kuhnetal14 and Getmanetal18. Bottom: Grey bars show the average ratio $\bar{f}_{C0/I}$ in each region, with small dotted horizontal dotted lines showing the limits $\bar{f}_{C0/I} \pm \sigma_{{f}_{C0/I}}$. Red (resp. black) dots show the median values of ${f}_{C0/I}$ from the relative risk maps in the area occupied by the convex hull of NESTs (resp. structures from Kuhnetal14 and Getmanetal18), with error bars representing the first and third quartile. Red (resp. black) solid lines join the median values of ${f}_{C0/I}$ for NESTs (resp. Kuhnetal14 and Getmanetal18) to help compare the values. Finally, vertical lines show separate between HRA, IRA, and LRA regimes.