MNRAS 000,1{30 (2018) Preprint 17 April 2018 Compiled using MNRAS LATEX style file v3.0 Young Star Clusters In Nearby Molecular Clouds K. V. Getman,1? M. A. Kuhn,2;3 E. D. Feigelson,1 P. S. Broos,1 M. R. Bate,4 G. P. Garmire5 1Department of Astronomy & Astrophysics, 525 Davey Laboratory, Pennsylvania State University, University Park PA 16802 2Instituto de Fisica y Astronomia, Universidad de Valparaiso, Gran Bretana 1111, Playa Ancha, Valparaiso, Chile 3Millenium Institute of Astrophysics, Av. Vicuna Mackenna 4860, 782-0436 Macul, Santiago, Chile 4Department of Physics and Astronomy, University of Exeter, Stocker Road, Exeter, Devon EX4 4QL, UK 5Huntingdon Institute for X-ray Astronomy, LLC, 10677 Franks Road, Huntingdon, PA 16652, USA Accepted for publication in MNRAS, 2018 February 19 ABSTRACT The SFiNCs (Star Formation in Nearby Clouds) project is an X-ray/infrared study < of the young stellar populations in 22 star forming regions with distances ∼ 1 kpc designed to extend our earlier MYStIX survey of more distant clusters. Our central goal is to give empirical constraints on cluster formation mechanisms. Using paramet- ric mixture models applied homogeneously to the catalog of SFiNCs young stars, we identify 52 SFiNCs clusters and 19 unclustered stellar structures. The procedure gives cluster properties including location, population, morphology, association to molecu- lar clouds, absorption, age (AgeJX ), and infrared spectral energy distribution (SED) slope. Absorption, SED slope, and AgeJX are age indicators. SFiNCs clusters are examined individually, and collectively with MYStIX clusters, to give the following results. (1) SFiNCs is dominated by smaller, younger, and more heavily obscured clusters than MYStIX. (2) SFiNCs cloud-associated clusters have the high elliptici- ties aligned with their host molecular filaments indicating morphology inherited from their parental clouds. (3) The effect of cluster expansion is evident from the radius-age, radius-absorption, and radius-SED correlations. Core radii increase dramatically from ∼ 0:08 to ∼ 0:9 pc over the age range 1 − 3:5 Myr. Inferred gas removal timescales are longer than 1 Myr. (4) Rich, spatially distributed stellar populations are present in SFiNCs clouds representing early generations of star formation. An Appendix com- pares the performance of the mixture models and nonparametric Minimum Spanning Tree to identify clusters. This work is a foundation for future SFiNCs/MYStIX studies including disk longevity, age gradients, and dynamical modeling. Key words: infrared: stars { stars: early-type { open clusters and associations: general { stars: formation { stars:pre-main sequence { X-rays: stars 1 INTRODUCTION But the formation mechanisms of rich clusters is still under study. Dozens of sophisticated numerical (radiation)- Most stars in the Galaxy were formed in compact bound arXiv:1804.05075v1 [astro-ph.GA] 13 Apr 2018 (magneto)-hydrodynamic simulations of the collapse and stellar clusters (Lada & Lada 2003) or distributed, unbound fragmentation of turbulent molecular clouds followed by stellar associations (Kruijssen 2012). Short-lived radioiso- cluster formation have been published (Krumholz et al. topic daughter nuclei in the Solar System meteorites indi- 2014; Dale 2015). The inclusion of stellar feedback is gen- cate that our Sun formed in a modest-sized cluster on the erally found to better reproduce important characteristics edge of a massive OB-dominated molecular cloud complex of star formation such as the stellar initial mass function (Gounelle & Meynet 2012; Pfalzner et al. 2015). It is well ac- (IMF)(Bate 2009; Offner et al. 2009; Krumholz et al. 2012), cepted that most clusters quickly expand and disperse upon gas depletion time, and star formation efficiency (Krumholz the removal of the residual molecular gas via stellar feed- et al. 2014; V´azquez-Semadeni et al. 2017). back so only a small fraction survive as bound open clusters (Tutukov 1978; Krumholz et al. 2014). Constraints can be imposed on the simulations through quantitative comparison with the detailed properties of large cluster samples. For instance, realistic simulations of stel- ? E-mail: [email protected] (KVG) lar clusters in the solar neighborhood, such as Bate(2009, c 2018 The Authors 2 K. V. Getman et al. 2012), are anticipated to obey the empirical cluster mass-size 2 CLUSTER IDENTIFICATION WITH relationship seen in infrared (IR) cluster catalogs Carpenter MIXTURE MODELS (2000); Lada & Lada(2003), X-ray/IR catalogs Kuhn et al. (2014, 2015a,b), and compilations of massive stellar clusters Identification and morphological characterization of SFiNCs Portegies Zwart et al.(2010); Pfalzner & Kaczmarek(2013); clusters uses the parametric statistical mixture model devel- Pfalzner et al.(2016). oped in MYStIX (Kuhn et al. 2014). The mixture model is the sum of multiple clusters fit to the sky distribution of young stellar objects (YSOs) using maximum likelihood es- Our effort called MYStIX, Massive Young Star-Forming timation. The number of clusters is one of the model param- Complex Study in Infrared and X-ray, produces a large eters optimized in the procedure. and homogeneous dataset valuable for analysis of clustered Briefly, this analysis investigates the distribution of a star formation (Feigelson et al. 2013; Feigelson 2018, http: set of points X = fx1; :::; xN g giving the (RA,Dec) coordi- //astro.psu.edu/mystix). MYStIX identified > 30; 000 nates of each YSO. Each point belongs to one of k clusters, young diskless (X-ray selected) and disk-bearing (IR excess or to an unclustered component of scattered stars charac- selected) stars in 20 massive star forming regions (SFRs) at terized by complete spatial randomness. Each cluster is de- distances from 0.4 to 3.6 kpc. Using quantitative statisti- scribed by the probability density function g(x; θj ), where θj cal methods, Kuhn et al.(2014) identify over 140 MYStIX is the set of parameters for the component. Mixture models clusters with diverse morphologies, from simple ellipsoids to of point processes commonly use a normal (Gaussian) func- elongated, clumpy substructures. Getman et al.(2014a,b) tion (McLachlan & Peel 2000; Fraley & Raftery 2002; Kuhn develop a new X-ray/IR age stellar chronometer and dis- & Feigelson 2017). But, following Kuhn et al.(2014), we cover spatio-age gradients across MYStIX SFRs and within choose g to be a two-dimensional isothermal ellipsoid defined individual clusters. Kuhn et al.(2015a,b) derive various clus- in their equation 4. This is the radial profile of a dynamically ter properties, discover wide ranges of the cluster surface relaxed, isothermal, self-gravitating system where the stel- stellar density distributions, and provide empirical signs of lar surface density having a roughly flat core and power-law dynamical evolution and cluster expansion/merger. halo. The effectiveness of the isothermal ellipsoidal model is shown in x4.2 below for SFiNCs and by Kuhn et al.(2014, More recently, the Star Formation in Nearby Clouds 2017) for other young clusters. The relative contribution of (SFiNCs) project (Getman et al. 2017) extends the MYStIX each cluster is given by mixing coefficients fa1; :::; akg, where Pk effort to an archive study of 22 generally nearer and smaller 0 ≤ aj ≤ 1 and j=1 aj = 1. The distribution of the full SFRs where the stellar clusters are often dominated by a sample f(x) is then the sum of the k cluster components, Pk+1 single massive star | typically a late-O or early-B | rather j=1 aj g(x; θj ), plus a constant representing the unclus- than by numerous O stars as in the MYStIX fields. Utiliz- tered component. ing the MYStIX-based X-ray and IR data analyses methods, The mixture model is fit to the (RA,Dec) point distri- Getman et al. produce a catalog of nearly 8500 diskless and bution for each SFiNCs field from Getman et al.(2017) by disk-bearing young stars in SFiNCs fields. One of our ob- maximum likelihood estimation. Each cluster has six param- jectives is to perform analyses similar to those of MYStIX eters, θ = fx0; y0;Rc; , φ, Σcg: coordinates of cluster center in order to examine whether the behaviors of clustered star (x0; y0), isothermal core radius Rc defined as the harmonic formation are similar | or different | in smaller (SFiNCs) mean of the semi-major and semi-minor axes, ellipticity , and giant (MYStIX) molecular clouds. position angle φ, and central stellar surface density Σc. The full model, including the unclustered surface density, has 6k + 1 parameters. The density in our chosen model never In the current paper, the MYStIX-based parametric vanishes at large distances from the cluster center. However, method for identifying clusters using finite mixture models the absence of a parameter for the outer truncation radius (Kuhn et al. 2014) is applied to the young stellar SFiNCs in our model does not strongly affect the results, because sample. Fifty two SFiNCs clusters and nineteen unclustered the spatial distribution of the MYStIX and SFiNCs stars is stellar structures are identified across the 22 SFiNCs SFRs. examined over a finite field of view. The core radius param- Various basic SFiNCs stellar structure properties are de- eter can be better constrained than the truncation radius rived, tabulated, and compared to those of MYStIX. In an (e.g., Kroupa, Aarseth, & Hurley 2001), and our previous Appendix, we compare our parametric method to the com- work shows that inclusion of a truncation radius parameter mon non-parametric method for identifying stellar clusters is not necessary to get a good fit (e.g., Kuhn et al. 2010). based on the minimum spanning tree (MST) (Gutermuth et To reduce unnecessary computation on the entire pa- al. 2009; Schmeja 2011). rameter space, an initial superset of possible cluster compo- nents is obtained by visual examination of the YSO adap- The method of finite mixture models for spatial point tively smoothed surface density maps. This initial guess then processes is briefly described in x2.