A Chandra Study of the Rosette Star-Forming Complex. III. The NGC 2237 Cluster and the Regions Star Formation History

arXiv:1004.1422v1 [astro-ph.SR] 8 Apr 2010 A Chandra Study of the Rosette Star-Forming Complex. III. The NGC 2237 Cluster and the Region’s Star Formation History Junfeng Wang,1,2 Eric D. Feigelson,1 Leisa K. Townsley,1 Patrick S. Broos,1 Carlos G. Rom´an-Z´u˜niga,3 Elizabeth Lada,4 and Gordon Garmire1 ABSTRACT We present high spatial resolution Chandra X-ray images of the NGC 2237 young stellar cluster on the periphery of the Rosette Nebula. We detect 168 X- ray sources, 80% of which have stellar counterparts in USNO, 2MASS, and deep FLAMINGOS images. These constitute the first census of the cluster members with 0.2 ≲M ≲2 M⊙. Star locations in near-infrared color-magnitude diagrams indicate a cluster age around 2 Myr with a visual extinction of 1 ≲AV ≲3 at 1.4 kpc, the distance of the Rosette Nebula’s main cluster NGC 2244. We derive the K-band luminosity function and the X-ray luminosity function of the cluster, which indicate a population ∼400–600 stars. The X-ray-selected sample shows a K-excess disk frequency of 13%. The young Class II counterparts are aligned in an arc ∼3 pc long suggestive of a triggered formation process induced by the O stars in NGC 2244. The diskless Class III sources are more dispersed. Several X-ray emitting stars are located inside the molecular cloud and around gaseous pillars projecting from the cloud. These stars, together with a previously unre- ported optical outflow originating inside the cloud, indicate that star formation is continuing at a low level and the cluster is still growing. This X-ray view of young stars on the western side of the Rosette Nebula complements our earlier studies of the central cluster NGC 2244 and the em- bedded clusters on the eastern side of the Nebula. The large scale distribution of the clusters and molecular material is consistent with a scenario in which the 1Department of Astronomy & Astrophysics, The Pennsylvania State University, 525 Davey Lab, Univer- sity Park, PA 16802 2Current Address: Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138; juwang@cfa.harvard.edu 3Centro Astron´omico Hispano Alem´an, Camino Bajo Hu´etor 50, Granada, Spain 18008 4Department of Astronomy, University of Florida, 211 Bryant Space Science Center, Gainesville, FL 32611 – 2 – rich central NGC 2244 cluster formed first, and its expanding HII region trig- gered the formation of the now-unobscured satellite clusters RMC XA and NGC 2237. A large swept-up shell material around the HII region is now in a second phase of collect-and-collapse fragmentation, leading to the recent formation of subclusters. Other clusters deeper in the molecular cloud appear unaffected by the Rosette Nebula expansion. Subject headings: Open clusters and associations: individual (NGC 2237, NGC 2244) - ISM: individual (Rosette Nebula) - stars: formation - stars: pre-main sequence - X-Rays: stars 1. Introduction The triggered formation of the lower mass stars in the vicinity of massive stars is a complex process that is only now being characterized in detail (see Brice˜no et al. 2007, for a recent review). In their immediate neighborhood, massive stars suppress further star for- mation by quickly ionizing and dispersing surrounding molecular material (Herbig 1962). At greater distances, OB stars are more constructive to star formation activity; the shocks driven by ionization or stellar winds are crucial in triggering the collapse of molecular cores (Whitworth et al. 1994; Lefloch & Lazareff1994). Triggered star formation events by mas- sive stars have been observed at different spatial scales, for example, small bright-rimmed clouds on the periphery of HII regions (Sugitani et al. 1995; Getman et al. 2007; Ogura et al. 2007), an embedded cluster in a molecular cloud core on the edge of the dispersed Cep OB3b (Getman et al. 2006), multiple generations of star formation in W5 (Koenig et al. 2008), a broad ridge of young stars along the southwestern boundary of M 17 (Jiang et al. 2002; Broos et al. 2007), and a rich secondary cluster on the edge of Sharpless 219 (Deharveng et al. 2006). The Rosette star forming complex has been considered an excellent candidate for trig- gered star formation (Cox et al. 1990; Phelps & Lada 1997) following the framework devel- oped by Elmegreen & Lada (1977). The massive young cluster NGC 2244 (d ∼1.4 kpc, t ∼2 Myr; Hensberge et al. 2000) powers a visually spectacular expanding HII region known as the Rosette Nebula. The ionized nebula is clearly interacting with the adjacent Rosette Molecu- lar Cloud (RMC) to the east of the Nebula which has a collection of embedded young stellar clusters, each with a few hundred pre-main sequence stars (Table 6 in Wang et al. 2009; see also Poulton et al. 2008). However, recent investigations of the ages and disk fractions of these embedded stellar populations at near-infrared (NIR), mid-infrared, and X-ray wave- lengths do not obviously support a sequential, triggered origin (Rom´an-Z´u˜niga et al.

…(Full text truncated)…

This content is AI-processed based on ArXiv data.