Protoplanetary Disk Evolution around the Triggered Star Forming Region Cepheus B

molecular clouds. The initial mass function (IMF) of the lightly obscured triggered population exhibits a standard Galactic field IMF shape. The unusually high apparent value of 70% star formation efficiency inferred from the ratio of star mass to current molecular gas mass indicates that most of the Cep B molecular cloud has been already ablated or transformed to stars. Contrary to the current RDI simulations, our findings indicate that star formation triggering by HII region shocks is not restricted to a single episode but can continue for millions of years. Other results include: 1. agreement of the disk fractions, their mass dependency, and fractions of transition disks with other clusters; 2. confirmation of the youthfulness of the embedded Cep B cluster; 3. confirmation of the effect of suppression of time-integrated X-ray emission in disk-bearing versus diskless systems. B) is a molecular core located at the edge of the Cepheus giant molecular cloud at a distance around 725 pc and lying 2.6 • above the Galactic Plane (Sargent 1977;Yu et al. 1996). A handful of embedded young stars were found in Cep B from radio continuum and infrared (IR) studies (Felli et al. 1978;Testi et al. 1995). The unobscured stellar OB association Cep OB3 lies around Cep B; the younger subgroup, Cep OB3b, lies closest to the cloud as shown in Figure 1 (Blaauw 1964;Kun et al. 2008). For many years, the Cep OB3 association has been considered to be a good example of large-scale sequential star formation in accord with the model of Elmegreen & Lada (1977) where stellar winds and supernova remnants of an older stellar cluster compress and trigger a second generation of star formation in nearby molecular cloud cores (Sargent 1979).

The interface between the molecular cloud and the Cep OB3b star association is clearly delineated by the optically bright HII region Sharpless 155 (S 155), where cloud material is ionized and heated by the radiation field of the O7 star HD 217086, B1 star HD 217061 and perhaps other cluster members (Panagia & Thum 1981;Beuther et al. 2000). Figure 1 shows the spatial relationship of the cloud, HII region, and exciting stars. Unlike in the Orion Nebula where the HII region lies between the cloud and ourselves, the photodissociation region at S 155 is favorably oriented to reveal the progression of star formation. Following the triggered star formation model, we expect the surface of the cloud to be eroded by the early-type stars so that the cloud edge moves eastward across the observer’s field of view with new stars emerging from the obscuring molecular cloud. Sources located within but near the edge of the molecular cloud would represent a new generation of star formation triggered by the HII region shock propagating into the cloud. This scenario of triggered star formation has been recently strengthened by the discovery using the Chandra X-ray Observatory of > 300 lightly-obscured low-mass pre-main sequence (PMS) members of the Cep OB3b cluster located outside of the cloud, and a rich population of ∼ 60 PMS stars in the cluster embedded within Cep B (Getman et al. 2006). X-ray surveys are particularly effective in discriminating disk-free PMS populations from Galactic field stars which often badly contaminate infrared (IR) surveys of young stellar clusters (see review by Feigelson et al. 2007). Using 2MASS counterparts of the X-ray sources, Getman et al. (2006) found that PMS stars in the embedded cluster are more likely (26% vs. 4%) to have K-band excesses from heated inner protoplanetary disks than stars in the unobscured Cep OB3 region. This supports both, youthfulness of the embedded Cep B population and the prevalence of planet forming disks in the embedded cluster.

We seek here to elucidate the relationships of disks and environments in this region using mid-IR photometry from the Spizer Space Telescope which is exceptionally well-suited for detecting disks around low-mass members of young clusters (e.g. Allen et al. 2004;Sicilia-Aguilar et al. 2006;Luhman et al. 2008b, and references therein). Measures of disk fractions as functions of stellar mass and age in different star-forming environments can provide better understanding of the evolution of circumstellar disks in different environments and thus have direct astrophysical importance in evaluating the conditions for planet formation. X-ray PMS selection is very complementary to IR surveys. As X-ray emission from PMS stars is based on enhanced solar-type magnetic reconnection events rather than disk or accretion processes, X-ray selection delivers rich and clean samples of diskless stars missed by IR selection (Feigelson et al. 2007). The X-ray stars identified by Getman et al. (2006) in combination with discovered here Spitzer disk-bearing stars constitute a valuable sample to measure protoplanetary disk properties in different radiative environments and to study the triggering process inside the Cep B molecular cloud. We use the spatial

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