Lawrence Morgan 1 , J.S.Urquhart 2 , C.Figura 3 , M.A.Thompson 4

1. Find me at this conference, or email me at lmorgan@ap.smu.ca 1St. Mary’s University, Halifax, 2ATNF, 3Wartburg College, 4University of Hertfordshire Abstract Observed characteristics of embedded star formation within potentially triggered regions are interpreted as bimodal in nature; a sample of protostars located close to photoionised regions has been separated into two groups, the first apparently unaffected by the relatively high temperatures and pressures associated with nearby HII regions. A second population of protostars within bright-rimmed clouds is indicated, through mass function analysis and other diagnostics, to be subject to significant photoionisation related effects. These effects produce a substantial influence upon the evolution of star formation in these clouds through the interaction of induced shocks. The production of consecutive generations of stars through photoionisation-related processes is known as radiatively-driven implosion. Models of this process have been proposed for some time. There have, however, been few successful programmes to date explicitly linking the predictions of such models to observed physical properties. The contribution of this mode of star formation to both the Galactic stellar population and the interstellar mass function is significant, yet poorly understood. Bright-rimmed clouds have long been suggested as the ideal candidates to investigate this particular mode of star formation and the last few years have yielded a wealth of new observations of these clouds. A multi-wavelength approach to the study of these potentially triggered star forming regions has revealed a population of dynamically active intermediate- to high-mass early-phase protostars. This sample has now been revealed to be host to multiple formation scenarios, at least two of which are now distinguishable through observational analysis. Refining a Sample of Triggered Regions Sugitani et al. (1991) searched the Sharpless HII region catalogue (Sharpless 1959) for bright-rimmed clouds associated with IRAS point sources indicating YSOs/protostars. Their catalogue of 44 BRCs has been the focus of an ongoing census of potential triggered star formation (Morgan et al 2004, 2006, 2008, 2009; Thompson et al 2004; Urquhart et al. 2004, 2006, 2007, 2009). Morgan et al. (2004) established the presence of IBLs and PDRs at the interface between the HII regions and the molecular material of the Sugitani BRCs. Previous studies (Morgan et al., 2004; Thompson et al., 2004; Urquhart et al., 2006, 2007) show these clouds are likely to be in a post-shocked state and therefore that any observed star-formation is a possible result of the cloud's interaction with the external ionising source Morgan et al. (2008) reported a Submillimetre Common User Array (SCUBA)1 imaging survey of the submillimetre continuum emission from the clouds, in order to reveal the presence of compact, potentially star-forming, dust cores within the clouds and to contrast the properties of these cores with those found in other star-forming regions. The kinematics and temperature of the dust cores revealed by their SCUBA observations show that a number of the SFO bright-rimmed clouds are associated with active star formation. A comparison of the data sets of Morgan et al. (2004) and Morgan et al. (2008) allows us to discard 20 of the Sugitani BRCs from a refined triggered sample. Many of these sources may be star-forming. However, our analysis indicates that any such activity has come about without significant interaction with local radiation fields. 1The JCMT is operated by the Joint Astronomy Centre on behalf of National Research Council of Canada, PPARC for the United Kingdom, and the Netherlands Organisation of Scientific Research Triggered Star Formation Bright-rimmed clouds are the potential results of radiatively-driven implosion (RDI), in which an ionised boundary layer (IBL) forms at the edge of small molecular clouds due to high levels of photoionising radiation (See Fig. 1). The formation of an IBL may allow for the propagation of shocks into the molecular interior of the cloud, triggering compression and star formation. Previous models of this process suggest that molecular clumps exposed to this process will undergo a morphological evolutionary sequence (Bertoldi & McKee, 1990, Lefloch & Lazareff, 1994, Kessel-Deynet & Burkert, 2003, Miao et al., 2006). Once an IBL forms, creating a `bright rim', the cloud will progress from `A' to `B' to `C' type, becoming more cometary in shape through time due to the erosion of cloud material through the progression and expansion of the ionised layer. Figure 1 DSS (optical) image of SFO 13 overlaid with contours of NVSS (free-free) emission in black dashed lines tracing the IBL (Morgan et al., 2004). Submillimetre emission at 850 microns (Morgan et al., 2008) is overlaid in white contours, tracing a star forming core embedded within the cloud, set back from the rim. Both sets of contours begin at a level of three times the r.m.s. noise of their respective images, then increase in increments of 20% of the relative peak fluxes. Lawrence Morgan1, J.S.Urquhart2, C.Figura3, M.A.Thompson4 Bimodal Star Formation within BRCs Morgan et al. (2008) suggested that BRCs were separated into triggered and non-triggered subsamples on a morphological basis. That is, the more cometary `B' type rims exhibited a correlation between the luminosity of embedded sources and the ionising flux impinging upon the rims themselves. `A' type rims do not show this correlation and therefore are presumably examples of spontaneous star formation. In addition to the correlation between ionising flux and protostellar luminosity, plotting the mass functions of these two subsets of BRCs indicates that `B' type rims contain star formation which trends more to more massive cores and/or clusters. Fig. 2 shows how these two sets of BRCs vary in mass function with relation to an independent sample of protostars. Morgan et al. (2009) has further refined the samples defined in Morgan et al. (2008). While the analysis of Morgan et al. (2008) was dependant solely upon morphology, using the NVSS results of Morgan et al. (2004), Morgan et al. (2009) has removed sources from these subsets that are not likely to be affected via photoionisation-related triggering. Figure 2 The mass spectra of protostars associated with `A' type (left) and `B' and `C' type (right) rims. Sources detected by Morgan et al. (2008) are shown as pluses while an independent sample of protostars from the survey of Mitchell et al. (2001) are shown as diamonds. The Y-axis represents the log of the cumulative number of sources (Ncum) per mass bin of size 0.05 MS as a fraction of N, where N is the total number of sources. The solid line is a best fit to the data of Morgan et al. (2008) above the mean sensitivity limit, the dash-dot line is a best fit to the data of Mitchell et al. (2001). Conclusions Our ongoing census of star formation in BRCs indicates that, in `B' and `C' type rims, relatively more massive objects are formed than low-mass stars in comparison to the Salpeter IMF and thus may make an important contribution to the overall intermediate to high-mass stellar population, with spectral classifications of intermediate to high mass stellar sources within the cores. A multi-wavelength analysis of our sample of BRCs indicates that `A' type rims are commonly associated with clouds unlikely to be associated with photoionisation-induced triggered star formation. This has significance for the morphological evolution hypothesis of Bertoldi & McKee (1990), Lefloch & Lazareff (1994), Kessel-Deynet & Burkert (2003) and Miao et al. (2006). The model of increasing collimation with time must now be re-evaluated as the observational evidence suggests that `A' and `B' type rims are fundamentally different in their formation processes. With `A' type rims typically not exposed to the levels of photoionisation required to bring about dynamical change within the clouds. There is, however, no reason to rule out the possibility that `A' type rims are also the result of triggering, though perhaps via a different mechanism i.e. the `collect and collapse' model (Elmegreen & Lada 1977). 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