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Astron. Astrophys. 318, 931-946 (1997)

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5. Discussion

Elmegreen & Lada (1977) and Thronson et al. (1985) proposed a model scenario where the W3 core had been the result of a super-critical collapse as part of a compressed postshock layer which became unstable, collapsed and fragmented. They speculated that the H II region W 4 expanded into the initially low density gas, driving a shock which created a dense layer of gas, the postshock layer. Bound by the H II region on one side, and the unshocked low density molecular gas on the other, the shock then progressed through the cloud. Larger scale CO images of Dickel et al. (1980) show this morphology. Bertoldi (1989) and Bertoldi & McKee (1990) have calculated the effect of expanding H II regions on molecular clumps. Given a certain critical density and size, molecular clumps will eventually be accelerated and compressed. Very diffuse and/or small clumps may be destroyed in this process. The C18 O data of Tieftrunk et al. (1995) and, at higher resolution, of Roberts et al. (1996) indeed show the molecular gas toward the W3 core to be highly clumped with only very diffuse intercloud medium.

Inside the dense fragments young stellar objects can now form. The stellar winds and radiation pressure from newly formed O/B stars begin to remove the surrounding molecular gas slowing accretion. Ionized bubbles form around the very young stars. These hypercompact H II regions will expand to become compact and ultracompact H II regions with more complex structures. Observations of these structures led to a family of morphological classes (Wood & Churchwell 1989). The simple classification scheme, however, has helped little to understand the origin and evolution of these H II regions. Models such as "champagne flows" (Bodenheimer et al. 1979, Tenorio-Tagle 1979, Tenorio-Tagle et al. 1979, Bedijn & Tenorio-Tagle 1981, Yorke et al. 1983) or "moving star bow shocks" (van Buren & Mac Low 1992, Mac Low et al. 1991, Churchwell 1991, van Buren et al. 1990) are aimed at explaining the frequency of edge-brightened H II regions. Model results are viewed from different perspectives assuming fitting lines-of-sight until the observations may be matched. However, velocity dispersions and gradients observed from many edge-brightened H II regions (Gaume et al. 1994, Cesaroni et al. 1991) can neither support the champagne flow model nor the moving star bow shock model conclusively (Fey et al. 1995, Fey et al. 1992). Whereas the parabolic structure of a bow shock exists only for as long as the O/B star moves supersonically through ambient molecular material, the champagne flow model has the shortcoming that it predicts a very short lifetime for ultracompact and hypercompact H II regions.

More "natural" results for the morphology of ultracompact H II regions are obtained from new models of subsonic and/or supersonic flows from photoionized clumpy clouds near newly formed stars (Dyson et al. 1996, Redman et al. 1996, Williams et al. 1996). Once the expanding shells have reached the "clump scale size" of the ambient molecular gas, the ionization fronts may encounter high density cores in the low intercloud medium, which affect their expansion, causing the observed electron density inhomogenuities. This may also result in edge-brightening for ultracompact H II regions with velocity gradients similar to those expected from a moving star bow shock or champagne flow. More recent observations of the dynamics of cometary H II regions (Gaume & Claussen 1990, Gaume et al. 1993, 1995, Fey et al. 1992, 1994, 1995) have been interpreted as the expansion of H II regions into a highly anisotropic medium. Depending on the spatial distribution of clumps and the distribution of the "mass loading" from neutral gas around the region, the new models can satisfactorily produce spherical, core-halo (Redman et al. 1996) and arc-like (Williams et al. 1996) ultracompact H II sources. The ultracompact H II regions may be expanding much slower through pressure equilibrium and the continuous mass loading from the ambient molecular gas.

De Pree et al. (1995) have also shown that ultracompact H II regions can exist longer if the molecular clump, in which an O star forms, is simply denser and warmer than in other models proposed. They calculate ages of the order of [FORMULA] years for several ultracompact H II regions from the data of Wood & Churchwell (1989) and Kurtz et al. (1994). Models of "photoevaporating disks" (Hollenbach et al. 1994) may explain how the ionized material within these H II regions could be continuously replenished during an early stage by a steadily photoevaporating neutral accretion disk surrounding the newly formed ionizing star. Also, photoionization and/or hydrodynamic ablation from neutral clumps of molecular gas in the vicinity of the ionizing stellar sources can act as localized sources of mass injection and create an ionization bound ultracompact H II region that will not expand very quickly (Dyson et al. 1996, Redman et al. 1996). Thus, the inferred lifetime of H II regions could be greatly increased in their early stage.

Once the high density gas has been dispersed, the ionized regions will no longer be bound by a recombination front. Beyond the ultracompact phase the surrounding molecular gas becomes more homogeneous as its density decreases. Beyond the "clump scale size" of the ambient molecular gas the total H II continuum brightness and the brightness asymmetries begin to diminish. Compact H II regions exhibit an overall spherical or shell-like morphology with only their edges showing turbulent elements (swirls, trunks and variations of the spectral index) in the vicinity of remnant dense neutral gas. The association of these diffuse H II regions with individual ionizing stars can now usually be inferred from direct observations of their visual extinction.

If we use the simple assumptions made by De Pree et al. (1995) to compare the time of evolution into the ambient gas (no "outside pressure") of the observed H II regions toward the W3 core, assuming the same initial hydrogen density for all natal molecular clumps, we find ages between a few [FORMULA] years for the hypercompact H II regions and a few [FORMULA] years for the diffuse H II regions. The latter number, indicating an age older than the expected life-time of the exciting star, shows that this numerical exercise, of course, fails to describe the true evolution of H II regions in time. Nevertheless, since W3 gives us the rare opportunity to observe a large variety of H II regions, probably originated from the same volume of neutral gas, a qualitative sequence for the evolution of the H II regions is reasonable. Thus, W3 M would be the youngest source, followed by W3 Ca, F, C, E, B, G, D, A, H, J and K.

Comparing the number and age of the larger compact H II regions to that of younger ultracompact and very young hypercompact continuum sources and their spatial distribution in accordance to the dense molecular gas, our observations of the W3 core favor a scenario where recent star formation toward IRS 5 and IRS 4 has not been triggered by the initial shock wave, but subsequently by the expansion of the firstly formed H II regions.

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© European Southern Observatory (ESO) 1997

Online publication: July 3, 1998