Pedagogía Tradicional

Once the central star has stopped accreting and moves on to the main sequence, UCHIIregions

begin to become detectable, as they expand outside their very dense core and the emitted optical radiation is no longer as obscured. There have been several surveys of UCHIIregions,

including those by Garay et al. (1993), Kurtz, Churchwell & Wood (1994), Miralles, Rodriguez & Scalise (1994) and Churchwell & Wood (1987).

Wood & Churchwell (1989b) based their survey on radio continuum brightness distribution and found that there are only a few morphological types: cometary, core-halo, shell, irregu- lar (Wood & Churchwell, 1989b) and bipolar (e.g. Campbell (1984) , Rodriguez, Canto & Moran (1988), De Pree, Rodriguez & Goss 1995). These are shown in the schematic figure 143 in Wood & Churchwell (1989b), along with brightness profiles for each type. Wood & Churchwell (1989b) find that of their catalogue of 75 UCHIIregions, 20% of UCHIIregions appear cometary in structure, 16% core-halo, 4% shell like, 17% irregular and 43% appear spherical or unresolved. They note that factors such as the resolution of the images will im- pact on their observation. It is worth noting that the resolution of these images also impacts on the structures observed; higher resolution observations would likely reveal more highly structured regions, leading to a reduction in the percentage of UCHII regions that appear

6.1. Introduction

Figure 6.1: Example UCHIIregion morphologies, clockwise from top left (a) spherical G45.07+0.13, (b) cometary G12.21.0.10, (c) shell G5.89.0.39, (d) core and halo G41.74+0.10, (e) irregular G10.15.0.34 and (f) NGC7538 IRS1. Panels a-e from Wood & Churchwell (1989b) and f from Campbell (1984). Reproduced with permission.

(a) spherical G45.07+0.13, (b) cometary G12.21.0.10, (c) shell G5.89.0.39, (d) core and halo G41.74+0.10, (e) irregular G10.15.0.34 and (f) NGC7538 IRS1. Figure 2 of (Urquhart et al., 2009) shows more recents VLA 6cm emission observations of several UCHIIregions including

the region CO51.5095+00.1679, which illustrates the clear spherical and shell like structure of some of these regions.

It is thought that the morphology of UCHIIregions is related to their age, dynamics, density of the surrounding media and the motion of the region relative to the ISM (Churchwell, 2002). Since there are so many factors that could potentially impact on the morphology, it is unclear which is the most important. It is thought that if an HII region expands at the rate of that expected from the ionised gas (∼10km s−1) then the regions should expand to greater than 0.1pc in approximately_∼104yrs. However many observed UCHIIregions appear to be older than 105years. Therefore, it is important that any explanation of the morphological state of these regions should also explain their unexpectedly long life times (_∼105yrs).

There are relatively few proposed scenarios to explain the morphology and lifetime of UCHII regions. Churchwell (1999) give a brief description of these, and this is summarised here. The first is the champagne flow or “blister” model to explain the cometary structure of UCHII regions. This requires that massive stars form near the edge of molecular clouds and the UCHIIregions therefore are confined on one side while being able to expand freely into the low density gas surrounding the cloud (Yorke, Tenorio-Tagle & Bodenheimer 1983). Forster et al. (1990) and Fey et al. (1995) produced models of these regions and showed that if a steep density gradient is present, then a cometary structure will form. While this theory is able to explain the cometary morphology, it does not explain the long lifetime of UCHIIregions, since they are able to expand on dynamical timescales (_∼104years).

Wood & Churchwell (1989b) suggest that there are two mechanisms to extend the life time of UCHIIregions. The first of these involves the dense dusty cocoon that surrounds the UCHII

regions collapsing towards the star while the ionised region expands. This would slow the rate of the ionised gas expansion due to the external pressure of the dust. Second, it is possible that cometary UCHIIregions are formed as an O star travels supersonically through the ISM forming a bow shock around the star. This shock and resulting build up of gas would result in an apparent UCHIIregion for the full life time of the star (∼106years). The all sky survey of Wood & Churchwell (1989a) suggests that the reason so many cometary UCHIIregions were

found is that these regions do intrinsically have a longer lifetime.

Zheng et al. (1985), Ho & Haschick (1986) and Keto, Ho & Reid (1987) presented models where UCHII regions are surrounded by dense, infalling molecular gas showing that in this

scenario the regions would remain compact far beyond the ages expected from the dynamical timescales. While this theory is able to explain the long lifetime of UCHIIregions, it does not

explain their morphologies.

Hollenbach, Johnstone & Shu (1993) and Hollenbach et al. (1994) produced a model in which the massive disc that surrounds an O star during the early stages of its evolution is photoevaporated to form a compact HII region which is constantly replenished by the disc.

This theory therefore naturally produces long lived regions, as long as the star is surrounded by a disc.

De Pree, Rodriguez & Goss (1995) and De Pree, Goss & Gaume (1998) suggested that the density of gas surrounding UCHII regions may be much higher than expected (_∼ 107cm−3).

6.1. Introduction Therefore the increased external pressure on confined HIIregions may maintain their small size for∼105 years. Similarly Xie et al. (1996) suggested that turbulent pressure from molecular clouds would provide sufficient increases in external pressure to explain the longevity of these regions. Again, while these models explain the long lifetimes of UCHII regions, they do not explain the morphologies.

Dyson, Williams & Redman (1995) propose a model in which the stellar wind from the mas- sive central star is mass loaded from the surface of pressure-confined self gravitating clumps in the UCHIIregion. This results in a recombination rather than an ionisation front surround-

ing the UCHII region since the ionised clumps from the interior of the region are moved to the edges, where recombination occurs. This confines the size of the regions thus extending the time HII regions spend in their ultra compact phase. Again, this does not explain the morphology of UCHIIregions.

Finally, van Buren et al. (1990) suggested that the cometary appearance of UCHIIregions could be a result of stellar wind bow shocks as the massive central star moves through the molecular cloud.

Churchwell (1999) suggested that each of these mechanisms may be important at different stages of the regions evolution. For example, “blister” models are likely to be relevant when the star leaves its parent cloud while the disc model could logically be important while the star has a disc. While each of these models is able to explain the morphologies or lifetime of UCHII

regions, none is able to explain both the range of different morphologies and the timescales. It is therefore clear that more work needs to be done to present a new theory of the evolution of these regions.

Surveys of UCHIIregions show that many are surrounded by photoionised extended low density gas, possibly resulting from external sources (e.g. Kurtz et al. 1999, Kurtz, Churchwell & Wood 1994, Wood & Churchwell 1989b, Kim et al. 2001, Ellingsen, Shabala & Kurtz 2005). While Kurtz et al. (1999) and Kim et al. (2001) found that many of their observed sources contained diffuse emission, Ellingsen, Shabala & Kurtz (2005) produced a survey based on much younger UCHII regions and found less extended emission, suggesting this emission is

related to the age of the region. Using 21cm emission line data, Kim et al. (2001) showed that the UCHIIregions and extended emission in these areas was at almost the same velocity,

explained in terms of a champagne flow in a non uniform structure that allows photons to travel out of the UCHII region in to the diffuse surroundings. If this is the case, it would require UCHIIregions to be close to the edge of their parent clouds, supporting the theory of blister regions to explain the cometary morphology. The origin of this emission is still not fully understood, and therefore more observations of the radio emission lines are required.