HII regions interact with their neighboring molecular clouds by pushing away the lower density material faster than the higher density cloud cores. This leads to bright rims and pillars. Most HII regions contain these shapes, as they are commonly observed in Hubble Space Telescope images of nebulae. Triggered star formation in the dense heads of pillars has been predicted (e.g., Klein, Sandford & Whitaker 1980) and observed for many years (e.g., Sugitani et al. 1989). Here we review some recent observations.
Recent simulations of bright rim and pillar formation are in Mellema et al. (2006), Miao et al. (2006), and Gritschneder et al. (2009). In a large HII region, there can be many bright rims with star formation in them. A good example is 30 Dor in the LMC, which has bright-rims that look like they have triggered star formation in many places (Walborn, Maíz-Apellániz & Barbá 2002).
IC 1396 is an HII region with a shell-like shape. The radius is 12 pc, and the expansion speed is 5 km s-1, making the expansion time 2.5 Myr (Patel et al. 1995). The shell contains several bright rims and pillars around the edge that all point to the sources of radiation. In optical light, few embedded stars can be seen, but in the infrared there are often embedded stars.
The stellar content of the large pillar in IC 1396 has been studied by Reach et al. (2009; see Figure 2). Several Class I protostars are located in the main head and in a shelf off the main pillar. Class II stars are scattered all over the region with no particular association to the cloud. The Class I stars recently formed in the pillar, and considering that they are much younger than the HII region, they could have been triggered.
Getman et al. (2007) observed IC 1396N, another bright rim in the same region, in x-ray and found an age sequence that suggests triggering from south to north, into the rim. There are class III and class II stars around the rim and class I/0 stars inside. Beltrán et al. (2009) did a JHK survey of the same bright rim and found few NIR-excess sources and no signs of clustering toward the southern part of the rim. They also found no color or age gradient in the north-south direction. They concluded there was no triggering but perhaps there was a gradient in the erosion of gas around protostars. Choudhury et al. (2010) observed the region with Spitzer IRAC and MIPs and suggested there was an age sequence with the younger stars in the center of the bright rim and the older stars near the edge. They derived a propagation speed into the rim of 0.1-0.3 km s-1.
The pillars of the Eagle Nebula, M16, are among the most famous cloud structures suggestive of triggering. Several young stars appear at the tips. It is difficult to tell if these stars were triggered by the pressures that made the pillars, or if they existed in the head regions before the HII pressure swept back the periphery. Triggering requires that the pillar stars are much younger than the other stars in the region. Some exposure of existing stars could be possible if there is a wide range of ages among the pillar and surrounding stars.
Sugitani et al. (2002) found Type I sources near the pillar heads in M16 and older sources all around the pillars. They suggested there was an age sequence within the pillar. Fukuda et al. (2002) observed M16 in 13CO, C18O, and 2.7 mm emission, finding a high density molecular core at the end of the pillar, as expected from HII region compression. However, Indebetouw et al. (2007) suggested that the young objects in the area are randomly distributed and not triggered. They showed the distribution of protostars in various stages of accretion and saw no clear patterns with age. Thus the issue of triggering in the M16 pillars seems unresolved.
Guarcello et al. (2010) found a different age sequence in M16: the stars in the northwest part of the whole HII region are younger than the stars in the southeast part. They suggested that a 200 pc shell triggered both M16 and M17 3 Myr ago on much larger scales.
IC 5146 is a filamentary cloud with low-level star formation at one end (Lada, Alves & Lada 1999). It is not in an HII region and the source of the structure and pressure to shape it is not evident. It looks like it was formerly a diffuse cloud that was compressed at one end by a supernova. The ends of a filamentary cloud are the most susceptible parts to this kind of random disturbance.
Hosokawa & Inutsuka (2007) studied molecule formation in compressed shells around HII regions and suggested that H2 could form without bright CO emission during the expansion of the shell into a cold neutral medium. They found an example of this in the shell around the W3-4-5 region. There is cold HI and perhaps unobserved H2, without any evident CO. They proposed that this is an intermediate stage in the collapse of a swept up shell and the site for future triggered star formation.
Figure 3. Large-scale dust map of the Ophiuchus region, from Lombardi, Lada & Alves (2008).
The Ophiuchus cloud core was swept back by pressures from the Sco-Cen association (de Geus 1992). Star formation in the rho Oph region could have been triggered at the same time. A large-scale dust map of the whole region is in Lombardi et al. (2008; see Figure 3). There are many protostars and dense cores (e.g., Kirk, Ward-Thompson & André 2005) in what looks like a giant pillar. Sco-Cen is off the field to the upper right.
The Carina nebula has many bright rimmed clouds and pillars that were recently studied by Smith (2010). They note that the young stars lag the bright rims, as if they were left behind in an advancing ionization front of cloud destruction.