Zavagno et al. (2006) studied star formation in the Milky Way source W 79 (Fig. 4). It has a 1.7 Myr old shell with gravitationally collapsed regions 0.1 Myr old along the perimeter. This is an example of star formation triggering by the gravitational collapse of swept-up gas around an older cluster or OB association. Sh2-219 is a similar region (Deharveng et al. 2006). There is O9.5V star in a centralized HII region, and a CO cloud, K-band embedded cluster, Ultra-compact HII region, and Herbig Be star at the edge.
Figure 4. Milky Way region W 79, consisting of a shell with dense clouds and star formation at the edge, from Zavagno et al. (2006).
Deharveng et al. (2010) recently studied 102 bubbles and triggered star formation using the Spitzer-GLIMPSE and MIPSGAL surveys for the IR, the MAGPIS and VGPS surveys for the radio continuum, and the ATLASGAL survey at 870 µm for cold dust emission. They found that 86% of the bubbles contain HII regions, and among those with adequate resolution, 40% have cold dust along their borders, presumably accumulated during the bubbles' expansions. Eighteen bubbles have either ultracompact HII regions or methanol masers in the peripheral dust, indicating triggering. They categorized their results into several types of triggering. Star formation that occurs in pre-existing cloud condensations is distinctive because the clouds protrude into the bubble cavity like bright rims; 28% of the resolvable shells are like this. Star formation that occurs by the collapse of swept-up gas does not protrude but is fully in the shell. That is because it is comoving with the shell. In fact, clumps forming by gravitational collapse in a shell could eventually protrude out of the front of the shell because their higher column densities makes them decelerate slower than the rest of the shell (Elmegreen 1989). If this is observed, then the relative position of a triggered clump and the shell around it should indicate their relative speeds and the time when the clump first formed.
Beerer (2010) studied Cygnus X North with Spitzer IRAC, classifying stars according to their IR spectral ages. They found that younger objects are in filaments that look compressed. Triggering of those stars was suggested.
Desai (2010) examined all 45 known supernova remnants in the LMC and looked for associations with young stellar objects and with GMCs that have no YSOs. Seven SNR were associated with GMCs and YSOs, 3 SNRs were with YSOs and no GMCs, and 8 SNRs were with GMCs and no YSOs. For the 10 SNRs with YSOs, only 2 have YSOs that are clearly associated with the SN shell, but in these cases, the SNe are younger than YSOs, so the YSOs could not have been triggered. Desai et al. concluded that SNe are too short-lived for triggering.