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The HII region IC 1848 harbors a lot of intricate elephant trunk-likestructures that look morphologically different from usual bright-rimmed clouds(BRCs). Of particular interest is a concentration of thin and long elephanttrunk-like structures in the southeastern part of IC 1848E. Some of them havean apparently associated star (or two stars) at their very tip. We conducted$VI_{c}$ photometry of several of these stars. Their positions on the$V/(V-I_{c})$ color-magnitude diagram as well as the physical parametersobtained by SED fittings indicate that they are low-mass pre-main-sequencestars having ages of mostly one Myr or less. This strongly suggests that theyformed from elongated, elephant trunk-like clouds. We presume that suchelephant trunk-like structures are genetically different from BRCs, on thebasis of the differences in morphology, size distributions, and the ages of theassociated young stars. We suspect that those clouds have been caused byhydrodynamical instability of the ionization/shock front of the expanding HIIregion. Similar structures often show up in recent numerical simulations of theevolution of HII regions. We further hypothesize that this mechanism makes athird mode of triggered star formation associated with HII regions, in additionto the two known mechanisms, i.e., collect-and-collapse of the shellaccumulated around an expanding HII region and radiation-driven implosion ofBRCs originated from pre-existing cloud clumps.

Stars at the Tip of Peculiar Elephant Trunk-Like Clouds in IC 1848E: A Possible Third Mechanism of Triggered Star Formation