Sequential Star Formation Triggered by Expansion of an H ii Region

Abstract

We show three-dimensional numerical simulations in which stars form sequentially in a filamentary molecular cloud. The star formation is triggered by expansion of an H II region. The H II region is distant from the filamentary cloud at the initial stage. As it expands, it interacts with the filamentary cloud. The cloud is pinched and separated into two parts. Subsequently, the gravitational instability is induced to form two cores of the first generation along the filament axis in a typical model. The separation of the two cores is several times larger than the filament diameter. It is comparable to the wavelength of the fastest growing fragmentation mode. The first-generation cores become isolated, and filamentary clouds shorten to widen the separation. New cores of second generation form at the edges of the shortened filamentary clouds. This core formation is recursive, and our model shows sequential star formation triggered by an expanding H II region. The age difference is several times that of the dynamical timescale between the first-and second-generation cores. This sequential star formation is similar to that observed in the filamentary cloud associated with the H II region NGC 2024. Our first-generation cores correspond to FIR 4 and FIR 5, while the second-generation cores correspond to FIR 3 and FIR 6.