Hα Emission Stars and Herbig-Haro Objects in the Vicinity of Bright-rimmed Clouds

Bright-rimmed clouds (BRCs) found in H II regions are probably sites of triggered star formation due to compression by ionization/shock fronts. Many BRCs harbor IRAS point sources of low dust temperature. They also frequently contain a small cluster of near-IR stars that is elongated along the axis of the BRC. Here we present the results of our Hα grism spectroscopy and narrowband imaging observations of BRCs in search of candidate pre–main-sequence stars of the T Tauri, Herbig Ae/Be and related types, and Herbig-Haro (HH) objects. We have detected a large number (460) of Hα emission stars down to a limiting magnitude of about R = 20 in and around all but two of the 28 BRCs observed. The present study has, for the first time, reached down nearly to the faintest classical T Tauri star population in OB associations. A total of 12 new HH objects have been found. Most are of small apparent size, emphasizing the need for deep searches at high spatial resolution, but HH 588 associated with BRC 37 represents a huge HH complex composed of two-staged bow shocks on both sides of a tiny central knot. These stars and HH objects are concentrated near the tip of bright rims (i.e., in the head of the BRCs and just outside the rims) and often make loose aggregates similar to those of near-IR stars, thus supporting our hypothesis of "small-scale sequential star formation." The presence of such a large number of Hα emission stars in the immediate vicinities of BRCs implies that second-generation formation of low-mass stars is relatively extensive and further supports the notion of cohabitation of high- and low-mass populations in OB associations.