  OB associations are the prevailing star forming sites in the Galaxy. Up to
now, the process of how OB associations were formed remained a mystery. A
possible process is self-regulating star formation driven by feedback from
massive stars. However, although a number of observational studies uncovered
various signposts of feedback-driven star formation, the effectiveness of such
feedback has been questioned. Stellar and gas kinematics is a promising tool to
capture the relative motion of newborn stars and gas away from ionizing
sources. We present high-resolution spectroscopy of stars and gas in the young
open cluster NGC 1893. Our findings show that newborn stars and the tadpole
nebula Sim 130 are moving away from the central cluster containing two O-type
stars, and that the timescale of sequential star formation is about 1 Myr
within a 9 parsec distance. The newborn stars formed by feedback from massive
stars account for at least 18 per cent of the total stellar population in the
cluster, suggesting that this process can play an important role in the
formation of OB associations. These results support the self-regulating star
formation model.
