Variable stars in the Sh 2-170 H II region

The period distribution of the PMS variables shows two peaks, one near ~1.5d and the other near ~4.5d. It is found that the younger stars with thicker discs and envelopes seem to rotate slower than their older counterparts. These properties of the PMS variables support the disc-locking mechanism.

Both the period and amplitude of PMS stars show decrease with increasing mass probably due to the effective dispersal of circumstellar discs in massive stars. Our results favour the notion that cool spots on weak line T-Tauri stars are responsible for most of their variations, while hot spots on classical T-Tauri stars resulting from variable mass accretion from an inner disc contribute to their larger amplitudes and irregular behaviours.