Variable stars in the Sh 2-170 HII region
  We present multi-epoch deep ($\sim$20 mag) $I_{c}$~band photometric
monitoring of the Sh 2-170 star-forming region to understand the variability
properties of pre-main-sequence (PMS) stars. We report identification of 47
periodic and 24 non-periodic variable stars with periods and amplitudes ranging
from $\sim$4 hrs to 18 days and from $\sim$0.1 to 2.0 mag, respectively. We
have further classified 49 variables as PMS stars (17 Class\,{\sc ii} and 32
Class\,{\sc iii}) and 17 as main-sequence (MS)/field star variables. A larger
fraction of MS/field variables (88\%) show periodic variability as compared to
the PMS variables (59\%). The ages and masses of the PMS variable stars are
found to be comparable with those of T-Tauri stars. Their variability
amplitudes show an increasing trend with the near-IR/mid-IR excess. The period
distribution of the PMS variables shows two peaks, one near $\sim$1.5 days and
the other near $\sim$4.5 days. 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.
