Abstract

This thesis presents an observational study of one of the nearest star-forming regions, the ρ Ophiuchi cluster, with the goal of characterising stellar activity in young stellar objects. Variability is a common characteristic of pre-main-sequence (PMS) stars and originates from mechanisms related to magnetic fields, accretion discs, and circumstellar extinction. Variability surveys in the infrared (IR) can probe stellar and circumstellar environments and provide information about the dynamics of the on going magnetic and accretion processes. To this end, a large-field (∼0.8 deg2), near-IR photometric survey of ρ Ophiuchi has been conducted using the Wide Field Camera (WFCAM) at the United Kingdom Infrared Telescope (UKIRT), which monitored the cluster over 14 nights in two consecutive years. Statistical tools, such as the multi-band cross correlation index and the reduced chi-square, were used to disentangle signals of variability from noise. Variability in the near-IR is found to be present in half of the known population of ρ Ophiuchi. The behaviours shown are several and can be associated with the existence of spots on the stellar surface, variations in circumstellar extinction, or changes in the geometry of an accretion disc. Furthermore, this thesis explores the feasibility of using photometric variability as a criterion for selection of new low-luminosity candidates of galactic star forming regions down to very low masses. The use of this new selection method could help remove the biases from other techniques. For example, it has the advantage of identifying young sources that do not show IR-excess or have small Hα emission. The use of this relatively new technique should contribute significantly to obtain a full census of the young objects in star formation regions, which remains one of the most important goals of galactic star formation studies. Using variability, a new population of objects has been uncovered, that is believed to be part of the ρ Ophiuchi cluster. Using low-resolution near-IR spectroscopy, 4 candidate members have been confirmed as PMS stars. Finally, the survey presented in this thesis is the first of its kind in the IR, both in large area surveyed and sensitivity, taking advantage of recent developments in IR astronomical instrumentation. Of future relevance for IR variability studies is the new IR-imager at the Very Large Telesope (VLT), HAWK-I (High-Acuity Wide-field K band Imager), the most advanced IR camera on an 8-meter telescope at the present moment. The instrument was assembled and commissioned during the progress of this thesis, and is presented here, giving emphasis to the contribution of the author to the project.