Deep near-IR variability survey of pre-main-sequence stars in ρ  Ophiuchi^*

European Southern Observatory (ESO), Karl-Schwarzschild-Strasse 2, 85748 Garching bei Muenchen, Germany e-mail: coliveir@eso.org

Received: 26 November 2007
Accepted: 1 April 2008

Abstract

Context. Variability is a common characteristic of pre-main-sequence stars (PMS). Near-IR variability surveys of young stellar objects (YSOs) can probe stellar and circumstellar environments and provide information about the dynamics of the ongoing magnetic and accretion processes. Furthermore, variability can be used as a tool to uncover new cluster members in star formation regions.

Aims. We hope to achieve the deepest near-IR variability study of YSOs targeting the ρ  Ophiuchi cluster.

Methods. Fourteen epochs of observations were obtained with the Wide Field Camera (WFCAM) at the UKIRT telescope scheduled in a manner that allowed the study of variability on timescales of days, months, and 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 characteristics are compared to existing models of YSOs in order to relate them to physical processes, and then used to select new candidate members of this star-forming region.

Results. Variability in the near-IR is found to be present in 41% of the known population of ρ  Ophiuchi recovered in our sample. 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. Using variability, a new population of objects has been uncovered that is believed to be part of the ρ  Ophiuchi cluster.