A near-infrared variability study in the cloud IC1396W: low star-forming efficiency and two new eclipsing binaries

Identifying the population of young stellar objects (YSOs) in high-extinction regions is a pre-requisite for studies of star formation. This task is not trivial, as reddened background objects can be indistinguishable from YSOs in near-infrared colour-colour diagrams. Here we combine deep JHK photometry with J- and K-band light curves, obtained with the United Kingdom Infrared Telescope, to explore the YSO population in the dark cloud IC1396W. We demonstrate that a colour-variability criterion can provide useful constraints on the star-forming activity in embedded regions. For IC1396W we find that a near-infrared colour analysis alone vastly overestimates the number of YSOs. In total, the globule probably harbours not more than 10 YSOs, among them a system of two young stars embedded in a small (similar to 10 000 au) reflection nebula. This translates into a star-forming efficiency (SFE) of similar to 1 per cent, which is low compared with nearby more massive star-forming regions, but similar to less massive globules. We confirm that IC1396W is likely associated with the IC1396 H ii region. One possible explanation for the low SFE is the relatively large distance to the ionizing O star in the central part of IC1396. Serendipitously, our variability campaign yields two new eclipsing binaries, and eight periodic variables, most of them with the characteristics of contact binaries.