K-band variability as a method to select young stellar object candidates

A study of the applicability of K- (2.2 mu m) band variability as a method to separate young stellar objects (YSOs) from held stars in young embedded clusters is presented. Deep K-band imaging of the central similar to 8' x 6' region of the Serpens Cloud Core was made at several epochs in 1995 and 1996. Variability in K was detectable at peak-to-peak amplitudes as small as typically Delta K = 0.15 at a conservative 3 sigma level for sources brighter than K approximate to 15. This level increases gradually to Delta K = 1.3 at K approximate to 18. From a multi-epoch sample of about 1000 stars (K < 18), a total of 55 stars have been found to exhibit variability in the K band at observed amplitudes from 0.15 to 2.2 mag, and nine of these have varied at several of the investigated epochs. Thirty-nine of the 55 variable sources were not previously recognized and are hereby proposed as new YSO candidates. Not less than 26% of the total sample of already known YSOs within this region were found to vary over a timescale of only 1 yr. We conclude that the K-variability method is efficient in finding low-luminosity young sources in deeply embedded clusters. In addition, deep J (1.25 mu m) and H (1.65 mu m) imaging of the same region revealed 37 near-IR excess sources from their position in the J-H/H-K diagram, and 17 of these are new. One additional YSO candidate was found from association with nebulosity. The total of 57 new YSO candidates increases the currently recognized YSO population by 85% within the observed field. The K luminosity function continues to rise down to K approximate to 16, and no turnover is found at K > 16 that might not be due to sample incompleteness. The new sources are probably very low mass pre-main-sequence stars and young brown dwarfs.