1994A&A...288..849A

We present J, H, K and narrow band L (nbL) near-IR (NIR) imaging photometry of the fields immediately surrounding the 16 very red objects (J-K>4) discovered by Barsony et al. (1991), in the immediate vicinity of the Herbig Ae/Be star Lk Hα 101. Based on their location in the K vs. J-K colour-magnitude diagram, these sources were provisionally designated Class I sources i.e. highly obscured accreting protostars. Due to a lack of photometry in three NIR passbands, no detailed investigation of the thermal emission characteristics and NIR excesses in the sources could be made. Our aims here are to quantify the NIR colours of these sources, determine whether they are truly Class I sources, and further investigate the pre-main sequence (PMS) stellar population around Lk Hα 101. At 2.2 µm, we find 51 stellar objects to a 5σ limiting magnitude of K=16.8 within the 16 (38"x36" or 0.14pcx0.15pc) fields around Lk Hα 101. Of these stars, 47 were detected at J, H and K. A total of nine sources (including Lk Hα 101) were found with J-K>4; two of these objects show a significant 2µm NIR thermal excess. At nbL, 17 of the 51 stellar objects were detected to a 5σ limiting magnitude of L=11.8. Of the nine J-K>4 sources, seven were detected at nbL, five show NIR 3µm thermal excess emission. In total, 15 out of the 51 sources (∼30%) show evidence suggesting they are in a PMS evolutionary state. We conclude that i) a cluster of young PMS stars does exist around Lk Hα 101, confirming the discovery of Barsony et al. (1991), ii) a population of highly reddened (J-K>4) PMS objects are present in this cluster; determining whether these objects are true accreting protostars requires a more complete definition of their respective spectral energy distributions, and iii) NIR J-H vs. H-K colour-colour diagrams are generally incomplete in their identification of the young PMS population showing thermal NIR excess. We finally note that a complete J, H, K and nbL survey is a good way to fully explore the membership and extent of the PMS population in a region of embedded star formation.