NEAR-IR IMAGING PHOTOMETRY OF THE J-K-GREATER-THAN-4 SOURCES IN THE LK H-ALPHA-101 INFRARED CLUSTER

We present J, H, K and narrow band L (nbL) near-IR (NIR) imaging photo metry of the fields immediately surrounding the 16 very red objects (J -K>4) discovered by Barsony et al. (1991), in the immediate vicinity o f the Herbig Ae/Be star Lk Halpha 101. Based on their location in the K vs. J-K colour-magnitude diagram, these sources were provisionally d esignated Class I sources i.e. highly obscured accreting protostars. D ue to a lack of photometry in three NIR passbands, no detailed investi gation 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, an d further investigate the premain sequence (PMS) stellar population ar ound LK Halpha 101. At 2.2 mum, we find 51 stellar objects to a 5sigma limiting magnitude of K=16.8 within the 16 (38'' x 36'' or 0.14pc x 0 .15pc) fields around Lk Halpha 101. Of these stars, 47 were detected a t J, H and K. A total of nine sources (including Lk Halpha 101) were f ound with J-K>4; two of these objects show a significant 2mum NIR ther mal excess. At nbL, 17 of the 51 stellar objects were detected to a 5s igma limiting magnitude of L=11.8. Of the nine J-K>4 sources, seven we re detected at nbL, five show NIR 3mum thermal excess emission. In tot al, 15 out of the 51 sources (approximately 30%) show evidence suggest ing they are in a PMS evolutionary state. We conclude that i) a cluste r of young PMS stars does exist around Lk Halpha 101, confirming the d iscovery 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 d efinition of their respective spectral energy distributions, and iii) NIR J-H vs. H-K colour-colour diagrams are generally incomplete in the ir identification of the young PMS population showing thermal NIR exce ss. We finally note that a complete J, H, K and nbL survey is a good w ay to fully explore the membership and extent of the PMS population in a region of embedded star formation.