THE LKH-ALPHA 101 INFRARED CLUSTER

An 8' x 12' area, corresponding to a 1.9 x 2.8 pc field centered on the Herbig Ae/Be star, LkH-alpha 101, has been mapped in the near-infrared J and K (1.2 and 2.2-mu-m) bandpasses at 1''.35 pixel-1 resolution using IRIM on the KPNO 1.3 m telescope. We report the discovery of an embedded cluster of more than 100 young stellar objects within this area. We estimate our completeness limits to be 16.5 mag at J and 14.5 mag at K, although sources as faint as K = 16.0 and J = 17.5 were detected. We have identified four types of cluster members, defined by their location in the K versus J-K color magnitude diagram: (1) accreting protostars (the Class I sources of Lada and Wilking, which are the sources with J-K > 4), (2) probable accreting protostars (sources with 3 < J-K < 4), (3) embedded, low-mass, pre-main-sequence (PMS) stars younger than 10(6) yr (sources with K < 13.4 and 2 < J-K < 3), and (4) young (almost-equal-to 10(6) yr), embedded, brown dwarf candidates (sources with K > 13.4 and 2 < J-K < 3). According to this classification scheme, the embedded cluster population of the LkH-alpha 101 cloud core consists of 16 Class I sources, 39 probable Class I sources, 40 young, embedded, low-mass, PMS stars, and 46 young, embedded, brown dwarf candidates. Only eight of the latter have J counterparts. From the relative numbers of sources of each type, we estimate an upper limit for the accreting protostar phase of 4 x 10(5) yr. The spatial distribution of the sources in the cloud core is highly nonuniform, such that the youngest sources are the most centrally concentrated. In keeping with this trend, it appears that the most massive star of the cluster, LkH-alpha 101, is probably the youngest. There is also a hint of mass segregation in the cluster, such that almost-equal-to 58% of the brown dwarf candidate sources are concentrated in 17% of the observed area. The star formation efficiency (SFE) in the central 2' x 2' of this cluster is almost-equal-to 18%. In the densest parts of the cluster, the observed stellar density places a limit of 1 star per almost-equal-to (2.7 x 10(17) cm)3 for the initial volume of gas which can eventually form a star. This volume corresponds to initial masses of 0.045, 0.45, and 4.5 M. for uniform densities of rho-H2 = 10(3), 10(4), and 10(5) cm-3, respectively. We find an upper limit of 4 x 10(5) yr for the protostellar collapse phase in this cluster and infall rates of 1.1 x 10(-7), 1.1 x 10(-6), and 1.1 x 10(-5) M. yr-1, corresponding to the above-quoted stellar masses if the collapse time is assumed independent of the final stellar mass.