1996A&A...307..775M

We present J, H, and K-band mosaics of a ∼1.5'x1.5' field in the W3 Main molecular core, a massive molecular core in the process of forming an OB association. Our mosaics include the HII regions W3 A and W3 B and a massive molecular clump surrounding W3 IRS 5. These data show a large, embedded population of intermediate to low mass stars co-existing with recently formed OB stars. The highest density of embedded stars is found in a compact cluster surrounding IRS 5; to a limiting absolute magnitude of M_K=5.1, we estimate a stellar density of 1300-4300pc^–3. A comparison of the m_K vs. H-K diagrams for the W3 population and for nearby YSOs shows that most of the stars have colors and magnitudes consistent with reddened T Tauri stars, i.e. low mass pre-main sequence stars. On the basis of their dereddened K-band magnitudes, we find that 60% of the stars detected in H and K have an upper mass limit of 2M_☉, supporting our claim that most of the embedded stars are low mass stars. The K-band luminosity function is consistent with the KLF of a 0.3Myr cluster with a Miller-Scalo initial mass function (IMF); however, an older cluster with an IMF flatter than the Miller-Scalo IMF is not excluded. Comparing the results of radio surveys of the IRS 5 clump with our infrared data, we find evidence that the >10M_☉ stars traced by the radio observations are more numerous than predicted by a Miller-Scalo IMF. We estimate a star formation efficiency in the molecular clump surrounding IRS 5 of 6-18%, suggesting that most of the molecular mass has not been converted into stars. We also report the detection of candidate exciting stars for the W3 B, W3 E and W3 F ultracompact HII regions.