2008A&A...488..987H

To understand low- to intermediate-mass star-formation in the nearby RCrA molecular cloud, we try to identify the stellar content that is accessible with near-infrared observations. We obtained a JHK_s band mosaic of ∼10'x60' covering the entire RCrA molecular cloud with unprecedented sensitivity. We present a catalogue of about 3500 near-infrared sources fainter than the saturation limit K_s∼10mag, reaching K_s∼18mag. We analysed the extended sources by inspecting their morphology and point sources by means of colour-colour and colour-magnitude diagrams. Additionally, we compared the extinction inferred from the NIR data with the line-of-sight dust emission at 1.2mm. Sources towards high dust emission but relatively low H-K_s show a projected mm-excess; these sources are either immediately surrounded by cold circumstellar material or, if too red to be a true foreground object, they are embedded in the front layer of the 1.2mm emitting dust cloud. In both cases they are most likely associated with the cloud. By means of the projected mm-excess technique we find 33 new faint near-infrared sources deeply embedded in the Coronet cluster around RCrA, for which so far about 20 bright infrared stars have been known. In contrast to the Coronet region, both the northwestern dust ridge and the southeastern cloud condensation ``C'' appear to be devoid of associated stars detectable with our near-infrared data. Furthermore, about a dozen sources, which are spread over the entire molecular cloud region, exhibit a possible K-band excess, but only with marginal statistical significance (<3σ), so that we do not consider the indicated K-band excess as real. Finally, while the Herbig-Haro-like objects seen on our maps are concentrated around the Coronet, we find four new nebulae also located farther down to the southeast. At the position of IRAS18595-3712, an X-shaped bipolar nebula is resolved; its exciting star is hidden behind an edge-on disc. The deep near-infrared survey of the entire RCrA molecular cloud strengthens the evidence for the Coronet being the region where most of the young stars are found. Our results are consistent with earlier predictions that the RCrA cloud has fragmented into sub-condensations at different star-forming stages.