2016MNRAS.459.1946P

The far-infrared IRAS 15507-5359 source is known to be a medium-mass star-forming region associated with a compact H ii region and a near-infrared embedded cluster. We present a survey of infrared-calibrated images ranging from 1.2 to 500 µm obtained with the Baade telescope at Las Campanas Observatory, and the Herschel space telescope with additional archive Spitzer data. We confirm the distance to the complex to be 5.0 kpc. Three Herschel far-infrared sources are found, I, II, III, identified with dense cores at different evolutionary stages. One (III) is a starless infrared dark cloud showing, near its edge, two infrared reflection nebulae (R1) and (R2) with dispersed young stellar populations, including a knot of shocked H₂ line emission. Both show considerable polycyclic aromatic hydrocarbon emission. Core II has associated a radio H ii region and a deeply embedded one-million-year-old cluster (Cl 1) that contains more than 45 young stellar objects, reddened by at least 20 visual magnitudes. About 20 per cent of them show considerable infrared excess emission. Core I appears void of a near-infrared population, and coincides with a long emission bar that resembles a photodissociation front. We determine the properties of the two most luminous Class I sources in the region by fitting models of young stars with accreting discs and envelopes to their 1-500 µm spectral energy distributions. This is another example of a medium-mass region with at least three well-defined active centres of star formation separated by about 1 pc and at different evolutionary stages.