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Tightening the belt: Constraining the mass and evolution in SDC335

Recent ALMA observations identified one of the most massive star-forming cores yet observed in the Milky Way; SDC335-MM1, within the infrared dark cloud SDC335.579-0.292. Along with an accompanying core MM2, SDC335 appears to be in the early stages of its star formation process. In this paper we aim to constrain the properties of the stars forming within these two massive millimetre sources. Observations of SDC335 at 6, 8, 23 and 25GHz were made with the ATCA. We report the results of these continuum measurements, which combined with archival data, allow us to build and analyse the spectral energy distributions (SEDs) of the compact sources in SDC335. Three HCHII regions within SDC335 are identified, two within the MM1 core. For each HCHII region, a free-free emission curve is fit to the data allowing the derivation of the sources' emission measure, ionising photon flux and electron density. Using these physical properties we assign each HCHII region a ZAMS spectral type, finding two protostars with characteristics of spectral type B1.5 and one with a lower limit of B1-B1.5. Ancillary data from infrared to mm wavelength are used to construct free-free component subtracted SEDs for the mm-cores, allowing calculation of the bolometric luminosities and revision of the previous gas mass estimates. The measured luminosities for the two mm-cores are lower than expected from accreting sources displaying characteristics of the ZAMS spectral type assigned to them. The protostars are still actively accreting, suggesting that a mechanism is limiting the accretion luminosity, we present the case for two different mechanisms capable of causing this. Finally, using the ZAMS mass values as lower limit constraints, a final stellar population for SDC335 was synthesised finding SDC335 is likely to be in the process of forming a stellar cluster comparable to the Trapezium Cluster and NGC6334 I(N).