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We present a composite model and radiative transfer simulations of themassive star forming core W33A MM1. The model was tailored to reproduce thecomplex features observed with ALMA at $\approx 0.2$ arcsec resolution inCH$_3$CN and dust emission. The MM1 core is fragmented into six compact sourcescoexisting within $\sim 1000$ au. In our models, three of these compact sourcesare better represented as disc-envelope systems around a central (proto)star,two as envelopes with a central object, and one as a pure envelope. The modelof the most prominent object (Main) contains the most massive (proto)star($M_\star\approx7~M_\odot$) and disc+envelope($M_\mathrm{gas}\approx0.4~M_\odot$), and is the most luminous($L_\mathrm{Main} \sim 10^4~L_\odot$). The model discs are small (a few hundredau) for all sources. The composite model shows that the elongated spiral-likefeature converging to the MM1 core can be convincingly interpreted as afilamentary accretion flow that feeds the rising stellar system. The kinematicsof this filament is reproduced by a parabolic trajectory with focus at thecenter of mass of the region. Radial collapse and fragmentation within thisfilament, as well as smaller filamentary flows between pairs of sources areproposed to exist. Our modelling supports an interpretation where what was onceconsidered as a single massive star with a $\sim 10^3$ au disc and envelope, isinstead a forming stellar association which appears to be virialized and toform several low-mass stars per high-mass object.

Radiative transfer modelling of W33A MM1: 3-D structure and dynamics of a complex massive star forming region