Cracking open the "dragon egg": ALMA 1.3mm observations of a compact, young high-mass core candidate at 30mas resolution

Massive prestellar cores are a central assumption of core-accretion models of high-mass star formation, yet convincing observational examples remain rare. The “dragon egg” (C2c1a) core in the infrared-dark cloud G028.37+00.07 has been proposed as a promising compact high-mass core candidate. We present new ALMA Band 6 observations at ${0.035}^{\mathrm{\prime }\mathrm{\prime }}$-${0.078}^{\mathrm{\prime }\mathrm{\prime }}$ ($\sim 175$-400 au at 5 kpc), providing one of the most stringent high-resolution tests of such a candidate. The region resolves into multiple fragments, but the brightest component, C2c1a(a), remains a single compact continuum peak down to $\sim 175$ au, with a deconvolved radius of $\sim 87$ au and a nearest neighbouring fragment at a projected separation of $\sim 2400$ au. The continuum peak reaches a brightness temperature of $\sim 40$ K, exceeding the $\sim 10$ K gas temperatures inferred from lower-resolution NH${}_3$ observations and indicating a strong radial temperature gradient. Continuum-based mass estimates range from $\sim 2$ to $\sim 30$ $M_{\odot }$ (implying extreme densities of $n_{\mathrm{H}2}>{10}^9$cm${}^{-3}$), depending on temperature, while lower-resolution data indicate a much larger mass reservoir of $\sim 110$ $M_{\odot }$ on $\sim {10}^4$ au scales. The spectrum shows no strong hot-core tracers and only weak, spatially extended H${}_2$CO and CO emission; no high-velocity outflow can be unambiguously associated with C2c1a(a). We conclude that C2c1a(a) is an exceptionally compact core embedded within a massive cold reservoir, with no additional compact fragments detected within the present resolution and sensitivity limits, potentially tracing the earliest formation of a central protostellar “seed” within a still-dominant reservoir. [abridged]