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

Abstract

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 $0.035''$-$0.078''$ ($\sim175$-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 $\sim175$ au, with a deconvolved radius of $\sim87$ au and a nearest neighbouring fragment at a projected separation of $\sim2400$ au. The continuum peak reaches a brightness temperature of $\sim40$ K, exceeding the $\sim10$ K gas temperatures inferred from lower-resolution NH$_3$ observations and indicating a strong radial temperature gradient. Continuum-based mass estimates range from $\sim2$ to $\sim30$ $M_\odot$ (implying extreme densities of $n_\mathrm{H2} > 10^9$cm$^{-3}$), depending on temperature, while lower-resolution data indicate a much larger mass reservoir of $\sim110$ $M_\odot$ on $\sim10^4$ 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]