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

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'' (∼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 ∼175 au, with a deconvolved radius of ∼87 au and a nearest neighbouring fragment at a projected separation of ∼2400 au. The continuum peak reaches a brightness temperature of ∼40 K, exceeding the ∼10 K gas temperatures inferred from lower-resolution NH₃ observations and indicating a strong radial temperature gradient. Continuum-based mass estimates range from ∼2 to ∼30 M_⊙ (implying extreme densities of nH2 > 10⁹cm⁻³), depending on temperature, while lower-resolution data indicate a much larger mass reservoir of ∼110 M_⊙ on ∼10⁴ au scales. The spectrum shows no strong hot-core tracers and only weak, spatially extended H₂CO 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]

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BibTeX

@misc{barnes2026cracking,
  title = {{Cracking open the "dragon egg": ALMA 1.3mm observations of a compact, young high-mass core candidate at 30mas resolution}},
  author = {A. T. Barnes and J. D. Henshaw and Á. Sánchez-Monge and P. Caselli and I. Jiménez-Serra and J. E. Pineda and K. Immer and M. De Simone and R. J. Parker and F. Fontani and E. Redaelli and S. N. Longmore and L. Böhm and Q. Zhang and C. Y. Law and G. Cosentino and J. C. Tan},
  howpublished = {arXiv},
  year = {2026},
  month = oct,
  doi = {10.48550/arxiv.2610.07138},
  eprint = {2610.07138},
  archivePrefix = {arXiv},
  url = {https://arxiv.org/abs/2610.07138},
  note = {CC BY 4.0},
}

RIS

TY  - UNPB
TI  - Cracking open the "dragon egg": ALMA 1.3mm observations of a compact, young high-mass core candidate at 30mas resolution
AU  - A. T. Barnes
AU  - J. D. Henshaw
AU  - Á. Sánchez-Monge
AU  - P. Caselli
AU  - I. Jiménez-Serra
AU  - J. E. Pineda
AU  - K. Immer
AU  - M. De Simone
AU  - R. J. Parker
AU  - F. Fontani
AU  - E. Redaelli
AU  - S. N. Longmore
AU  - L. Böhm
AU  - Q. Zhang
AU  - C. Y. Law
AU  - G. Cosentino
AU  - J. C. Tan
PY  - 2026
DA  - 2026/10/05
PB  - arXiv
DO  - 10.48550/arxiv.2610.07138
UR  - https://arxiv.org/abs/2610.07138
AB  - 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'' (∼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 ∼175 au, with a deconvolved radius of ∼87 au and a nearest neighbouring fragment at a projected separation of ∼2400 au. The continuum peak reaches a brightness temperature of ∼40 K, exceeding the ∼10 K gas temperatures inferred from lower-resolution NH_3 observations and indicating a strong radial temperature gradient. Continuum-based mass estimates range from ∼2 to ∼30 M_⊙ (implying extreme densities of n_H2 > 10^9cm^−3), depending on temperature, while lower-resolution data indicate a much larger mass reservoir of ∼110 M_⊙ on ∼10^4 au scales. The spectrum shows no strong hot-core tracers and only weak, spatially extended H_2CO 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]
ER  -

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