What hides inside nebula G333.6‑0.2? Unveiling the hot cores of newborn stars

Using ALMA, the world’s most powerful telescope, astronomers pointed at the G333.6‑0.2 region, a stellar nursery about 10,000 light‑years away. There they found three hot “cores” – A, B and C – where methyl cyanide (CH₃CN) and methanol (CH₃OH) glow like neon. These molecules, seen in high‑energy transitions, reveal temperatures between 380 °C and 430 °C, almost a cosmic sauna. Velocity maps show all three cores rotating, hinting at protoplanetary disks in formation, while core A also appears to be expanding, as if a newborn star is blowing away its cocoon. Moreover, the ionized hydrogen line (H29α) indicates gas moving faster than 30 km/s, a sign of turbulence and intense heat. From the electron density (≈5 × 10⁵ cm⁻³) and Lyman photon flux, scientists infer a massive O5‑O6 star at the centre, still in the zero‑age main‑sequence phase. In short, G333.6‑0.2 is an intermediate stage between hyper‑compact and ultra‑compact H II regions, where massive star birth is in full swing. This study sheds light on how the biggest stars form, spin and ionize their surroundings, opening a window onto processes that happen millions of light‑years away but shape the whole universe.

The rotational diagram analysis of CH₃CN was performed using the Boltzmann method, which derives the excitation temperature (T_ex) from observed line intensities. The ten components of the J=14→13 ladder provided a well‑linear diagram, indicating near local thermodynamic equilibrium (LTE). The derived temperature, between 380 K and 430 K, is typical of hot cores, where heating is dominated by UV radiation and thermal shocks. Velocity gradients seen in the first‑moment maps of CH₃CN and CH₃OH show shear values of about 2–3 km s⁻¹ arcsec⁻¹, compatible with Keplerian rotation around stellar masses of 10–20 M_⊙, assuming a radius of 0.01 pc. Core A also displays an asymmetric line profile with a red wing, characteristic of expansion or outflow motion. The H29α recombination line, with a width of 30.2 km s⁻¹, is dominated by thermal broadening (≈7 km s⁻¹ at 10⁴ K) and dynamical broadening (turbulence, ionized gas flow). The emission measure (EM) of 1.23 × 10⁹ pc cm⁻⁶ and electron density ne≈4.8 × 10⁵ cm⁻³ indicate an extremely dense ionized region, typical of ultracompact H II zones. The Lyman photon flux (Q₀≈10⁴⁹ s⁻¹) corresponds to an O5‑O6 V star, with mass ≈40 M_⊙ and luminosity ≈5 × 10⁵ L_⊙. These results place G333.6‑0.2 in a transitional stage between hyper‑compact and ultracompact H II regions, where ionized pressure begins to dominate molecular gas pressure, yet hot cores remain confined, favoring the formation of massive multiple star systems. The coexistence of rotation and outflow suggests simultaneous accretion and feedback processes, providing an ideal testbed for high‑mass star formation models that must account for angular momentum transfer, ionization, and molecular dissociation in extreme environments.