What mystery hides the young star Mol 12? Meet the new extreme Herbig Be!

At the heart of a dense cloud 1.6 kpc from Earth shines Mol 12, a massive young star still wrapped in a protoplanetary disk. Astronomers observed its light from visible to near‑infrared (0.6‑2.4 µm) and found a forest of emission lines from iron, hydrogen, carbon, nitrogen and calcium, typical of very hot gas swirling rapidly around the star. No photospheric absorption lines appear, because the core is hidden behind a dust veil with AV≈12. A striking feature is the helium line at 1.083 µm, showing a P‑Cygni profile: evidence of a stellar wind blowing material away. Moreover, CO overtone bands in emission at 2.3 µm reveal a still‑forming disk. From line strengths, scientists estimated an accretion rate of about 10⁻⁵ M☉ yr⁻¹ and an accretion luminosity of 10³ L☉. The most intriguing part is the variability: over the past decade the star has suffered brightness drops exceeding four magnitudes in the r‑band, becoming shallower at longer wavelengths and disappearing beyond 3 µm. Colour‑magnitude diagrams suggest small dust cloudlets, larger than sand grains, crossing our line of sight, causing temporary eclipses and adding up to 10 magnitudes of extra extinction. In short, Mol 12 is a “fireball” hidden behind a veil of clouds, offering a rare glimpse of how the most massive stars and their protoplanetary disks form.

Mol 12 is classified as a Class I object, meaning a protostar still deeply embedded in its accretion phase. Its total luminosity of 1.74×10³ L☉ stems from both nascent nuclear fusion and the energy released by material accreting from the disk. The P‑Cygni profile seen in the He I 1.083 µm line is typical of stellar winds with velocities of order 200‑400 km s⁻¹, indicating strong mass loss that can shape the disk structure. The CO overtone bands (Δv=2) imply temperatures of 2000‑3000 K in the inner disk region, where gas is dense enough to excite molecular vibrations. The estimated accretion rate (~10⁻⁵ M☉ yr⁻¹) matches models of >8 M☉ star formation, which require very high mass inflow to overcome radiative pressure. The >4‑mag variability in the r‑band, with shallower dips at longer wavelengths, is characteristic of occultations by sub‑micron dust particles. An extra extinction of 10 mag suggests column densities around 10²³ cm⁻², compatible with small dust clumps forming spontaneously in the disk or surrounding corona. These eclipse events offer a rare chance to map dust and gas distribution at sub‑AU scales, shedding light on disk evolution toward massive planet or multiple‑system formation. Moreover, the presence of metal lines such as Fe II and Ca II indicates partial ionization, likely driven by UV radiation from the star, which can trigger complex chemical pathways in the disk, fostering the synthesis of complex organic molecules. In summary, Mol 12 serves as a natural laboratory to study the interplay of accretion, stellar winds, and disk dynamics during a critical stage of massive star birth.