Fragmentation at the Earliest Phase of Massive Star Formation

We present 1.3mm continuum and spectral line images of two massive molecular clumps P1 and P2 in the G28.34+0.06 region with the Submillimeter Array. While the two clumps contain masses of 1000 and 880 \msun, respectively, P1 has a luminosity $< 10^2$ \lsun, and a lower gas temperature and smaller line width than P2. Thus, P1 appears to be at a much earlier stage of massive star formation than P2. The high resolution SMA observations reveal two distinctive cores in P2 with masses of 97 and 49 \msun, respectively. The 4 GHz spectral bandpass captures line emission from CO isotopologues, SO, CH$_3$OH, and CH$_3$CN, similar to hot molecular cores harboring massive young stars. The P1 clump, on the other hand, is resolved into five cores along the filament with masses from 22 to 64 \msun and an average projected separation of 0.19 pc. Except $^{12}$CO, no molecular line emission is detected toward the P1 cores at a 1$\sigma$ rms of 0.1 K. Since strong $^{12}$CO and C$^{18}$O emissions are seen with the single dish telescope at a resolution of 11$’’$, the non-detection of these lines with the SMA indicates a depletion factor upto $10^3$. While the spatial resolution of the SMA is better than the expected Jeans length, the masses in P1 cores are much larger than the thermal Jeans mass, indicating the importance of turbulence and/or magnetic fields in cloud fragmentation. The hierarchical structures in the P1 region provide a glimpse of the initial phase of massive star and cluster formation.

💡 Research Summary

The paper presents high‑resolution (∼1″) 1.3 mm continuum and spectral line observations of two massive molecular clumps, P1 and P2, located in the infrared‑dark cloud G28.34+0.06, using the Submillimeter Array (SMA). Both clumps have comparable total masses (∼1000 M☉ for P1 and ∼880 M☉ for P2) but differ dramatically in evolutionary status. P2 exhibits a luminosity of order 10³ L☉, higher gas temperature, and broader line widths, together with a rich spectrum of hot‑core tracers (multiple transitions of CH₃OH, CH₃CN, SO, and CO isotopologues). The SMA resolves P2 into two compact cores of 97 M☉ and 49 M☉, separated by ≲0.1 pc, confirming that massive star formation is already underway. In contrast, P1 is faint in the infrared (L < 10² L☉), has a low kinetic temperature (~13 K from NH₃ and N₂H⁺), and narrow linewidths (~1.7 km s⁻¹). The SMA continuum image reveals five filamentary cores with masses ranging from 22 to 64 M☉ and an average projected spacing of 0.19 pc. No molecular line emission, except for ¹²CO, is detected toward these cores down to a 1σ rms of 0.1 K. Single‑dish observations, however, show strong ¹²CO and C¹⁸O emission on 11″ scales, indicating that the interferometer resolves out the extended gas and that CO is severely depleted (by factors up to 10³) in the dense cores.

The authors compare the observed core masses with the thermal Jeans mass calculated from the measured temperature and density. The thermal Jeans mass is only ~1–2 M☉, far below the observed core masses, implying that thermal pressure alone cannot support the fragmentation. The larger core separations relative to the thermal Jeans length (∼0.07 pc) point to additional support from supersonic turbulence (linewidths ≈1.7 km s⁻¹) and possibly magnetic fields. The depletion of CO, together with the absence of complex organic molecules, suggests that the P1 cores are chemically young (∼10⁵ yr) and still in a pre‑stellar phase. Conversely, the rich hot‑core chemistry in P2 demonstrates that it has already entered a stage where protostellar heating drives ice mantle sublimation and complex molecule formation.

The study therefore provides a direct observational contrast between an early, turbulence‑dominated fragmentation regime (P1) and a later, thermally heated hot‑core regime (P2) within the same giant molecular filament. It highlights the importance of high‑resolution interferometry for uncovering the initial conditions of massive star and cluster formation, and it underscores that both turbulence and magnetic fields must be considered when interpreting the fragmentation scale in massive clumps. The authors suggest that future polarization measurements and broader molecular line surveys will be essential to quantify magnetic field strengths and to map the turbulent power spectrum, thereby refining our understanding of how massive stars and stellar clusters emerge from their natal clouds.