A new study of formaldehyde (H₂CO) absorption lines toward 73 Planck cold cores reveals the dynamic conditions of early star formation. The study found most cores remain gravitationally unbound, supporting the hypothesis that Planck cold cores are in the early stages of star formation. 

In some cores, the inner gas is cold and quiescent—poised for gravitational collapse—while the surrounding gas remains turbulent, revealing how gravity begins to take hold amid chaotic motions.

The study, led by Yernar Imanaly, a doctoral student in the star formation and evolution group at the Xinjiang Astronomical Observatory (XAO) of the Chinese Academy of Sciences (CAS), was recently published in Monthly Notices of the Royal Astronomical Society.

Star formation starts in cold, dense molecular cores. Observing these objects is therefore essential for uncovering the initial conditions and early evolutionary processes of star formation, and for testing and refining theoretical models.

The vast number of cold cores detected by Planck provides an exceptional sample for probing the earliest stages of star birth. Molecular spectral-line observations offer a powerful means to investigate the internal gas excitation states and dynamical evolution of these regions. Among these lines, the H₂CO absorption line at 6cm band is highly sensitive to cold dense gas, making it ideal for exploring the internal structure and dynamical states of cold cores.

In this work, the researchers conducted an observational study of H₂CO absorption lines at the 6 cm band toward 73 Planck cold cores using the Nanshan 26-meter radio telescope. They systematically analyzed the physical properties, turbulent states, and early gravitational evolution characteristics of dense gas within the cold cores.

They detected H₂CO absorption lines in 51 of the 73 target sources, corresponding to a detection rate of 69.9%. Among these, 24 sources exhibited resolved hyperfine structure (HFS), accounting for 32.9% of the total sample.

The analysis shows that non-thermal motions are generally stronger than thermal motions in cold cores with detected H₂CO absorption lines. Approximately 96% of the sources exhibit supersonic turbulence, indicating that turbulence plays a significant role in the early star-forming environment.

For the 24 sources with HFS structure, the researchers precisely calculated their H₂CO excitation temperatures, which range from 2.08 to 2.59 K, with an average of approximately 2.37 K. HFS sources generally exhibit narrower line widths, lower Mach numbers, higher column densities, and greater optical depths, suggesting that these regions are dynamically quiescent and may be evolving toward the early gravitational collapse phase.

The virial parameterαvir is a key indicator of the gravitational binding state of molecular cloud cores, with a critical value of about 2; values below this limit suggest that the system may be in a state of gravitational collapse. The figureshows the distribution of the virial parameter α_vir in the Planck cold core subsample for which virial parameters could be estimated.

The results show that most cold cores have α_vir > 2, indicating that they have not yet entered global gravitational collapsing stage. Only a subset of cold cores with detected HFS structure exhibits relatively low α_vir values, approaching a gravitationally bound state. Among them, G168.13−16.39, G174.08−13.24, and G177.97−09.72 have α_vir values below the critical value of 2, suggesting that they may have already entered the early gravitationally bound or collapsing stage.

Based on H₂CO absorption line observations with the Nanshan 26‑meter radio telescope, the researchers calculated and analyzed the excitation temperatures, turbulence, dynamical states, and virial parameters of Planck cold cores, concluding that most are non‑gravitationally bound. These findings support the hypothesis that Planck cold cores are in the early stages of star formation.

According to the researchers, these results provide new observational evidence for understanding the initial conditions of star formation, the role of turbulence, and the early gravitational collapse process.