2023A&A...669A.100M

Context. Star formation is underway in the W49N molecular cloud (MC) at a high level of efficiency, with almost twenty ultra-compact (UC) HII regions observed thus far, indicating a recent formation of massive stars. Previous works have suggested that this cloud is undergoing a global contraction.
Aims. We analyse the data on OH masers in the molecular cloud W49N, observed with the VLBA at the 1612, 1665, and 1667 MHz transitions in left circular polarization (LCP) and right circular polarization (RCP) with an aim to study the global kinematics of the masers.
Methods. We carried out our study based on the locations and observed velocities of the maser spots, V_obs. We found the location (α, δ)_m of the maximum correlation between V = V_obs-V_sys (with V_sys the systemic velocity) and distance to it. The velocities were fitted to the straight line of V_obs-V_sys versus d_(α,δ)m, resulting in V_ftd. The difference between the fitted values and those obtained from observations is ∆ V = (V_obs-V_sys)-V_ftd. The V_obs-V_sys velocity shows a gradient as a function of the distance to (α, δ)_m, where the closer spots have the largest velocities. Spots with similar velocities are located in different sectors, with respect to (α, δ)_m. Then, we assumed that the spots are moving towards a contraction centre (CC_OH), which is at the apex of a CONUS. We also assumed that the distance of each spot to CC_OH is d_cc = √2 d_(α,δ)m and that they fall with a velocity V_cc = √2V_ftd, with the total velocity being V_Tot = V_cc + √2 Δ V. Using this velocity, we estimated the free-fall velocity.
Results. The coordinates of (α, δ)_m are effectively (α₂₀₀₀ = 19:10:13.1253, δ₂₀₀₀ = 9:6:13.570). The observed dispersion with respect to the global trend against d_cc, shows a maximum at 0.12 pc, with a decay from 0.12 to 0.19 pc, which is faster than that taking place between 0.19 and 0.42 pc. Based on V_Tot, an inner mass of M_inn = 2500 M_⊙ was estimated. In addition, the estimated accretion rate is Ṁ = 1.4×10^–3M_⊙yr^–1, which requires a time of t_inn = 1.8×10⁶ yr to accumulate M_inn. The free-fall time, assuming n = 1×10^–4 cm^–3, is t_ff = 3.4×10⁵ yr. Performing the same procedure with published data that are of lower spatial resolution (than the VLBA data) produces similar results. For example, based on the available data, we find that (α, δ)_m = (19:10:13.1392, 9:6:13.4387) J2000, which is at ≲ 0.3 asec from what has been calculated with the VLBA data, with an estimated inner mass of 2700 M_⊙. A sub-collapse appears to be taking place in the region traced by the OH maser spots. Based on methanol maser cloudlets data, which lie in a smaller region, another possible centre of contraction is identified, which could be due to a sub-collapse towards a 75 M_⊙ inner mass.
Conclusions. The velocities of the OH spots at W49N, along with their positions with respect to (α, δ)_m, make it possible to trace a global kinematics that is apparently due to a sub-collapse in the W49N MC.