Global kinematics study of OH masers in W49N

Abstract

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, Vobs. We found the location (α, δ)m of the maximum correlation between V = Vobs−Vsys (with Vsys the systemic velocity) and distance to it. The velocities were fitted to the straight line of Vobs−Vsys versus d(α,δ)m, resulting in Vftd. The difference between the fitted values and those obtained from observations is ∆ V = (Vobs−Vsys)-Vftd. The Vobs−Vsys 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 (CCOH), which is at the apex of a CONUS. We also assumed that the distance of each spot to CCOH is dcc = √2 d(α,δ)m and that they fall with a velocity Vcc = √2Vftd, with the total velocity being VTot = Vcc + √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 dcc, 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 VTot, an inner mass of Minn = 2500 M⊙ was estimated. In addition, the estimated accretion rate is Ṁ = 1.4×10−3 M⊙yr−1, which requires a time of tinn = 1.8×106 yr to accumulate Minn. The free-fall time, assuming n = 1×10−4 cm−3, is tff = 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.