An ALMA view of the Galactic super star cluster RCW38 at 270-AU resolution

Abstract

We report millimeter/submillimeter continuum and molecular line observations of the Galactic super star cluster RCW 38, obtained from the Atacama Large Millimeter/Submillimeter Array with a minimum angular resolution of $0''.17\times0''.15$ ($\simeq289\,{\rm AU}\times255\,{\rm AU}$). The C$^{18}$O image reveal many massive condensations embedded within filamentary structures extending along the northwest-southeast direction in the center of cluster. The condensations have sizes of 0.01-0.02 pc, H$_2$ column densities of $10^{23}$-$10^{24}$ cm$^{-2}$, and H$_2$ masses of 10-130 $M_\odot$. In addition, the 233-GHz continuum image reveals two dense, small millimeter-sources with radii of 460 and 200 AU (Source A and Source B). Source A is embedded within the most massive C$^{18}$O condensation, whereas no counterpart is seen for Source B. The masses of Source A and Source B are estimated as 13 and 3 $M_\odot$ at the optically-thin limit, respectively. The C$^{18}$O emission shows a velocity gradient of 2 km s$^{-1}$ at the central 2000 AU of Source A, which could be interpreted as a Keplerian rotation with a central mass of a few $M_\odot$ or infall motion of gas. Further, the ALMA $^{12}$CO data reveal that Source A and Source B are associated with molecular outflows exhibiting maximum velocities of $\sim$30-70 km s$^{-1}$. The outflows have short dynamical timescales of $<$1000 yr and high mass outflow rates of $\sim10^{-4}$-$10^{-3}$ $M_\odot$ yr$^{-1}$. These observational signatures suggest an early evolutionary phase of the massive star formation in Source A and Source B.