We present observations towards a high-mass ($rm >40,M_{odot}$), low luminosity ($rm <10,L_{odot}$) $rm 70,mu$m dark molecular core G 28.34 S-A at 3.4 mm, using the IRAM 30 m telescope and the NOEMA interferometer. We report the detection of $rm SiO$ $J=rm 2rightarrow1$ line emission, which is spatially resolved in this source at a linear resolution of $sim$0.1 pc, while the 3.4 mm continuum image does not resolve any internal sub-structures. The SiO emission exhibits two W-E oriented lobes centring on the continuum peak. Corresponding to the red-shifted and blue-shifted gas with velocities up to $rm 40,km,s^{-1}$ relative to the quiescent cloud, these lobes clearly indicate the presence of a strong bipolar outflow from this $rm 70,mu$m dark core, a source previously considered as one of the best candidates of starless core. Our SiO detection is consistent with ALMA archival data of $rm SiO$ $J=rm 5rightarrow4$, whose high-velocity blue-shifted gas reveals a more compact lobe spatially closer to the dust center. This outflow indicates that the central source may be in an early evolutionary stage of forming a high-mass protostar. We also find that the low-velocity components (in the range of $rm V_{lsr}$$rm_{-5}^{+3},km,s^{-1}$) have an extended, NW-SE oriented distribution. Discussing the possible accretion scenarios of the outflow-powering young stellar object, we argue that the molecular line emission and the molecular outflows may provide a better indication of the accretion history when forming young stellar object, than that from a snapshot observations of the present bolometric luminosity. This is particularly significant for the cases of episodic accretion, which may occur during the collapse of the parent molecular core.
تحميل البحث