Current core accretion planet formation theories prohibit the formation of gas giant planets around very low mass (VLM) stars (M <∼0.2 M⊙). However, some VLM stars have gas giant planets. These formation mechanisms could be either a disk fragmentation scenario or a cloud collapse formation scenario like binary stars. Recently, a planet mass accretion rate is proposed as a diagnosis to distinguish these two scenarios. Here we propose IRCS grism spectroscopy of two gas giant planets to study the formation mechanism of gas giant planets around VLM stars. For this purpose, we will observe Paβ at λ = 1.282 µm to derive the mass accretion rate. We also use NGS AO to reduce the contamination from central stars. The sensitivity will reach one order of magnitude lower than the predicted location of disk fragmentation in Ṁ−M diagram. Thus our observations will distinguish the disk fragmentation scenario and the cloud collapse formation scenario. If we find that planets have a large mass accretion rate of ∼10−11 M⊙/yr, these planets could form via disk fragmentation. Conversely, if not, our results might suggest that they could form via cloud collapse formation like binary stars. In other words, gas giant planets around VLM stars cannot form in the protoplanetary disk.
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