ALMA Observations of the Protostellar Disk around the VeLLO IRAS16253-2429

We present ALMA long-baseline observations toward the Class 0 protostar IRAS 16253-2429 (hereafter IRAS 16253) with a resolution down to 0.'' 12 (similar to 15 au). The 1.3 mm dust continuum emission has a deconvolved Gaussian size of 0.'' 16 x 0.'' 07 (20 au x 8.8 au), likely tracing an inclined dusty disk. Interestingly, the position of the 1.38 mm emission is offset from that of the 0.87 mm emission along the disk minor axis. Such an offset may come from a torus-like disk with very different optical depths between these two wavelengths. Furthermore, through CO (2 - 1) and (CO)-O-18 (2 - 1) observations, we study rotation and infall motions in this disk-envelope system and infer the presence of a Keplerian disk with a radius of 8-32 au. This result suggests that the disk could have formed by directly evolving from a first core, because IRAS 16253 is too young to gradually grow a disk to such a size considering the low rotation rate of its envelope. In addition, we find a quadruple pattern in the CO emission at low velocity, which may originate from CO freeze out at the disk/envelope midplane. This suggests that the "cold disk" may appear in the early stage, implying a chemical evolution for the disk around this proto-brown dwarf (or very-low-mass protostar) different from that of low-mass stars.