ALMA observations of the protostellar disk around the VeLLO IRAS
  16253-2429
  We present ALMA long-baseline observations toward the Class 0 protostar IRAS
16253-2429 (hereafter IRAS 16253) with a resolution down to 0.12" (~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 C18O (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 IRAS16253 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.
