2023ApJ...948..127O

We have observed the low-mass protostellar source IRAS 15398-3359 at a resolution of 0.″2-0.″3, as part of the Atacama Large Millimeter/Submillimeter Array Large Program FAUST, to examine the presence of a hot corino in the vicinity of the protostar. We detect nine CH₃OH lines including the high-excitation lines with upper-state energies up to 500 K. The CH₃OH rotational temperature and the column density are derived to be ${119}_{-26}^{+20}$ K and ${3.2}_{-1.0}^{+2.5}\times {10}^{18}$ cm^–2, respectively. The beam filling factor is derived to be ${0.018}_{-0.003}^{+0.005}$, indicating that the emitting region of CH₃OH is much smaller than the synthesized beam size and is not resolved. The emitting region of three high-excitation lines, 18_3,15-18_2,16, A (E_u = 447 K), 19_3,16-19_2,17, A (E_u = 491 K), and 20_3,17-20_2,18, A (E_u = 537 K), is located within the 50 au area around the protostar and seems to have a slight extension toward the northwest. Toward the continuum peak, we also detect one emission line from CH₂DOH and two features of multiple CH₃OCHO lines. These results, in combination with previous reports, indicate that IRAS 15398-3359 is a source with hybrid properties showing both hot corino chemistry rich in complex organic molecules on small scales (∼10 au) and warm carbon-chain chemistry rich in carbon-chain species on large scales (∼100-1000 au). A possible implication of the small emitting region is further discussed in relation to the origin of the hot corino activity.