FAUST VII. Detection of A Hot Corino in the Prototypical Warm Carbon-Chain Chemistry Source IRAS 15398-3359

Abstract

We have observed the low-mass protostellar source, IRAS 15398$-$3359, at a resolution of 0.${}^{\prime \prime }$2-0.${}^{\prime \prime }$3, as part of the Atacama Large Millimeter/Submillimeter Array Large Program FAUST, to examine the presence of a hot corino in the vicinity of the protostar. We detect nine CH${}_3$OH lines including the high excitation lines with upper state energies up to 500 K. The CH${}_3$OH rotational temperature and the column density are derived to be 119${}_{-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 CH${}_3$OH 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=$537 K), is located within the 50 au area around the protostar, and seems to have a slight extension toward the northwest. Toward the continuum peak, we also detect one emission line from CH${}_2$DOH and two features of multiple CH${}_3$OCHO lines. These results, in combination with previous reports, indicate that IRAS 15398$-$3359 is a source with hybrid properties showing both hot corino chemistry rich in complex organic molecules on small scales $\sim$10 au) and warm carbon-chain chemistry (WCCC) rich in carbon-chain species on large scales ($\sim$100-1000 au). A possible implication of the small emitting region is further discussed in relation to the origin of the hot corino activity.

20pages, 3figures

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• Parsec scales of carbon chain and complex organic molecules in AFGL 2591 and IRAS 20126
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