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Low-mass protostars have been suggested to show highly variable accretionrates through-out their evolution. Such changes in accretion, and relatedheating of their ambient envelopes, may trigger significant chemical variationson different spatial scales and from source-to-source. We present images ofemission from C17O, H13CO+, CH3OH, C34S and C2H toward the low-mass protostarIRAS 15398-3359 on 0.5" (75 AU diameter) scales with the Atacama LargeMillimeter/submillimeter Array (ALMA) at 340 GHz. The resolved images show thatthe emission from H13CO+ is only present in a ring-like structure with a radiusof about 1-1.5" (150-200 AU) whereas the CO and other high dipole momentmolecules are centrally condensed toward the location of the central protostar.We propose that HCO+ is destroyed by water vapor present on small scales. Theorigin of this water vapor is likely an accretion burst during the last100-1000 years increasing the luminosity of IRAS 15398-3359 by a factor of 100above its current luminosity. Such a burst in luminosity can also explain thecentrally condensed CH3OH and extended warm carbon-chain chemistry observed inthis source and furthermore be reflected in the relative faintness of itscompact continuum emission compared to other protostars.

A recent accretion burst in the low-mass protostar IRAS 15398-3359: ALMA imaging of its related chemistry