A Recent Accretion Burst in the Low-mass Protostar IRAS 15398-3359: ALMA Imaging of Its Related Chemistry

Low-mass protostars have been suggested to show highly variable accretion rates throughout their evolution. Such changes in accretion, and related heating of their ambient envelopes, may trigger significant chemical variations on different spatial scales and from source-to-source. We present images of emission from C¹⁷O, H¹³CO⁺, CH₃OH, C³⁴S and C₂H toward the low-mass protostar IRAS 15398-3359 on 0.''5 (75 AU diameter) scales with the Atacama Large Millimeter/submillimeter Array at 340 GHz. The resolved images show that the emission from H¹³CO⁺ is only present in a ring-like structure with a radius of about 1-1.''5 (150-200 AU) whereas the CO and other high dipole moment molecules are centrally condensed toward the location of the central protostar. We propose that HCO⁺ is destroyed by water vapor present on small scales. The origin of this water vapor is likely an accretion burst during the last 100-1000 yr increasing the luminosity of IRAS 15398-3359 by a factor of 100 above its current luminosity. Such a burst in luminosity can also explain the centrally condensed CH₃OH and extended warm carbon-chain chemistry observed in this source and furthermore be reflected in the relative faintness of its compact continuum emission compared to other protostars.