Abstract
Based on astrochemical theory, the complex molecular composition around high-mass YSOs should evolve from the outer envelope in toward the central hot region as a sequence of temperature dependent chemical pathways are activated in ices and in the gas-phase. The resulting complex molecules have been broadly classified into three generations dependent on the temperature (<25, >25, and >100 K) required for formation. We combine IRAM 30m and Submillimeter Array observations to explore the spatial distribution of organic molecules around the high-mass young stellar object NGC 7538 IRS9, whose weak complex molecule emission previously escaped detection, quantifying the emission and abundance profiles of key organic molecules as a function of distance from the central protostar. We find that emission from N-bearing organics and saturated O-bearing organics present large increases in emission around 8000 AU and R<3000 AU, while O-bearing molecules and hydrocarbons do not. The increase in flux from some complex molecules in the envelope, around 8000 AU or 25 K, is consistent with recent model predictions of an onset of complex ice chemistry at 20-30 K. The emission increase for some molecules at R<3000 AU suggests the presence of a weak hot core, where thermal ice evaporation and hot gas-phase chemistry drives the chemistry. Complex organics thus form at all radii and temperatures around this protostar, but the composition changes dramatically as the temperature increases, which is used to constrain the chemical generation(s) to which different classes of molecule belong.
Highlights
The early phases of high-mass (Mzams > 8 M ) star formation take place in highly obscured regions, where the central young stellar object (YSO) is embedded in a large envelope of cold, lukewarm, and hot gas and dust
Complex organic molecules are common probes of the “hotcore” stage of massive young stellar objects (MYSOs), which is characterized by intense emission of different organic molecules from a dense and hot region close to the protostar (Blake et al 1987; Helmich & van Dishoeck 1997; Schilke et al 2001)
Complex organic molecules are proposed to form through ice chemistry in the colder parts of the protostellar envelope and evaporate as material flows toward the protostar
Summary
The early phases of high-mass (Mzams > 8 M ) star formation take place in highly obscured regions, where the central young stellar object (YSO) is embedded in a large envelope of cold, lukewarm, and hot gas and dust. Because the YSO is deeply obscured, observations of trace species, i.e., dust and gasphase molecules other than H2, often provide the best and sometimes only constraints on the early evolution of these stars (van Dishoeck & Blake 1998). The utility of such molecular probes depends strongly, on how well their chemistry is understood, and especially on how their formation and destruction efficiencies depend on the environment. Complex organic molecules are proposed to form through ice chemistry in the colder parts of the protostellar envelope and evaporate as material flows toward the protostar. While organic ices evaporate over a range of temperatures, most of the ice is expected to be released at ∼100 K, forming the observed hot core
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