The Extraordinary Outburst in the Massive Protostellar System NGC 6334 I-MM1: Strong Increase in Mid-Infrared Continuum Emission

Abstract

<jats:title>Abstract</jats:title> <jats:p>In recent years, dramatic outbursts have been identified toward massive protostars via infrared and millimeter dust continuum and molecular maser emission. The longest lived outburst (&gt;6 yr) persists in NGC 6334 I-MM1, a deeply embedded object with no near-IR counterpart. Using FORCAST and HAWC+ on SOFIA, we have obtained the first mid-IR images of this field since the outburst began. Despite being undetected in pre-outburst ground-based 18 <jats:italic>μ</jats:italic>m images, MM1 is now the brightest region at all three wavelengths (25, 37, and 53 <jats:italic>μ</jats:italic>m), exceeding the UCHII region MM3 (NGC 6334 F). Combining the SOFIA data with ALMA imaging at four wavelengths, we construct a spectral energy distribution of the combination of MM1 and the nearby hot core MM2. The best-fit Robitaille radiative transfer model yields a luminosity of (4.9 ± 0.8) × 10<jats:sup>4</jats:sup> <jats:italic>L</jats:italic> <jats:sub>⊙</jats:sub>. Accounting for an estimated pre-outburst luminosity ratio MM1:MM2 = 2.1 ± 0.4, the luminosity of MM1 has increased by a factor of 16.3 ± 4.4. The pre-outburst luminosity implies a protostar of mass 6.7 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>, which can produce the ionizing photon rate required to power the pre-outburst HCHII region surrounding the likely outbursting protostar MM1B. The total energy and duration of the outburst exceed the S255IR-NIRS3 outburst by a factor of ≳3, suggesting a different scale of event involving expansion of the protostellar photosphere (to ≳20 <jats:italic>R</jats:italic> <jats:sub>⊙</jats:sub>), thereby supporting a higher accretion rate (≳0.0023 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> yr<jats:sup>−1</jats:sup>) and reducing the ionizing photon rate. In the grid of hydrodynamic models of Meyer et al., the combination of outburst luminosity and magnitude (3) places the NGC 6334 I-MM1 event in the region of moderate total accretion (∼0.1–0.3 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>) and hence long duration (∼40–130 yr).</jats:p>