The Extraordinary Outburst in the Massive Protostellar System NGC 6334 I-MM1: Strong Increase in Mid-Infrared Continuum Emission
T. R. Hunter, C. L. Brogan, James M. De Buizer, A. P. M. Towner, C. D. Dowell, Gordon MacLeod, B. Stecklum, C. J. Cyganowski, Samer J. El-Abd, Brett A. McGuire
Abstract
Abstract 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 (>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 μ m images, MM1 is now the brightest region at all three wavelengths (25, 37, and 53 μ 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 4 L ⊙ . 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 M ⊙ , 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 R ⊙ ), thereby supporting a higher accretion rate (≳0.0023 M ⊙ yr −1 ) 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 M ⊙ ) and hence long duration (∼40–130 yr).