Exploring the Mass-loss Histories of the Red Supergiants

Abstract

We report mid- to far-infrared imaging and photometry from 7 to 37 μm with SOFIA/FORCAST and 2 μm adaptive optics imaging with LBTI/LMIRCam of a large sample of red supergiants (RSGs) in four Galactic clusters: RSGC1, RSGC2 = Stephenson 2, RSGC3, and NGC 7419. The RSGs in these clusters cover their expected range in luminosity and initial mass from ≈9 to more than 25 M_⊙. The population includes examples of very late-type RSGs such as MY Cep, which may be near the end of the RSG stage, high-mass-losing maser sources, yellow hypergiants, and post-RSG candidates. Many of the stars and almost all of the most luminous have spectral energy distributions (SEDs) with extended infrared excess radiation at the longest wavelengths. To best model their SEDs, we use the DUSTY code with a variable radial density distribution function to estimate their mass-loss rates. Our $\dot{M}$ -luminosity relation for 42 RSGs basically follows the classical de Jager curve, but at luminosities below 10⁵ ${L}_{\odot }$ , we find a significant population of RSGs with $\dot{M}$ below the de Jager relation. At luminosities above 10⁵ L_⊙, there is a rapid transition to higher mass-loss rates that approximates and overlaps the de Jager curve. We recommend that instead of using a linear relation or single curve, the empirical $\dot{M}$ -luminosity relation is better represented by a broad band. Interestingly, the transition to much higher $\dot{M}$ at about 10⁵ L_⊙ corresponds approximately to an initial mass of 18-20 M_⊙, which is close to the upper limit for RSGs becoming Type II supernovae. ^* Based on observations obtained with (1) the NASA/DLR Stratospheric Observatory (SOFIA). SOFIA is jointly operated by the Universities Space Research Association, Inc. (USRA) under NASA contract NAS2-97001 and the Deutsches SOFIA Institut (DSI) under DLR contract 50OK 0901 to the University of Stuttgart, and (2) LMIRCam/LBTI on the Large Binocular Telescope (LBT), an international collaboration among institutions in the United States, Italy, and Germany. LBT Corporation partners include The University of Arizona on behalf of the Arizona university system; Istituto Nazionale di Astrofisica, Italy; LBTBeteiligungsgesellschaft, Germany, representing the Max-Planck Society, the Astrophysical Institute Potsdam, and Heidelberg University; The Ohio State University, and The Research Corporation, on behalf of The University of Notre Dame, University of Minnesota, and University of Virginia.