Exploring the Mass-loss Histories of the Red Supergiants

We report mid- to far-infrared imaging and photometry from 7 to 37 mu m with SOFIA/FORCAST and 2 mu m 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 approximate to 9 to more than 25 M-circle dot. 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 (M) over dot-luminosity relation for 42 RSGs basically follows the classical de Jager curve, but at luminosities below 10(5) L-circle dot, we find a significant population of RSGs with (M) over dot below the de Jager relation. At luminosities above 10(5) L-circle dot, 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 (M) over dot-luminosity relation is better represented by a broad band. Interestingly, the transition to much higher (M) over dot at about 10(5) L-circle dot corresponds approximately to an initial mass of 18-20 M-circle dot, which is close to the upper limit for RSGs becoming Type II supernovae.