Exploring the Mass Loss Histories of the Red Supergiants
  We report mid- to far-infrared imaging and photomety from 7 to 37 microns
with SOFIA/FORCAST and 2 micron 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 red supergiants in these clusters
cover their expected range in luminosity and initial mass from approximately 9
to more than 25 Solar masses. 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 DUSTY with a variable radial
density distribution function to estimate their mass loss rates. Our mass loss
rate -- luminosity relation for 42 RSGs basically follows the classical de
Jager curve, but at luminosities below 10^5 Solar luminosities we find a
significant population of red supergiants with mass loss rate below the de
Jager relation. At luminosities above 10^5 Solar luminosities 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 mass loss rate -- luminosity relation is better
represented by a broad band. Interestingly, the transition to much higher mass
loss rates at about 10^5 Lsun corresponds approximately to an initial mass of
18 --20 Msun which is close to the upper limit for RSGs becoming Type II SNe.
