Stephenson 2 DFK 49

The open cluster Stephenson 2 was discovered by American astronomer Charles Bruce Stephenson in 1990 in the data obtained by a deep infrared survey. The cluster is also known as RSGC2, one of several massive open clusters in Scutum, each containing multiple red supergiants. The 49th brightest star in the K band was given an identifier number of 49. The study mentions a weak CO emission with radial velocities similar to Stephenson 2 DFK 49, but it is said to be unrelated due to being too intense for a red supergiant at Stephenson 2 DFK 49’s distance. Another study observed and studied 57 red supergiant stars across the galaxy and gave estimates of the stars' properties based on their Spectral Energy Distributions, like luminosity and temperature. In 2016, it was compared to the yellow hypergiant star IRAS 18357-0604, which can be found in the same general region as Stephenson 2. A recent study on red supergiant mass loss rates and histories notes it as the most interesting object in the cluster, because its spectral energy distribution, which has a significant infrared excess, is similar to that of the famous and extreme red hypergiant VY Canis Majoris. However, Stephenson 2 DFK 49 is hotter. The study also estimates the possible mass loss rates of the star, as well as its other properties. ==Properties==

Stephenson 2 DFK 49 was known to be an interesting object since its home cluster was first studied in depth. Using the SED, Humphreys (2020) estimates a luminosity of . However, it is noted that the estimated luminosity may be an underestimate because it does not include excess radiation from warm dust. Temperature and Spectrum Davies (2007) estimated its temperature of , but with an uncertainty in the measurement of ± 112 K. 5 years later, Fok (2012) estimated a slightly cooler temperature at . A more recent study estimated the star's temperature at a hotter , based on its spectral type of K4. Size Davies (2007) estimates a temperature of , with a luminosity of . Applying the Stefan-Boltzmann law, Stephenson 2 DFK 49’s radius would be , making it one of the largest stars known. A 2012 study published the estimated properties of numerous red supergiants and other supergiant stars using Spectral Energy Distributions, including Stephenson 2 DFK 49. The study estimates its temperature at a slightly cooler but its luminosity at a much lower . This would imply a radius of only , smaller than the radius implied by Davies (2007). Humphreys (2020) estimates Stephenson 2 DFK 49's temperature at , but a higher luminosity of . Applying the Stefan-Boltzmann law, its radius would be , larger than the other two estimates. Mass loss and maser emissions While Davies (2007) does not estimate a mass loss rate for Stephenson 2 DFK 49, the study mentions that it would be interesting to estimate its mass loss rate. In 2012, it was noted for having maser emissions at certain spectral lines. A 2020 study later determines a mass loss rate of per year based on several computer models, but it is noted that these rates are not high for other hypergiant stars such as VY Canis Majoris, NML Cygni, and IRC +10420. As a possible post-red supergiant star, Stephenson 2 DFK 49 likely experiences both constant and variable mass-loss rates. ==Evolution and future==

A star with the properties of Stephenson 2 DFK 49 would imply an initial mass of more than . Because Stephenson 2 DFK 49 has lost so much mass and continues to do so, and its likely status as a post-red supergiant star, it is likely on the verge of shedding all of its outer layers and becoming a luminous blue variable or a Wolf-Rayet star. ==See also==