Stephenson 2 DFK 1

Stephenson 2 DFK 1 was first catalogued in 1988. It was given the designation IRAS 18363-0607 in the IRAS Point Source Catalog. 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 brightest star in the region of the cluster was given the identifier 1 in the first analysis of cluster member properties. However, it was not considered to be a member of Stephenson 2 due to its outlying position, abnormally high brightness, and slightly atypical proper motion, instead being categorized as an unrelated red supergiant. The designation St2-18 (short for Stephenson 2-18) is often used for the star, following the numbering from Deguchi (2010). In 2012, Stephenson 2 DFK 1, along with 56 other red supergiants, was observed in a study regarding the maser emissions from red supergiants across the galaxy. The study derived the properties of those red supergiants using the Australia Telescope Compact Array (ATCA) and the DUSTY model. Stephenson 2 DFK 1 was among the red supergiants mentioned. It was also noted in Humphreys et al. (2020), albeit mistakenly referred to as RSGC1-01, another very large and luminous red supergiant in the same constellation of Scutum and as well as part of another open cluster, RSGC1. ==Distance==

When the cluster was originally discovered in 1990, Stephenson 2, and therefore Stephenson 2 DFK 1, was originally estimated to have a distance of around , much farther than the cluster is thought to reside today. A study in 2007 determined a kinematic distance of kiloparsecs ( light-years) from comparison with the cluster's radial velocity, considerably closer than the original distance quoted by Stephenson (1990). However, because of Stephenson 2 DFK 1's doubtful membership, its distance was not directly estimated. This value was later adopted in a recent study of the cluster. A similar kinematic distance of 5.5 kiloparsecs (18,000 light-years) was reported in a 2010 study, derived from the average radial velocity of four of the cluster's members (96 kilometers per second) and from an association with a clump of stars near Stephenson 2, Stephenson 2 SW, locating it near the Scutum–Centaurus Arm of the Milky Way. This value was later adopted in a 2012 study to calculate the star's luminosity. It is noted that the uncertainty in the distance was greater than 50%. Despite this, it is also stated that distances to massive star clusters will be improved in the future. Verheyen et al. (2013) used the average radial velocity of the cluster (+109.3 ± 0.7 kilometers per second) to derive a kinematic distance of roughly six kiloparsecs (20,000 light-years) for the cluster. However, Stephenson 2 DFK 1's radial velocity is calculated to be only 89 kilometers per second and therefore leading to the study's statement that the star is a field red supergiant unassociated with the cluster. ==Physical properties==

Evolutionary stage was used to derive Stephenson 2 DFK 1's 2012 bolometric luminosity and effective temperature estimates. Stephenson 2 DFK 1 is usually classified as a red supergiant, like the other stars in the cluster. Spectral type In 2007, Davies et al. estimated Stephenson 2 DFK 1's spectral type at M5 or M6, unusual and very late for even a red supergiant star, based on its CO-bandhead absorption. Due to the unlikely temperature and the luminosity used, the estimate is likely unreliable. For comparison, Saturn orbits the Sun at a distance of . Mass loss Stephenson 2 DFK 1 has been estimated to have a mass loss rate of roughly per year, which is among the highest known for any red supergiant star. It is possible that Stephenson 2 DFK 1 underwent an extreme mass loss episode recently due to its significant infrared excess. In 2013, an article describing the red supergiants in Stephenson 2 stated that Stephenson 2 DFK 1 (referred to as D1) and D2 (another member of Stephenson 2) have maser emissions indicating that they have the highest mass loss in the cluster. Only the stars with the highest bolometric luminosities in the cluster seem to present maser emissions. Stephenson 2 DFK 1 displays strong silicate emission especially at wavelengths of 10 μm and 18 μm. Water masers were detected in the star as well. ==Membership==

DR1 It has been debated for a while if this star is actually part of its supposed cluster. Due to its radial velocity being below the other cluster stars but with some signs of membership, some sources state that the star is unlikely to be a foreground giant; however, more recent papers considered the star an unlikely member due to its extreme and inconsistent properties. Using radial velocities determined from silicon oxide (SiO) maser emission and IR CO absorption, a study of red supergiant masers in massive clusters considered Stephenson 2 DFK 1 as a field red supergiant, unrelated to Stephenson 2. This is due to its lower radial velocity that is significantly different compared to other stars from Stephenson 2. Another possibility is that Stephenson 2 DFK 1 is actually a member, because its radial velocity is offset by an expanding optically thick envelope. The velocity difference between this star’s radial velocity and Stephenson 2 itself (20 kilometers per second) is a typical outflow speed for red supergiants. One study suggests that Stephenson 2 DFK 1 is part of a cluster related to Stephenson 2, Stephenson 2 SW, which is assumed to be at the same distance as the core cluster itself. This proposed cluster contains several other massive stars and red supergiants, including Stephenson 2 DFK 49. ==Uncertainties in measurements==

The distance of Stephenson 2 DFK 1 has been stated to have a relative uncertainty greater than 50%, and the star's doubtful membership, uncertain distance and differing radial velocities compared to the rest of the stars in Stephenson 2 have led to some authors to consider the star as a red supergiant unrelated to Stephenson 2 or any of the red supergiant clusters at the base of the Scutum–Centaurus Arm. The radius of could possibly be an overestimation due to the method used and the fact that the limit for star size predicted by stellar evolutionary theory is estimated to only be roughly . Luminosity estimates for the star are uncertain as well, as another estimate of the luminosity gave a value of , however, it relies on a narrow range of fluxes, making it a likely underestimate. ==See also==