Stephenson 2 DFK 49 or St2-11 is a putative post red supergiant[3] star in the constellation Scutum. It is located in the massive open cluster Stephenson 2. It is possibly one of the largest known stars with radius estimates ranging from almost 1,000 solar radii[5] to 1,300 solar radii.[3] If the upper estimate is correct, then Stephenson 2 DFK 49 has a volume 2.2 billion times that of the Sun. If it was placed at the center of the Solar System, its photosphere will potentially approach or engulf Jupiter's orbit. It loses mass at a very high rate, resulting in large amounts of Infrared excess.
| Observation data Epoch J2000 Equinox J2000 | |
|---|---|
| Constellation | Scutum |
| Right ascension | 18h 39m 05.586s[1] |
| Declination | −06° 04′ 26.58″[1] |
| Characteristics | |
| Evolutionary stage | Red supergiant[2] or post red supergiant[3] |
| Spectral type | K4[2] |
| Apparent magnitude (K) | 7.324[1] |
| Astrometry | |
| Radial velocity (Rv) | 101[4]-109.4[2] km/s |
| Details | |
| Radius | 1,074[2] (884[5]-1,300[3]) R☉ |
| Luminosity | 245,000[2] (132,000[5]-390,000[3])) L☉ |
| Temperature | 3920±112[2] (3700[5]-4000[3]) K |
| Other designations | |
Stephenson 2 DFK 49, St2-11,[6] DENIS J183905.5-060425 2MASS J18390558-0604265, MSX6C G026.1215-00.0345
TIC 7759844 | |
| Database references | |
| SIMBAD | data |
Observation history
The open cluster Stephenson 2 was discovered by American astronomer Charles Bruce Stephenson in 1990 in the data obtained by a deep infrared survey.[2][7] The cluster is also known as RSGC2, one of several massive open clusters in Scutum, each containing multiple red supergiants.[8] The 49th brightest star in the K band was given an identifier number of 49.[2] The authors noted that the star had significant circumstellar and interstellar extinction, higher than even the other cluster members, and noted that its spectral type places it near yellow hypergiants on the Hertzsprung–Russell diagram (HR Diagram), though not as hot.[2]
In a later study from 2010, the same star was given the identifier number 11, and was grouped with a proposed cluster assumed to be associated with Stephenson 2, Stephenson 2-SW. The star showed maser emissions at some spectral lines. A later study corroborates this.[4] The study mentions a weak CO emission with radial velocities similar to Stephenson 2 DFK 49, but it is said to be unrelated, because it is too intense for a red supergiant at Stephenson 2 DFK 49’s distance.[6]
Another study 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.[9]
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 an infrared excess is similar to that of the famous and extreme red hypergiant VY Canis Majoris, although Stephenson 2 DFK 49 is hotter. This it indicates that the star has circumstellar extinction. The study also estimates the possible mass loss rates of the star, as well as its other properties.[3]
Properties
Stephenson 2 DFK 49 was known to be an interesting object since its home cluster was first studied in depth.[2] Interestingly, Stephenson 2 DFK 49 appears to be at the center of a bow-shock structure in infrared images.[2] Its properties, an earlier than usual spectral type compared to other stars in Stephenson 2, its luminosity and position on the HR Diagram indicate that it is somewhat similar to the star IRC+10420 and Variable A in the Triangulum Galaxy (M33).[3] Stephenson 2 DFK 49 is somewhat comparable to another extreme star in the vicinity of Stephenson 2, IRAS 18357-0604. Both are possible post-red supergiant stars,[9] and both of them are comparable to the yellow hypergiant IRC+10420.
Luminosity
In 2007, a study of the red supergiants in Stephenson 2 estimated its properties and determined a bolometric luminosity of 245,000 solar luminosities. [2] A 2012 study estimated and published the properties of numerous red supergiants and other supergiant stars. The study estimated Stephenson 2-11's luminosity at a much lower 132,000 solar luminosities, using Spectral Energy Distributions.[5] Using spectral energy distributions, Humphreys (2020) estimates a luminosity of 390,000 solar luminosities. However, it is noted that the estimated luminosity may be an underestimate, because it does not include excess radiation from warm dust. [3]
Temperature and Spectrum
Davies (2007) estimated its temperature of 3,920 K, but with an uncertainty in the measurement of ± 112 K.[2] 5 years later, Fok (2012) estimated a slightly cooler temperature at 3,700 Kelvin. [5] A more recent study estimated the star's temperature at a hotter 4,000 Kelvin, based on its spectral type of K4. [3] Stephenson 2 DFK 49's spectrum is similar to that of VY Canis Majoris, but it is somewhat hotter.[3]
Size
Davies (2007) estimates a temperature of 3,920 Kelvin, with a luminosity of 245,000 solar luminosities. Applying the Stefan-Boltzmann law, Stephenson 2 DFK 49’s radius would be 1,074 solar radii, making it one of the largest stars known. [2] 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 3,700 Kelvin but its luminosity at a much lower 132,000 solar luminosities. [5] This would imply a radius of only 884 solar radii, smaller than the radius implied by Davies (2007). Humphreys (2020) estimates Stephenson 2 DFK 49's temperature at 4,000 Kelvin, but a higher luminosity of 390,000 solar luminosities, but the luminosity estimate is stated to be conservative and underestimated. Applying the Stefan-Boltzmann law, its radius would be 1,300 solar radii, larger than the other two estimates. [3]
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 some spectral lines. [4] A 2020 study later determines a mass loss rate of 1.3-7*10-4 per year, 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. Its properties, an earlier than usual spectral type, its luminosity and position on the HR Diagram indicate that it is somewhat similar to the star IRC+10420 and Variable A in the Triangulum Galaxy (M33).[3]
Evolution and future
A star with the properties of Stephenson 2 DFK 49 would imply an initial mass of more than 25 solar masses.[3]
Because Stephenson 2 DFK 49 has lost so much mass and continues to do so as well as 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.[2]
References
- ^ a b c Cutri, Roc M.; et al. (2003). "VizieR Online Data Catalog: 2MASS All-Sky Catalog of Point Sources (Cutri+ 2003)". CDS/ADC Collection of Electronic Catalogues. 2246: II/246. Bibcode:2003yCat.2246....0C.
- ^ a b c d e f g h i j k l m n o Davies, B.; Figer, D. F.; Kudritzki, R. P.; MacKenty, J.; Najarro, F.; Herrero, A. (2007). "A Massive Cluster of Red Supergiants at the Base of the Scutum‐Crux Arm". The Astrophysical Journal. 671 (1): 781–801. arXiv:0708.0821. Bibcode:2007ApJ...671..781D. doi:10.1086/522224. S2CID 1447781.
- ^ a b c d e f g h i j k l m n Humphreys, Roberta M.; Helmel, Greta; Jones, Terry J.; Gordon, Michael S. (2020). "Exploring the Mass Loss Histories of the Red Supergiants". The Astronomical Journal. 160 (3): 145. arXiv:2008.01108. Bibcode:2020AJ....160..145H. doi:10.3847/1538-3881/abab15. S2CID 220961677.
{{cite journal}}: CS1 maint: unflagged free DOI (link) - ^ a b c Verheyen, L.; Messineo, M.; Menten, K.M. (March 2012). "SiO maser emission from red supergiants across the Galaxy". Astronomy and Astrophysics. 541: A36. arXiv:1203.4727. Bibcode:2012A&A...541A..36V. doi:10.1051/0004-6361/201118265. S2CID 55630819.
- ^ a b c d e f g Fok, Thomas K. T; Nakashima, Jun-ichi; Yung, Bosco H. K; Hsia, Chih-Hao; Deguchi, Shuji (2012). "Maser Observations of Westerlund 1 and Comprehensive Considerations on Maser Properties of Red Supergiants Associated with Massive Clusters". The Astrophysical Journal. 760 (1): 65. arXiv:1209.6427. Bibcode:2012ApJ...760...65F. doi:10.1088/0004-637X/760/1/65. S2CID 53393926.
- ^ a b Deguchi, Shuji; Nakashima, Jun-Ichi; Zhang, Yong; Chong, Selina S. N.; Koike, Kazutaka; Kwok, Sun (2010). "SiO and H2O Maser Observations of Red Supergiants in Star Clusters Embedded in the Galactic Disk". Publications of the Astronomical Society of Japan. 62 (2): 391–407. arXiv:1002.2492. Bibcode:2010PASJ...62..391D. doi:10.1093/pasj/62.2.391. S2CID 24396370.
- ^ Stephenson, C. B. (1990). "A possible new and very remote galactic cluster". The Astronomical Journal. 99: 1867. Bibcode:1990AJ.....99.1867S. doi:10.1086/115464.
- ^ Negueruela, I.; González-Fernández, C.; Marco, A.; Clark, J. S.; Martínez-Núñez, S. (2010). "Another cluster of red supergiants close to RSGC1". Astronomy and Astrophysics. 513: A74. arXiv:1002.1823. Bibcode:2010A&A...513A..74N. doi:10.1051/0004-6361/200913373. S2CID 118531372.
- ^ a b Clark, J. S.; Negueruela, I.; González-Fernández, C. (2014-01-01). "IRAS 18357-0604 – An analogue of the galactic yellow hypergiant IRC +10420?". Astronomy and Astrophysics. 561: A15. arXiv:1311.3956. Bibcode:2014A&A...561A..15C. doi:10.1051/0004-6361/201322772. ISSN 0004-6361. S2CID 53372226.