Stephenson 2

Stephenson 2, also known as RSGC2 (Red Supergiant Cluster 2), is a young massive open cluster belonging to the Milky Way galaxy. It was discovered in 1990 as a cluster of red supergiants in a photographic, deep infrared survey by the astronomer Charles Bruce Stephenson, after whom the cluster is named.[2][3] It is located in the constellation Scutum at the distance of about 6 kpc from the Sun. It is likely situated at the intersection of the northern end of the Long Bar of the Milky Way and the inner portion of the Scutum–Centaurus Arm—one of the two major spiral arms.[2]

Stephenson 2
2MASS image of Stephenson 2 as the cluster of stars in the upper left, with Stephenson 2 DFK 1 at the center.
Credit: Université de Strasbourg/CNRS (2003)
Observation data (J2000 epoch)
Right ascension18h 39m 21.12s[1]
Declination−06° 01′ 44.4″[1]
Distance(5.83+1.91
−0.78kpc[2])
Apparent magnitude (V)not visible[2]
Apparent dimensions (V)1.8'[1]
Physical characteristics
Mass3–5 × 104[2] M☉
Radius~4 pc[2]
Estimated age14–20 my[2][1]
Other designationsRSGC2
Associations
ConstellationScutum

Description
Spitzer image centred on Stephenson 2 SW, with the main cluster towards upper left

26 red supergiants have been confirmed as members of the cluster, far more than any other known cluster, both in and out of the Milky Way. This includes Stephenson 2 DFK 1, which is one of the largest stars currently known. A more recent study has identified around 80 red supergiants in the line of sight of Stephenson 2, approximately 40 of them with radial velocities consistent with being cluster members. However these stars are spread over a wider area than a typical cluster, indicating an extended stellar association similar to that found around the nearby cluster RSGC3.[4]

The age of Stephenson 2 is estimated at 14–20 million years. The observed red supergiants with the mass of about 12–16 solar masses are type II supernova progenitors. The cluster is heavily obscured and has not been detected in the visible light. It lies close to other groupings of red supergiants known as RSGC1, RSGC3, Alicante 7, Alicante 8, and Alicante 10. The mass of the open cluster is estimated at 30–50 thousand solar masses,[2] which makes it the second most massive open cluster in the Galaxy.

Stephenson 2 SW

Some of the stars in the vicinity of the cluster lie in a loose grouping near the cluster, including Stephenson 2 DFK 1, Stephenson 2 DFK 49 and Stephenson 2-26. This grouping was first mentioned in Deguchi (2010) and was named Stephenson 2 SW because it lies south-west of the main cluster. While the radial velocities of its members are somewhat different from the main cluster's radial velocity,(by about 7.7 km/s) The difference between the 2 velocities is still relatively small, and not enough to rule out its association with the main cluster. Thus, it was assumed that it is possibly related to Stephenson 2 itself.[5]

Members

Stars whose rows are colored in yellow are stars supposed to be part of Stephenson 2 SW.

DFK #[2] Spectral type Magnitude (K band) Temperature (effective, K) Absolute magnitude (K band) Luminosity (L☉) Radius (R☉)
1M6 2.900 3,200[6] 440,000[6] (90,000[5]–630,000[7]) 2,150[6]
2M3/M7[7]4.1203,200[7] – 3,700[6]−11.12160,000[7] 969[6] – 1,301[7]
3M4 4.499 3,535[2] (3,400[7]–3,700[6]) −10.72 88,000[7] (85,100[6]–110,000[2]) 854[7] (710[6]–883[2])
5M4 4.822 3,400[7] (3,500[6]–3,535[2]) −11.02 100,000[7] (91,200[6]–145,000[2] 911[7] (805[6]–1,014[2])
6M5 5.072 3,450[2]–3,600 [7] −9.95 50,100[2]–53,000 [7] 591[7]–627[2]
8K55.1063,840[2] (3,800[6]―3,900[7])−10.2384,000[7] (38,900[6]–87,100[2]) 635[7] (455[6]–667[2])
9M5 5.233 3,300[6]–3,450[2] −10.28 35,500[6]–69,200[2] 576[6]–736[2]
10M5 5.244 3,400[6]–3,450[2],3,500[7] −10.03 53,700[2] (46,800[6]–72,000[7]) 623[6]–649[2],730[7]
11M45.2563,500[6]–3,535[2],3,600[7]−10.0849,000[7] (41,700[2]–60,300[6]) 551[2]–654[6],569[7]
13M4 5.439 3,535[2]–3,700[7] −9.85 47,000[7]–49,000[2] 499[7]–590[2]
14M3 5.443 3,600[7]–3,605[2] −9.77 27,000[7]–47,900[2] 422[7]–561[2]
15M2 5.513 3,660[2]–3,700[7] −9.59 14,000[7]–42,700[2] 287[7]–514[2]
16M3 5.597 3,605[2] −9.50 37,200[2] 494[2]
17K3 5.619 4,000[7]–4,015[2] −9.99 47,000[7]– 79,400[2] 451[7]–582[2]
18M4 5.632 3,535[2]– 3,800 [7] −9.36 31,600[2]–53,000 [7] 474[2]–531[7]
19M3 5.801 3,605[2] −9.17 27,500[2] 425[2]
20M2 5.805 3,660[2] −9.32 33,100[2] 453[2]
21M2 5.824 3,660[2] −9.81 51,300[2] 563[2]
23M4 5.840 3,500[7]–3,535[2] −10.35 59,000[7]–77,600[2] 661[7]–743[2]
26M3 6.003 3,605[2] −9.16 27,500[2] 425[2]
27M2 6.130 3,660[2] −9.19 29,500[2] 427[2]
29M0 6.146 3,790[2] −8.86 24,000[2] 359[2]
30M1 6.200 3,745[2] −8.82 22,400[2] 355[2]
31M1 6.244 3,745[2] −9.24 32,400[2] 427[2]
49K4 7.324 4,000[7] (3,700[6]–3,920 [2]) −11.30 245,000[2] (132,000[6]–390,000[7]) 1,074[2] (884[6]–1,300[7])
52M0 7.419 3,790[2]–3,800[7] −8.72 20,900[2]–24,000[7] 335[2]–357[7]
72M0 7.920 3,790[2] −8.62 19,000[2] 320[2]

See also

References
  1. Froebrich, D.; Scholz, A. (2013). "The main sequence of three red supergiant clusters". Monthly Notices of the Royal Astronomical Society. 436 (2): 1116–1122. arXiv:1308.6436. Bibcode:2013MNRAS.436.1116F. doi:10.1093/mnras/stt1633. ISSN 0035-8711.
  2. 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.
  3. 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.
  4. Negueruela, I.; Marco, A.; González-Fernández, C.; Jiménez-Esteban, F.; Clark, J. S.; Garcia, M.; Solano, E. (2012). "Red supergiants around the obscured open cluster Stephenson 2". Astronomy & Astrophysics. 547: A15. arXiv:1208.3282. Bibcode:2012A&A...547A..15N. doi:10.1051/0004-6361/201219540. S2CID 42961348.
  5. 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.
  6. 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.
  7. 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.
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