IC 1396 is the most prominent part of the Cepheus bubble. The reddish star towards the top of the nebula in this image is the enormous cool supergiant Mu Cephei.
Source: Davide De Martin - http://www.skyfactory.org
The Cassiopeia arc, the brightest part of the Perseus arm visible from Earth, reaches its westernmost extent with the end of the Cep OB1 association in this sector. Objects continue to appear in the Perseus arm west of the Cassiopeia arc, however, including Sh 2-124 and the giant filamentary HII region CTB 102. This sector also includes the spectacular Sh 2-131 (IC 1396) and the beginning of the Rift dust clouds.
IC 1396
The Cepheus molecular clouds located at a distance of about 500 to 900 parsecs are an important part of this sector, and include the dramatic HII region Sh 2-131 (IC 1396). IC 1396 is the most prominent part of the Cepheus bubble, the expanding ring of gas and dust surrounding the Cep OB2 association. The Cepheus bubble is created by stellar winds from the hot stars in Cep OB2 and was most likely triggered by a supernova about 2 or 3 million years ago. [1], [2]
IC 1396 contains about 12 thousand solar masses of gas and dust and is ionised by the star cluster Trumpler 37. [3] There are about 480 visible stars in Trumpler 37 [4] including about 20 B-class stars. [5] Kharchenko lists 10 ionising stars - including 9 B-class stars and one remarkable O-class multiple star: HD 206267 (O6.5V+O9). She gives the age as 4.8 million years and the distance as 835 parsecs. [6] A recent study based on data from the Hipparcos astrometry satellite found the mean distance to Cep OB2 to be 615 ± 35 parsecs, so the Kharchenko estimate may be too large. [7]
In addition to the "A" component of HD 206267, which is actually a close O6.5V+O9 class binary, the system contains three other dimmer components. [8].
A second O-class star that may be in Trumpler 37 (although not listed as a probable member by Kharchenko) is the O9 class HD 204827. HD 206267 is by far the hottest and most influential star in the region, however. [9]
IC 1396 includes at least ten globules - dark clouds of dust and gas in which new stars are forming. The size and rate of star formation in these globules is strongly affected by their distance from HD 206267. [10] The best known of these globules is LBN 452 (IC 1396a), often called the Elephant Trunk nebula. This 200 solar mass globule contains two T Tauri stars (which have created a prominent cavity in the globule) and several dimmer protostars, with much of the star formation being driven by intense radiation from the nearby HD 206267. [5]
IC 1396 appears to contain Mu Cephei, also called Herschel's Garnet Star. This cool M1I class semi-regular pulsating supergiant has a radius about 1420 times the solar radius, making it one of the largest known stars in the Milky Way. [11]
IC 1396 has no common name, but given the location of the Elephant Trunk nebula, perhaps IC 1396 as a whole could be called the Elephant nebula?
Kharchenko and her colleagues have discovered two new ionising star clusters, ASCC 1114 and ASCC 1115, which respectively lie at distances of 550 parsecs and 600 parsecs, between Cep OB2 and the Cyg OB7 molecular clouds. ASCC 1114 has one B2 class ionising star, DM +53 2683, and is 56.2 million years old. ASCC 1115 also has one B2 class ionising star, DM +50 3496, and is 389 million years old. [12] These ages, especially the age for ASCC 1115, are unusually old for clusters that contain ionising stars.
Cyg OB7 and the beginning of the Rift
Cyg OB7 was the name of a proposed OB association. Several investigations have failed to provide strong evidence that it actually exists. [13], [7]. The only O-class star in the direction of Cyg OB7 in this sector is the O7.5 class LS III +50 28, which is believed to ionize Sh 2-124 at a distance of 2600 parsecs. [14] There is one B1I class supergiant, BD+48 3437, which lies beyond the Perseus arm at a distance of 6420 parsecs. [15] De Zeeuw, Hoogerwerf, and de Bruijne point out that "the line of sight toward Cygnus runs parallel to the local spiral arm. Some of the previous claims for OB associations based on large numbers of supergiants and/or early-type stars in this direction, e.g., Cyg OB4 and Cyg OB7, may therefore be the result of chance projections." [7]
Stars embedded deeply in the Cyg OB7 dust clouds are visible in this microwave image, which also shows the major ionising stars, clusters and associations in this sector.
Source: Galactic Plane Explorer microwave image
What certainly does exist in this direction is a complex of molecular clouds at a distance of about 800 parsecs that continues the Cepheus clouds seen to the east. By tradition, this complex is called the Cyg OB7 molecular clouds after the OB association that was once believed to lie in this direction. The Cyg OB7 clouds are very dusty. They obscure the sky so effectively in this direction that the central dark nebula [K60] 141 (less formally called Khavtassi 141) is often called the "Northern Coalsack" [7], [16] although this term is also used for several other dark clouds in Cygnus as well. Cyg OB7 begins the complex of local dust clouds that stretch from about 95° to 25° in galactic longitude and obscure large parts of the Milky Way in visual light. This complex is usually called the Rift because it appears to tear the Milky Way asunder. Fortunately, observations in radio, microwave and infrared frequencies make it possible to pierce the Rift clouds and observe what lies beyond.
Embedded within the [K60] 141 dark nebula and prominent at microwave frequencies is a 900 solar mass molecular cloud that contains the HII region IRAS21078+5211 (WB89 43) and the protostar GH2O092.67+03.07. The protostar is very young (about 3500 years old) and is surrounded by a thick rotating disk that is feeding material falling onto the central object. Despite its very young age, the material appears to have been accumulating so quickly that the central object may already be about 6 solar masses in size. [16]
SNR G093.7-00.3
The supernova remnant SNR G093.7-00.3 (also sometimes informally, but not officially, called CTB 104A) has a diameter of 35 parsecs and is located between the Cyg OB7 molecular clouds and the Perseus arm at a distance of about 1500 parsecs. No age estimate is available but it is noted that the remnant is "relatively old". [17] SNR G093.7-00.3 has several projections that extend well beyond the typically spherical shape of a supernova remnant. One explanation is that the remnant is expanding into an interstellar medium with portions that have an unusually low gas density that allows the expanding remnant gas to leak more quickly into the surrounding space. [18]
The prominent radio source 4C 50.55 at the northwest edge of SNR G093.7-00.3 is not associated with the supernova remnant - it is an extremely bright nearby radio galaxy. [19]
A second prominent radio source some distance to the west of SNR G093.7-00.3, 3CR 431 (NRAO 654), is well studied, with references in at least 35 scientific papers. None of these papers appear to suggest what it is, however.
Perseus arm
The Cassiopeia arc, the brightest part of the Perseus arm visible from Earth, reaches its westernmost extent with the end of the Cep OB1 association in this sector. The western region of Cep OB1 includes two B-class supergiants (LS III +53 32 (B2Ib) and the variable BD+52 3088 (B2Iab)) as well as the O-star HD 235673 (O 6.5 V). [20],[SIMBAD]
There are other objects in this sector that occur at the distance of the Perseus arm, including the mysterious HII region Sh 2-124, located some distance to the west of Cep OB1. Although very little appears in the scientific literature on this region, recent detailed hydrogen-alpha imaging done for the IPHAS project reveals the structure of this violin-shaped nebula in spectacular detail. Blitz, Fitch and Stark give a distance estimate of 2600 parsecs. [21] Chini and Wink identify two uncatalogued ionising stars, with classes O7V and B2V, and give a distance estimate of 3600 parsecs. [22] Crampton, Georgelin and Georgelin give a kinematic distance (based on gas velocity) of 4400 parsecs. [23] Felli and Harten give a list of 5 ionising stars, including the O7.5 class LS III +50 28, and support the distance estimate of 2600 parsecs. [14]
Kharchenko identifies a new ionising cluster, ASCC 1116, just north of the traditional boundary of Cep OB1. The 10.7 million year old ASCC 1116 contains the B-class supergiant HD 235679 (B2Ia) and is located at a distance of about 5000 parsecs. [12]
CTB 102 (KR 1) is an enormous filamentary HII region approximately 200 parsecs in size, lying at a distance of about 4500 parsecs. Although visually obscured by dust in the local Cyg OB7 molecular clouds, KR 1 has been detected in great detail at radio frequencies by the Canadian Galactic Plane Survey. [24]
The 25 thousand-year-old supernova remnant SNR 94.0+1.0 to the southeast of KR 1 is also located at a distance of about 4500 parsecs. [25]
The enormous filamentary HII region CTB 102, a major component of the Perseus arm, is visible at radio frequencies behind the Cyg OB7 dust clouds. This image also shows the major HII regions and supernova remnants in this sector.
Source: Galactic Plane Explorer radio image
According to scientists with the Canadian Galactic Plane Survey, the supernova remnant SNR G96.0+2.0 lies at a distance of 4000 parsecs "in the Perseus arm, which is a quite reasonable assumption in this direction of our Galaxy. The SNR would be located at the western edge of a large H I cavity ... probably a stellar wind bubble created by its progenitor and maybe a few other stars some of which may have not exploded yet and others may even have contributed with their explosion to the shape of this bubble." They also note that "The visible part of the remnant has a diameter of about 30 pc ... is a shell-type remnant in an early stage of development." [26]
Beyond the Perseus arm
The massive and luminous star formation region IRAS 21413+5442, in the same direction as the dark nebula LDN 1084 and the CO source WB89 114, is located at a distance of 5390 [27] or 7400 [28] parsecs. The brightest star is a massive O6 class young stellar object. The complex also includes several hot B or late O class stars. [29]
The faint nebulae Sh 2-120 and Sh 2-121 both appear to be located beyond the Perseus arm at a distance of about 6300 parsecs. [30] Chan and Fich give much larger distant estimates of 8590 ± 770 and 8080 ± 750 parsecs for Sh 2-120 and Sh 2-121 respectively. [31]
A radio map of Sh 2-120 shows a shell-like structure around a central source. [32] Very little information is available in the literature apart from this map.
The HII region Sh 2-121 is located at a distance of 6500 parsecs and is surrounded by an expanding shell containing about 39 solar masses of gas. The shell is at least 650 thousand year old and is likely driven by stellar winds from an O8.5 class star. [33] A more recent paper reduces the distance to Sh 2-121 to 4500 ± 1000 parsecs, which would make it part of the Perseus arm. [34]
The distant (about 10 thousand parsecs) HII region BG 2107 + 49 is located near in the sky to Sh 2-121. It is surrounded by a large 150 parsec shell containing 120 thousand solar masses of gas. The shell has been expanding for about 1.9 million years and is likely driven by solar winds from a central O4 class star. [35]
The HII regions Sh 2-127 and Sh 2-128 are located towards the outer edge of the Perseus arm if not far beyond. Sh 2-127 contains the infrared stellar group [BDS2003] 24 and is located at a distance of 9700 [30], 8700 [36], 7300 [37] or 6000 x/- 1500 [34] parsecs. Radio analysis reveals that Sh 2-127 consists of two distinct components - a larger weaker and more diffuse source (WB89 85A) to the northeast, consistent with ionisation by an O7 class star, and a stronger but smaller source to the southwest (WB89 85B), consistent with ionisation by an O8.5 class star. (However, it is also possible that both components are ionised by a single star.) Both components are located near the northwestern edge of a molecular cloud and are embedded in the molecular gas. [37] SIMBAD associates Sh 2-127 with the radio source KR 17, but a recent paper identifies KR 17 with Sh 2-187 in a completely different region of sky. [24]
Sh 2-128 contains the infrared cluster candidates [BDS2003] 26 and [BDS2003] 27 and is located at a distance of 9400 [38], 8300 [36] or 4800 ± 500 [34] parsecs. A central O7 class star ionises 25 solar masses of gas and is about one million years old. A second fainter and probably younger component, Sh 2-128N, is ionised by a O9.5 class star. Both components are embedded in a 4000 solar mass molecular cloud. [38] Russeil associates Sh 2-128 with the nebulae BFS6 and BFS7, and gives a distance of 9400 ± 400 parsecs for the entire star formation region. [39]
Russeil concludes that BFS8, which is near BFS6 in the sky, is ionised by an O5V class star at a distance of 8800 ± 610 parsecs. [39]
The HII region NRAO 655 lies far beyond the Cyg OB7 molecular clouds at a distance of about 8800 parsecs. It is ionised by a group of central stars of class O8.5 or cooler. [40]
Nebulae in this sector
Notes
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2. ^ Hoogerwerf, R., de Bruijne, J. H. J., & de Zeeuw, P. T. 2001, Astronomy and Astrophysics, On the origin of the O and B-type stars with high velocities. II. Runaway stars and pulsars ejected from the nearby young stellar groups
3. ^ Weikard, H., Wouterloot, J. G. A., Castets, A., Winnewisser, G., & Sugitani, K. 1996, Astronomy and Astrophysics, The structure of the IC1396 region.
4. ^ Schulz, N. S., Berghoefer, T. W., & Zinnecker, H. 1997, Astronomy and Astrophysics, The X-ray view of the central part of IC 1396.
5. ^ Reach, William T., Rho, Jeonghee, Young, Erick, Muzerolle, James, Fajardo-Acosta, Sergio, Hartmann, Lee, Sicilia-Aguilar, Aurora, Allen, Lori, Carey, Sean, Cuillandre, Jean-Charles, Jarrett, Thomas H., Lowrance, Patrick, Marston, Anthony, Noriega-Crespo, Alberto, & Hurt, Robert L. 2004, Astrophysical Journal Supplement Series, Protostars in the Elephant Trunk Nebula
6. ^ Kharchenko, N. V., Piskunov, A. E., Röser, S., Schilbach, E., & Scholz, R.-D. 2005, Astronomy and Astrophysics, Astrophysical parameters of Galactic open clusters
7. ^ de Zeeuw, P. T., Hoogerwerf, R., de Bruijne, J. H. J., Brown, A. G. A., & Blaauw, A. 1999, Astronomical Journal, A HIPPARCOS Census of the Nearby OB Associations
8. ^ Stickland, D. J. 1995, The Observatory, Spectroscopic binary orbits from ultraviolet radial velocities. Paper 17: HD 206267
9. ^ Patel, Nimesh A., Goldsmith, Paul F., Snell, Ronald L., Hezel, Thomas, & Xie, Taoling 1995, Astrophysical Journal, The Large-Scale Structure, Kinematics, and Evolution of IC 1396
10. ^ Froebrich, D., Scholz, A., Eislöffel, J., & Murphy, G. C. 2005, Astronomy and Astrophysics, Star formation in globules in IC 1396
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12. ^ Kharchenko, N. V., Piskunov, A. E., Röser, S., Schilbach, E., & Scholz, R.-D. 2005, Astronomy and Astrophysics, 109 new Galactic open clusters
13. ^ Garmany, C. D. & Stencel, R. E. 1992, Astronomy and Astrophysics Supplement Series, Galactic OB associations in the northern Milky Way Galaxy. I - Longitudes 55 deg to 150 deg
14. ^ Felli, M. & Harten, R. H. 1981, Astronomy and Astrophysics, A high-resolution search for small-scale structure in Sharpless H II regions at 4.995 GHz. II - General properties of the entire sample. III - Description of selected sources
15. ^ Kimeswenger, S. & Weinberger, R. 1989, Astronomy and Astrophysics, An optical spiral arm beyond the Perseus arm
16. ^ Bernard, J. P., Dobashi, K., & Momose, M. 1999, Astronomy and Astrophysics, Outflow and disk around the very young massive star GH2O 092.67+03.07
17. ^ Uyaniker, Bülent, Kothes, Roland, & Brunt, Christopher M. 2002, Astrophysical Journal, The Supernova Remnant CTB 104A: Magnetic Field Structure and Interaction with the Environment
18. ^ Landecker, T. L., Higgs, L. A., & Roger, R. S. 1985, Astronomical Journal, High-resolution radio observations of the supernova remnants CTB 104A (G93.7-0.3) and 3C 434.1 (G94.0 + 1.0)
19. ^ Molina, M., Giroletti, M., Malizia, A., Landi, R., Bassani, L., Bird, A. J., Dean, A. J., de Rosa, A., Fiocchi, M., & Panessa, F. 2007, Monthly Notices of the Royal Astronomical Society, Broad-band X-ray spectrum of the newly discovered broad-line radio galaxy IGR J21247+5058
20. ^ Humphreys, R. M. 1978, Astrophysical Journal Supplement Series, Studies of luminous stars in nearby galaxies. I. Supergiants and O stars in the Milky Way. and the unpublished catalogs available here.
21. ^ Blitz, L., Fich, M., & Stark, A. A. 1982, Astrophysical Journal Supplement Series, Catalog of CO radial velocities toward galactic H II regions
22. ^ Chini, R. & Wink, J. E. 1984, Astronomy and Astrophysics, The galactic rotation outside the solar circle
23. ^ Crampton, D., Georgelin, Y. M., & Georgelin, Y. P. 1978, Astronomy and Astrophysics, First optical detection of W51 and observations of new H II regions and exciting stars
24. ^ Kerton, C. R. 2006, Monthly Notices of the Royal Astronomical Society, A sharper view of the outer Galaxy at 1420 and 408 MHz from the Canadian Galactic Plane Survey - I. Revisiting the KR catalogue and new Gigahertz Peaked Spectrum sources
25. ^ Foster, T. 2005, Astronomy and Astrophysics, The Galactic plane region near ℓ = 93°. III. Multi-wavelength emission from SNR 3C 434.1
26. ^ Kothes, R., Uyanıker, B., & Reid, R. I. 2005, Astronomy and Astrophysics, Two new Perseus arm supernova remnants discovered in the Canadian Galactic Plane Survey
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28. ^ Shepherd, D. S. & Churchwell, E. 1996, Astrophysical Journal, Bipolar Molecular Outflows in Massive Star Formation Regions
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30. ^ Russeil, D. 2003, Astronomy and Astrophysics, Star-forming complexes and the spiral structure of our Galaxy
31. ^ Chan, Gordon & Fich, Michel 1995, Astronomical Journal, An IRAS Survey of H II Regions
32. ^ Fich, Michel 1993, Astrophysical Journal Supplement Series, A VLA survey of optically visible galactic H II regions
33. ^ Vallee, J. P. 1983, Astronomical Journal, Spectral observations and physical modeling of Sharpless 121
34. ^ Foster, T. & Routledge, D. 2003, Astrophysical Journal, A New Distance Technique for Galactic Plane Objects
35. ^ van der Werf, Paul P. & Higgs, L. A. 1990, Astronomy and Astrophysics, Radio and infrared observations of the H II complex BG 2107 + 49
36. ^ Avedisova, V. S. & Palous, Jan 1989, Bulletin of the Astronomical Institutes of Czechoslovakia, Kinematics of star forming regions
37. ^ Rudolph, Alexander L., Brand, Jan, de Geus, Eugene J., & Wouterloot, Jan G. A. 1996, Astrophysical Journal, Far Outer Galaxy H II Regions
38. ^ Bohigas, Joaquín & Tapia, Mauricio 2003, Astronomical Journal, Sh 2-128: An H II and Star-forming Region in the Galactic Outback
39. ^ Russeil, D., Adami, C., & Georgelin, Y. M. 2007, Astronomy and Astrophysics, Revised distances of Northern HII regions
40. ^ Foster, T. & Routledge, D. 2001, Astronomy and Astrophysics, The galactic plane region near l=93°. I. HII region NRAO 655