A&A 371, 675–697 (2001) Astronomy DOI: 10.1051/0004-6361:20010387 & c ESO 2001 Astrophysics The Cygnus superbubble revisited B. Uyanıker1,2,E.F¨urst1,W.Reich1,B.Aschenbach3, and R. Wielebinski1 1 Max–Planck–Institut f¨ur Radioastronomie, Postfach 2024, 53010 Bonn, Germany 2 Dominion Radio Astrophysical Observatory, Herzberg Institute of Astrophysics, Penticton, B.C., Canada V2A 6K3 3 Max–Planck–Institut f¨ur Extraterrestrische Physik, Postfach 1603, 85740 Garching, Germany Received 18 January 2001 / Accepted 8 March 2001 Abstract. The Orion local spiral arm is seen tangential towards the Cygnus region. Intense radio emission with quite a complex morphology is observed, which appears to be surrounded by strong soft X–ray emission. This remarkable X–ray structure is known as the Cygnus superbubble. We compare a recent 1.4 GHz radio continuum and polarization map from the Effelsberg 100-m telescope with X–ray data from the ROSAT all–sky survey of this area. Including available survey data of the infrared, H i and CO emission, we investigate a number of high latitude features, which are physically related to one of the Cygnus OB associations. These OB associations, however, are located along the local arm at different distances. Our results support the view that the Cygnus superbubble is not a physical unity, but results from a projection of unrelated X–ray emitting features at different distances blown out from the local arm seen along the line of sight. Key words. ISM: Cygnus superbubble – radio continuum: ISM – X–ray: ISM – galaxy: structure 1. Introduction interstellar matter (Castor et al. 1975; Weaver et al. 1977; Abbott et al. 1981) and single or sequential SN explosions The radiation and wind from massive stars and super- (Elmegreen & Lada 1977; Tomisaka et al. 1980; Higdon nova (SN) explosions determine the structure and energy 1981). content of the interstellar medium (ISM). Topologically, When the extent of a superbubble becomes comparable the ISM appears to be highly inhomogeneous. Much of its to the thickness of the Galactic disc, it may break out mass is concentrated in clouds with a small filling factor, of the disc and the hot gas will escape. A superbubble whereas much of its volume is filled with warm inter-cloud remains bound to the disc if its mechanical luminosity is gas (see Spitzer 1990 for a review). A hot gas compo- − around L ≤ 31037 erg s 1 in a uniform magnetic field nent exists as well, which is generated by SN explosions SN as large as 5 µG (Tomisaka 1990; Tomisaka 1998; see also and strong winds from massive stars (Cox & Smith 1974; Ferri`ere et al. 1991, for the effect of magnetic fields on McKee & Ostriker 1977; Ferri`ere 1995). This tenuous gas the evolution of superbubbles). Thus, the formation of a has a density of about 10−3 cm−3 and a temperature of gaseous halo around a spiral galaxy is connected to the OB about 106 K. Cool, dense clouds reaching a density of associations with a high star formation rate. The escaped 103 cm−3 with a temperature around 100 K float in this hot gas might rise several kpc above the Galactic plane, hot material. These clouds are birth places for new stars. where it cools radiatively. The cooled material falls back The new stars, when of type O or B, have strong winds. to the disc and can be observed as high velocity clouds They burn their energy rapidly to become a supernova. (Bregman 1980). A study of superbubbles extending to In the vicinity of a massive star, either because of the high latitudes, therefore, provides useful information on strong wind or due to the explosion of the star, the sur- themutualrelationbetweenthediscandthehaloofthe rounding ISM is blown off. In particular, the combined Galaxy. effects from SN explosions and stellar winds from OB as- Three giant bubbles have been identified so far in sociations produce quite large cavities filled with hot gas the local arm of the Milky Way. These structures are and are called “superbubbles”. They are observationally known as Orion-Eridanus (Reynolds & Ogden 1979; Guo traced at optical, radio and X–ray wavelengths. The origin et al. 1995; Brown et al. 1995), Scorpio-Centaurus (Weaver of this hot gas has been the topic of numerous investiga- 1979; Egger 1998) and the Cygnus superbubble (hereafter tions including the interaction of the stellar winds with CSB, Cash et al. 1980) according to the constellations Send offprint requests to:W.Reich, where they are located. Recently Heiles (1998) reported ◦ ◦ e-mail: [email protected] a possible bubble towards ` ∼ 238 and b ∼ 9 . While Article published by EDP Sciences and available at http://www.aanda.org or http://dx.doi.org/10.1051/0004-6361:20010387 676 B. Uyanıker et al.: The Cygnus superbubble revisited Table 1. OB associations in Cygnus region. The table is Table 2. The list of H ii regions, whose angular diameters are a compilation from Humphreys (1978), Garmany & Stencel ∼>100, selected from the Sharpless catalogue (1959). The dis- (1992), Bochkarev & Sitnik (1985), Sitnik & Mel’nik (1996) tances and VLSR values are taken from the list of kinematically and Mel’nik & Efremov (1995) distinct H ii regions of Brand & Blitz (1993). The distance es- timate of S117, 0.5 kpc, is adopted from Straiˇzys et al. (1989) name `b∆` ∆b dista ageb (◦)(◦)(◦)(◦) (kpc) (Myr) name `bdiam dist VLSR ◦ ◦ 0 −1 OB1 75.50 1.17 3.0 3.4 1.25–1.83 7.5 ( )()()(kpc)(kms) OB2 80.10 0.90 – – 1.44–2.10 5.0 S 92 064.08 +01.65 50 OB3 72.55 2.20 2.5 2.2 1.58–2.51 8.3 S 94 064.93 +06.77 25 OB4 82.50 −4.30 3.0 2.0 1.0 c S 96 066.10 +07.18 25 OB5 67.10 2.10 5.8 9.6 c1.61 c S 97 066.87 +00.91 10 3.90 21.0 OB6 86.00 1.00 6.0 8.0 c1.70 c S 98 068.15 +01.02 15 OB7 90.00 2.05 12.0 13.9 0.74–0.80 13.0 S101 071.58 +02.84 20 2.50 13.7 − OB8 77.75 3.75 2.9 3.3 2.19–2.32 3.0 S102 071.41 05.31 40 − OB9 78.00 1.50 2.0 1.4 1.17–1.20 8.0 S103 074.12 08.33 210 S105 075.46 +02.43 18 a This column indicates the distance interval for a given OB S108 078.19 +01.81 180 1.5 association. S109 079.49 +00.15 1080 1.4 b Age obtained from the Hertzsprung-Russel diagram. S110 079.60 −12.17 50 c Doubtful. S111 081.19 −17.01 90 S112 083.78 +03.29 15 2.10 −4.0 S113 083.71 −08.25 15 S115 084.84 +03.91 50 3.0 the Orion-Eridanus bubble has been identified by its Hα S117 085.49 −00.99 240 0.5–0.8 0.0 emission and the Sco-Cen bubble by its H i filaments, the S118 087.49 −08.92 480 CSB is identified by its strong X–ray emission. The CSB S119 087.60 −03.84 160 0.7 3.5 is the largest bubble. Its angular size is 18◦ × 13◦ along S123 091.15 −06.35 13 Galactic longitude and latitude, respectively. It is very S124 094.48 −01.54 70 − − complex and extends up to latitudes well above the lo- S126 095.39 16.80 160 0.6 0.2 S129 098.50 +07.97 140 0.4 −13.9 cal spiral arm. Numerous OB associations at different dis- S131 099.29 +03.73 170 tances are located in its direction. The physical connection S132 102.79 −00.65 90 to the CSB is unclear. Therefore, the distance to the CSB is poorly known and its size and energetics remain un- certain (Ruprecht et al. 1981; Bochkarev & Sitnik 1985; Garmany & Stencel 1992; Comer´on et al. 1998; de Zeeuw 2. The relation of the Cyg OB associations et al. 1999). Since the line of sight is along a spiral arm, to the Cygnus superbubble the confusion of the various objects is large and the dis- crimination of individual structures is difficult. The CSB was discovered by Cash et al. (1980) in the In this paper we investigate the CSB and try to dis- soft X–ray regime using the A-2 experiment on board the entangle the superposition of structures along the local HEAO-1 satellite. They found a temperature of the emit- spiral arm by using new, sensitive radio continuum and ting gas of T =(1.6–2.5) 106 K and a column density of 21 −2 polarization data at 1.4 GHz (Uyanıker et al. 1999; Reich NH =(5.6–9) 10 cm , assuming a thermal spectrum et al. 1990), ROSAT all–sky survey data (Aschenbach from an optically thin hot plasma. They adopted a dis- 1993), IRAS 60 and 100 µm images (Beichmann et al. tance of 2 kpc for the X–ray structure, based on the high 1985), H i data from the Dwingeloo Survey (Hartmann value of NH obtained from the spectral analysis and a pos- & Burton 1997) and CO data at 115 GHz (Dame et al. sible relation of the X–ray structure with the Cyg OB2 1987). The paper is organized as follows: in Sect. 2 we association (see Table 1). At 2 kpc the size of the CSB discuss the OB associations towards the CSB, their place- is 600 × 450 pc and the total energy content is 1054 erg, ment in the region and possible relations to the observed which is equivalent to the energy of about 1000 SN explo- X–ray structures.