Astron. Astrophys. 354, 645-656 (2000)

1. Introduction

Analysing the IRAS 60µm and 100µm maps of Cepheus, Kun, Balázs and Tóth (1987, hereafter KBT) reported that the area to , to is faint at far-infrared wavelengths, and is encircled by a ring-shaped region of enhanced infrared emission, the `Cepheus Bubble'. They compared the IRAS maps with Dubout-Crillon's (1976) H photographs, and demonstrated that the brightest parts of the infrared ring correspond to the HII regions IC 1396, Sh2-129, Sh2-133, Sh2-134, and Sh2-140 as well as to fainter or smaller HII regions not listed in Sharpless' (1959) catalogue. Since most of the listed bright HII regions were placed at about 900 pc from the Sun in the literature, KBT accepted this value for the Cepheus Bubble as well.

The infrared ring is probably related to the loop structure discovered at by Brand and Zealey (1975) as semi-circular filamentary dust lanes and faint emission nebulosity on the POSS prints. They associate this structure with Cep OB2 by noting that to the west the dust seems to interact with IC1396, although in their Table 1 erroneously Cepheus OB1 is given as related OB association. In the area encircled by the infrared ring a deficiency in HI column density was observed by Simonson and van Someren Greve (1976, hereafter SVSG) in the  kms^-1 radial velocity range. The infrared ring coincides positionally with the OB association Cepheus OB2, which consists of two subgroups: the older and more dispersed one, Cep OB2a ( yrs, de Zeeuw and Brand 1985) occupies the interior of the ring, while the younger and smaller subgroup, Cep OB2b ( yrs, SVSG) is situated at the edge of the ring. Since the distance of Cepheus OB2 is also about 900 pc, KBT proposed a physical link between the association and the bubble. A similar link was assumed earlier by SVSG, interpreting the low HI content in the region as a sign of the full ionization of the interstellar gas by the Cep OB2 association.

Table 1. Results of the principal components analysis. Columns 3 and 4 give the explained percentage and the cumulative percentage of the factors, respectively. Column 5 shows the radial velocity ranges where the corresponding factor is dominant.

KBT proposed that the Cepheus Bubble had been created by the strong stellar wind/UV radiation and the subsequent supernova explosion of the most massive star in the older subgroup Cep OB2a. The exploded star was perhaps the former companion of the runaway star  Cep whose proper motion points backwards approximately to the centre of the bubble. From tracing back this motion, KBT estimated an age of 310⁶ yrs for the bubble. The new proper motion measured by the Hipparcos satellite does not significantly change this age estimate. The formation of the younger subgroup, Cep OB2b could have been triggered by stars of Cep OB2a via stellar wind and propagating ionization fronts as proposed by KBT and Patel et al. (1995). This trigger could also be responsible for the birth of OB stars and cold embedded IRAS sources along the periphery of the Cepheus Bubble (KBT, Balázs & Kun 1989).

Recently de Zeeuw et al. (1999) determined the distances of nearby OB associations using proper motions and parallaxes measured by Hipparcos. Their work on 49 members of Cep OB2 resulted in a distance of 55930 pc, significantly lower than the 900 pc assumed before. However, the recalibration of the upper main sequence, suggested by Hipparcos, would also lessen the distance of those objects along the periphery of the bubble (HII regions and reflection nebulae) which were used as distance indicators for the Cepheus Bubble by KBT. Thus the arguments for the physical connection of the bubble with Cep OB2 are probably not affected by the new distance values. Therefore further investigations are needed before accepting the shorter Hipparcos scale.

A better understanding of the history of this Cepheus region requires more detailed mapping of the interstellar matter (including better distance estimates) as well as information on the large scale motions. Velocity information can also help to separate distinct interstellar features projected on the IRAS images. Recently, Patel et al. (1998) conducted a CO (1-0) spectral line survey in Cepheus and discussed the origin and evolution of the Cepheus Bubble on the basis of the overall distribution of molecular gas. They also performed an HI survey and concluded that the bulk of the interstellar gas associated with the bubble is in atomic form. In this paper we investigate the large scale morphology and kinematics of the Cepheus Bubble by a detailed analysis of the distribution of atomic hydrogen.

In the following we (1) analyse HI 21 cm measurements taken from the Leiden/Dwingeloo survey, in order to identify the atomic gas component of the Cepheus Bubble; (2) compare the hydrogen maps with IRAS and X-ray data; and (3) determine the distance of a prominent part of the bubble by using a Wolf-diagram built up from our optical observations. In addition to the more traditional methods, the analysis of the HI maps is also performed by using multivariate statistical methods. After connecting these new pieces of information into a coherent picture, we speculate about the possible origin of the Cepheus Bubble.

© European Southern Observatory (ESO) 2000

Online publication: February 9, 2000
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