3. Comparison with observations at other wavelengths
In order to relate the HI structures, identified in the previous section, to the infrared ring discovered by KBT, we created a optical depth map of the region from the IRAS 60 and 100 micron images (Fig. 6). Optical depth maps reflect the spatial distribution of dust particles more reliably than the original brightness images, because the variance in dust emissivity due to the different dust temperatures has been eliminated. The IRAS images were taken from the Infrared Sky Survey Atlas (Wheelock et al. 1994). In order to increase the signal-to-noise ratio, we convolved both images with a Gaussian filter to a lower spatial resolution of . The galactic background emission has been subtracted using the following procedure: (1) assuming that the large-scale background emission depends only on the galactic latitude, we determined the lowest pixel values in strips parallel to the galactic plane; (2) plotting these minimum pixel values versus b, the background emission for was approximated by a 2nd order polynomial fitted to the lower envelope of the distribution of the points on the plot, and by a constant value for .
A colour temperature map has been derived from the 60 and 100 micron maps by using a dust emissivity law as a blackbody intensity B multiplied by (Hildebrand, 1983). Assuming that the 100µm emission is optically thin over the region and that the dust temperature is constant along the line of sight, we calculated the 100µm optical depth for each pixel as =I(100)/B(100,T), where is the 100µm brightness. Since the optical depth is proportional to the column density of dust grains (Jarrett et al. 1989), the resulted contour map, shown in Fig. 6, reflects the distribution of the interstellar dust in the studied region.
The distribution of interstellar dust in Fig. 6 displays a closed ring of dust clouds (the Cepheus Bubble) which surrounds an almost dust-free region at to to . The inner boundary of the dust ring is relatively well defined unlike the outer boundary, which is often contaminated with other dust complexes. The comparison of Fig. 2 and Fig. 6 demonstrates that - in addition to the general similarity of the two ring patterns - most parts of the infrared ring can be directly associated with HI clouds, making it possible to assign radial velocities to the dust structures.
Many of the infrared structures revealed by the optical depth map are positionally associated with reflection nebulae and HII regions. These associations were used to derive the distance value of the Cepheus Bubble by KBT. Having radial velocity values for the HI clouds and assuming that the dust, emitting in the infrared and reflecting the starlight in the optical, is associated with the hydrogen gas along the line of sight, we can now confirm these associations in the velocity space. We made a literature search for radial velocities measured at various wavelengths towards the reflection nebulae and HII regions, and compared the results with the radial velocity values of the associated HI clouds. Table 2 summarizes the results.
Table 2. Distances and velocities of HII regions and reflection nebulae and coinciding HI clouds along the Cepheus Bubble
Fig. 6 reveals a prominent dust cloud at . It does not stand out from the background in the 100µm IRAS image. This structure coincides in projection with a strong HI structure in the velocity range kms-1 (Figs. 1 and 2). The 12CO survey of the Milky Way (Dame et al. 1987) reveals 12CO emission at the velocities of about -2.4 kms-1 and +1.2 kms-1 (Dame 1999) in this direction. No conspicuous optical object can be associated with this interstellar feature. In order to get an estimate of its distance we examined a Wolf-diagram, displaying the cumulative distribution of field star distance moduli along the line of sight to this cloud. In order to obtain the distance moduli of stars we combined objective prism spectral types from plates obtained with the Schmidt telescope of Konkoly Observatory (mirror: 90 cm, correction plate: 60 cm) with V magnitudes from the Guide Star Catalog. For details of this method see Kun (1998). Fig. 7 shows the logN(V) curve of B0-A2 type stars (M(V) 2 mag). Due to the absorbing clouds along the line of sight the slope of this diagram changes with respect to the comparison curve displaying the distribution of distance moduli without interstellar extinction. The distance of the absorbing layers can be obtained by using the points of the diagram at which it turns parallel to the reference curve. The distance is read from the reference curve, by intersecting the two curves with horizontal lines, as the dotted arrows indicate. The distortions in the shape of the curve in Fig. 7 suggest the presence of absorbing dust layers at 420 and 540 pc and possibly another one at around 700 pc, though this latter is uncertain due to the magnitude limit of the spectral classification. Assuming association between dust and atomic hydrogen, this result indicates the existence of several distinct interstellar structures along the line of sight, as also suggested by the factor maps in Fig. 3 (three factors, Facts. 2, 4, and 5, show emission in this direction) and by the double peak in the CO spectrum (Dame 1999).
The HI, optical and star count results, listed above, indicate that although several sections of the ring (mainly at the lower galactic longitude side) are superpositions of several distinct interstellar features, for about three-quarter of the ring the best available distance estimate is 800-900 pc. We conclude that - in agreement with KBT's proposal - the Cep OB2 association is surrounded by a ring of physically connected interstellar clouds. In addition, even in the section there are indications for the existence of interstellar matter beyond 400 pc, and the possibility that the Cepheus Bubble is completely closed seems to be plausible.
We also examined the maps of the soft X-ray diffuse background from the ROSAT XRT/PSPC All-Sky Survey (Snowden et al. 1995) and found no excess emission towards the interior of the Bubble in any of the three energy bands. The absence of X-ray emission indicates the lack of a significant amount of very hot gas inside the HI-cavity.
Finally we calculated distances for the two O9-type stars of Cep OB2a, HD207198 and HD209975. Using Simonson's (1968) spectral classification, Schmidt-Kaler's (1982) absolute magnitudes and colours, and the Tycho (ESA 1997) photometry, the derived values for the two stars are 930 and 870 pc, respectively. Both distance values suggest that the stars probably lie inside the Bubble. In addition, HD207198 appears close to the centre of the Cepheus Bubble (Fig. 6), suggesting that its UV emission and stellar wind might have played an important role in creating the ring-like structure.
© European Southern Observatory (ESO) 2000
Online publication: February 9, 2000