3. Morphology and physical conditions
For clearness, we show in Figs. 1 and 2 a summary with the main features observed in the molecular lines and HI maps. In Figs. 3, 4 and 5 we show the complete spectral maps of 12CO, 13CO and HI emissions respectively. In all the figures, the offsets are given relative to the position R.A.(1950)=21:01:00 Dec(1950)=67:58:00.0, offset (1.7", 4.7") from the star position. The spectra toward some selected positions are shown in Figs. 6, 7 and 8.
3.1. The cavity
Both molecular and atomic emission show that the star is located in a large ( 1.5 pc 0.8 pc) cavity of the molecular cloud which is spatially coincident with the optical nebula. The shape of this cavity is biconical with the western lobe much larger than the eastern one, and the star located 50 " east from the apex of the cavity (see Figs. 1 and 3). Within the cavity, the emission is not uniform. It presents a symmetric multi-shell structure that is better observed in the 12CO emission (see Figs. 1 and 3). Close to the velocity of the ambient molecular cloud the 12CO map shows the existence of a molecular filament that divides the western lobe in two subcavities. The first one has similar size to the eastern lobe and is located symmetric to it relative to the apex of the biconical cavity. The second subcavity seems also to have a counterpart beyond the intense eastern wall. But because of the complexity of this region, this tentative subcavity is not so clearly observed. The cavity is bordered by a bright rim in the 13CO J=10 emission. Intense 13CO clumps are found in this rim with the most intense located in the waist of the cavity and beyond the eastern wall. In this wall, the cavity shows two tunnels that penetrates into the molecular cloud (see Figs. 2 and 4). The straightness and direction of these tunnels suggest that they have been excavated by the outflow.
We have derived the gas column densities towards some selected positions in the walls of the cavity assuming a kinetic temperature of TK = 30 K and a hydrogen density of n(H2) = 5 103 cm-3 (as derived by Fuente et al. (1990, 1993) from NH3 and CO data) and fitting the observed lines with an LVG code. These positions are marked in Figs. 1 and 2 and the results are shown in Table 1. The derived 13CO column densities are 1 - 3 1016 cm-2. Within the cavity, the 13CO column densities have been estimated assuming optically thin emission and a rotation temperature of 15 K (Rogers et al. 1995, Gerin et al. 1998). The obtained 13CO column densities are also shown in Table 1 and the selected positions are marked in Figs. 1 and 2. The 13CO column densities are 6 1015 cm-2 over the whole cavity, with a mean value of 1.6 1015 cm-2. Then, the contrast between the walls and the interior of the cavity is of a factor 10.
Table 1. Physical conditions
Table 2. Energetics of the outflow
Both, in 12CO and 13CO emissions, the region present a bipolar structure. But the bipolarity turns into monopolarity in the HI image. The HI emission is formed by two intense filaments arranged in a "" shape feature with its apex spatially coincident with the apex of the molecular cavity (see Figs. 1, 2 and 5). These filaments have a length of 1 pc and run in straight lines adjacent to the walls of the eastern lobe of the cavity until they penetrate into the molecular cloud. Beyond the eastern wall, the HI filaments are spatially coincident with the tunnels detected in 13CO emission. Assuming that the size of the filaments along the line of sight is the same as its thickness, the mean and peak hydrogen densities in the filaments are respectively 103 cm-3 and 4 103 cm-3. These densities are two orders of magnitude larger than the mean hydrogen density estimated by Rogers et al. (1995) from a lower resolution HI image (synthesized beam 63.5"58.4"). Therefore, the atomic gas is not filling the eastern lobe of the cavity with a uniform density. On the contrary, it is concentrated in a hollow cone adjacent to the molecular walls. The HI filaments are the walls of this cone.
3.2. The bow shock
Our 12CO maps show the existence of a very hot spot (TMB (12CO 21) 60 K) that is located at the tip of the northern HI filament (see the panel at 3.7 kms-1 in Fig. 3). The shape, high brightness temperature and location of this feature reminds a bow-shock. Since this hot spot is a relatively small region (thickness 27"), we will use the main beam temperature to determine the physical conditions of the gas. In Fig. 8 we present the HI, 12CO and 13CO spectra towards this position [offset (200", 140")]. A narrow ( = 0.6 kms-1) and intense line (TMB 60 K) appears in the 12CO J=21 spectrum at a velocity of 4 kms-1, i.e., the velocity of the HI emission. The comparison between the 12CO and 13CO spectra shows that although the 12CO is self-absorbed at the velocity of the ambient molecular cloud (2.5 kms-1), the spike at 4 kms-1 is not due to self-absorption. In fact, this feature appears as a weak peak in the 13CO J=10 spectrum. It is a well differentiated velocity component with a characteristic velocity (v 4 kms-1) and a well defined spatial distribution (see the panel at 3.7 kms-1 in Fig. 3). Assuming a rotation temperature of 60 K and fitting this component in the 13CO spectra, we derive a 13CO column density of 1015 cm-2. Therefore, this component corresponds to a extremely warm (Tk 80 K) and thin (Av 1 mag) layer of molecular gas, located at the tip of one of the HI filaments at a distance of 0.6 pc from the star. Following the bright rim that delineates the edge of the cavity, the bow-shock is located at the most distant position from the star. However, its kinetic temperature is the highest. This shows that this gas is not heated only by the UV radiation from the star. An additional heating mechanism is required. The morphology and the kinematical structure suggest that it has been heated by the shock produced when the high velocity HI gas impinges into the molecular cloud. Hereafter we will refer to this feature as the "bow-shock" and it is marked with this name in Figs. 2 and 8.
© European Southern Observatory (ESO) 1998
Online publication: October 21, 1998