SpringerLink
Forum Springer Astron. Astrophys.
Forum Whats New Search Orders


Astron. Astrophys. 339, 575-586 (1998)

Previous Section Next Section Title Page Table of Contents

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.

[FIGURE] Fig. 1. Scheme with the main features observed in the HI and 12CO emissions towards NGC 7023. The grey scale and dashed contours corresponds to the integrated intensity of the 12CO J=2[FORMULA]1 line at a velocity of 1.86 kms-1. The thick contours indicates the HI filaments and the multi-shell structure observed in 12CO. The star position is marked with a star. Crosses mark the selected positions for which we have estimated the physical conditions (see Table 1). The lines indicate the strips shown in Fig. 9. The (0,0) position is RA: [FORMULA] Dec: [FORMULA].

[FIGURE] Fig. 2. Scheme with the main features observed in the HI and 12CO and 13CO emissions. The grey scale corresponds to the integrated intensity of the 13CO J=1[FORMULA]0 line at a velocity of 1.86 kms-1. The cavity and the tunnels are clearly seen in this map. The thick contours indicates the HI filaments which are spatially coincident with the 13CO tunnels. The thick dashed contours corresponds to the integrated intensity of the 12CO J=2[FORMULA]1 line at 3.79 kms-1 and clearly show the bow shock found at the tip of the northern HI filaments. The star and crosses mean the same as in Fig. 1.

[FIGURE] Fig. 3. Map of the integrated intensity map of the 12CO J=2[FORMULA]1 line for different velocity intervals. The velocity interval is 0.644 kms-1. Contour levels are 1, 2, 3 to 14 by 3 K kms-1.

[FIGURE] Fig. 4. The same as Fig. 3 for the 13CO J=1[FORMULA]0 line. Contour levels are 0.5, 1.5 to 6 by 1 K kms-1.

[FIGURE] Fig. 5. Map of the HI column density for different velocity intervals towards NGC 7023. Contour levels are from 5.6 1020 to 2.2 1021 cm-2 by 5.6 1020 cm-2.

[FIGURE] Fig. 6. Spectra of the HI, 12CO J=2[FORMULA]1, 13CO J=1[FORMULA]0 and C18O J=1[FORMULA]0 lines towards the offset (0",0"). The dashed line indicate the velocity of the ambient cloud (1.5 kms-1).

[FIGURE] Fig. 7. Spectra of the 12CO J=2[FORMULA]1, 13CO J=1[FORMULA]0 and C18O J=1[FORMULA]0 lines towards the offset (-40",80"). This position is located in the walls of the cavity.

[FIGURE] Fig. 8. The same as Fig. 6 for the offset (200",140"). This offset is the position of the bow-shock. Dashed lines indicate the velocity of the ambient cloud (2.5 kms-1) and that of the bow shock feature (4.0 kms-1). The temperature scale is TMB.

3.1. The cavity

Both molecular and atomic emission show that the star is located in a large ([FORMULA] 1.5 pc [FORMULA] 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 [FORMULA] 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=1[FORMULA]0 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 [FORMULA] 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 [FORMULA] 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 [FORMULA] 10.


[TABLE]

Table 1. Physical conditions
Notes:
1) Within the cavity, the 13CO column densities have been estimated assuming optically thin emission and a rotation temperature of 15 K.
2) Position in the molecular filament that divides the western lobe in two subcavities.
3) Physical conditions derived by fitting the 13CO and C18O lines with an LVG code and assuming the physical conditions shown in the table.



[TABLE]

Table 2. Energetics of the outflow
Notes:
(*) 12CO for V[FORMULA] -1 and V[FORMULA]6 kms-1 and 13CO for -1[FORMULA]V[FORMULA]6 kms-1.


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 "[FORMULA]" 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 [FORMULA] 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 [FORMULA] 103 cm-3 and [FORMULA] 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 [FORMULA] 63.5"[FORMULA]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 2[FORMULA]1) [FORMULA] 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 [FORMULA] 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 ([FORMULA] = 0.6 kms-1) and intense line (TMB[FORMULA] 60 K) appears in the 12CO J=2[FORMULA]1 spectrum at a velocity of [FORMULA] 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 [FORMULA] 4 kms-1 is not due to self-absorption. In fact, this feature appears as a weak peak in the 13CO J=1[FORMULA]0 spectrum. It is a well differentiated velocity component with a characteristic velocity (v [FORMULA] 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 [FORMULA] 80 K) and thin (Av[FORMULA] 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.

Previous Section Next Section Title Page Table of Contents

© European Southern Observatory (ESO) 1998

Online publication: October 21, 1998
helpdesk.link@springer.de