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

4. The outflow

4.1. Kinematics and geometry of the region

The bipolarity observed in the morphology of ¹²CO and ¹³CO emissions suggests that, at least in an earlier evolutionary stage, a bipolar outflow with its axis spatially coincident with the axis of the cavity has been developed in this region. At the present stage, we have no evidence for the existence of atomic and/or molecular high velocity gas along the axis of the cavity. There are some observational problems that could contribute to this fact. Since the outflow is seen almost edge-on, the gas close to the axis would be observed at velocities very close to that of the ambient cloud. This will increase the confusion problems with the foreground gas. However, even in this case one expects to observe the signs of the interaction between the high velocity gas and the molecular cloud. The fact that the bow shock is detected at the tip of one of the HI filaments instead of at the end of the axis of the cavity, support the model in which the high velocity gas is running along the edges of the cavity instead of inside it. Furthermore, two tunnels have been detected in the ¹³CO integrated intensity emission with the same directions as the HI filaments. As illustrated in Fig. 2, there is a perfect match between the direction and spatial location of the HI filaments and the ¹³CO tunnels. This strongly suggests that the high velocity gas is mainly flowing along the HI filaments and pushing away the ambient material.

In Fig. 9 we present the velocity-position diagrams of the HI, ¹²CO and ¹³CO emission along the strips marked in Fig. 1. Emission at red-shifted and blue-shifted velocities is detected at every position. This is the expected behavior for an edge-on outflow. The highest velocities are observed in the atomic gas located in the vicinity of the star. Within the cavity the high velocity gas is mainly atomic. At the edge of the cavity, intense ¹²CO emission appears with the same terminal velocity as the HI emission. The bow-shock is located at the interface between the atomic and molecular emission in Strip 2. Deeper into the cloud, the HI emission disappears and the width of the ¹²CO lines is the same as in the foreground molecular cloud. The width and terminal velocity of both HI and ¹²CO lines decrease outwards from the star. This kinematical structure is characteristic of a decelerated outflow, in which the velocity of the outflowing gas decreases when it interacts with the ambient cloud.

Fig. 9. Velocity-position diagram of the emission of the 12CO J=21 and 13CO J=10 lines and the HI column density for strip 1 and 2 (see Fig. 1). Contour levels are: 1.6 to 20.8 by 1.6 K for the 12CO J=21 line; 1 to 10 by 1 K for the 13CO J=10 line and 3.4 1020 to 2.2 1021 by 3.4 1020 cm-2. To have the same velocity resolution for the three emissions, we have degraded the resolution of the 12CO and 13CO data to 0.644 kms-1.

As commented above, the high velocity HI filaments are only observed towards the eastern lobe. This is also the lobe in which the star is located. Towards the western lobe, we have not detected any HI filaments and there is no evidence of interaction between the ambient gas and high velocity atomic gas. The ¹²CO lines are narrow and a bow-shock is not found in this lobe. The narrowness of the ¹²CO lines suggests that the non-detection of the HI filaments in this lobe is not due to observational problems (missing flux in the VLA image) but to a lack of high velocity HI gas in this region.

Summarizing, at the present stage the outflow is mainly formed by atomic gas that is running along the walls of a hollow cone adjacent to the eastern lobe of the cavity. The star is found 40" - 50" East from the apex of this cone. At the edge of the cavity the atomic gas impinges in the ambient cloud and is decelerated. A layer of shocked molecular gas is formed in the interface between the atomic gas and the ambient cloud.

One expects to find a layered structure in the walls of the cavity with the atomic gas in the inner layers and the molecular gas in the outer ones. Then, the comparison between the velocities of the atomic and molecular lines could provide important information about the kinematical structure of the walls. In Fig. 6 we present the HI and molecular spectra towards the star position. The ¹³CO emission shows a double-peak spectrum with the peaks at 0.7 and 2.0 kms^-1 and the dip at 1.5 kms^-1. This profile is characteristic of an expanding shell with an expansion velocity of 0.6 kms^-1. The HI line is wider than the molecular lines. In fact, the HI emission ranges from -5.6 to 8.6 kms^-1, with the emission peak at 4 kms^-1. This is also the peak of the CII emission (Gerin et al. 1998). However the velocity of the atomic absorption lines is -2 kms^-1 (Federman et al. 1997). The atomic absorption lines are tracing the atomic layer in front of the star. If we consider the the velocity of the dip in the ¹³CO spectrum as the reference velocity for the expansion, this layer is expanding at a velocity of 3.5 kms^-1. The peaks of the CII and HI emission lines corresponds very likely to the atomic column density peak. According with its velocity, this peak is located in the back wall and is expanding at a velocity of 2.5 kms^-1. Comparing the expansion velocities derived from atomic and molecular lines, we conclude that across the walls, the expansion velocity decreases from inside to outside. The aperture of the biconical cavity is . Assuming axial symmetry for the lobes of the cavity and correcting by the inclination angle, we derive that the atomic gas is outflowing with a velocity of 7 kms^-1 relative to that of the ambient molecular gas at the star position.

4.2. Energetics

We have estimated the mass, momentum and energy of the high velocity atomic and molecular gas of this outflow. The HI masses have been estimated assuming optically thin emission. To estimate the mass, momentum and energy of the molecular gas we have used our ¹²CO and ¹³CO data. Although the ¹²CO J=21 line is self-absorbed at the velocities of the molecular cloud, it is the best tracer for the high velocity gas. We have used the ¹²CO to estimate the mass of the gas with velocities -1 and 6 kms^-1, and ¹³CO for -1 V 6 kms^-1. To estimate the mass of the molecular gas we have assumed optically thin emission, a rotation temperature of 30 K, a standard CO abundance of 8 10^-5, and a ¹²CO/¹³CO isotopic ratio of 40. The intensity scale is antenna temperature. The main uncertainties in this calculation comes from the assumed rotation temperature and the unknown beam filling factor. The rotation temperature is uncertain by a factor of 2. Relative to the beam-filling factor, although the ¹²CO is very extended, some high velocity features, like the bow-shock, can have a small angular size. We have used the antenna temperature scale because the contribution to the energetics of the whole region of this thin layer of gas is not very important. In Table 1 we present the mass, momentum and kinetic energy estimates for different velocity intervals.

The momentum in HI is almost 2 orders of magnitude lower than that of the CO outflow. Large uncertainties are involved in these estimates. First of all, an extended component of the HI emission can be missed in our VLA data. Rogers et al. (1995) estimated a total HI mass of 3 toward this nebula. Compared with the mass derived from our VLA data, 1.6 , we could have missed about the 50 % of the total HI flux. Thus, the momentum in HI can be underestimated by a factor of 2. On the other hand, we have not corrected our calculations by the inclination angle relative to the plane of sky. Since the spatial distribution of the ¹²CO and HI emission are different, we could have a different correction factor for the molecular and the atomic emission. Finally, the HI emission could be optically thick. The HI column densities in the filaments ranges between 3 10²⁰ - 2.2 10²¹ cm^-2. Assuming a standard value of the spin temperature (T_s = 125 K), the opacity of the HI line would range between 1 and 9. The correction to the HI column densities due to the opacity could be a factor 2 - 9. But even taking into account all these uncertainties, the momentum in the molecular outflow is still an order of magnitude larger than that in HI. The momentum of the HI flux is enough to drive the highest velocity ¹²CO gas (V -1 and V 6 kms^-1) but clearly insufficient to drive the moderate velocity dense gas in the walls of the ¹³CO cavity. This gas has been very likely accelerated in a previous evolutionary stage when the cavity was excavated by a bipolar ouflow.

From our ¹³CO data we estimate that the mass in the bright rim that borders the cavity is 800 . Assuming that the mass was uniformly distributed in the region before the outflow onset, the outflow has swept up 300 of molecular gas. We have estimated a dynamical age of 10⁵ yrs for the molecular outflow (adopting a size of 0.6 pc and a terminal velocity of 3.5 kms^-1). Then, the outflow luminosity is 20 L. Cabrit & Bertout (1992) computed the mechanical luminosity of the swept up molecular gas in a sample of outflows driven by sources of very different bolometric luminosity (L_bol 1 - 4 10⁵ ) and found that both quantities are correlated. The mechanical luminosity estimated in HD 200775 is typical of sources with a bolometric luminosity L_bol 10³ (the bolometric luminosity of HD 200775 is L_bol 8 10³ ; van den Ancker et al. 1997). The outflows driven by GL 490, NGC 2071-IRS1 and S140-IRS1 are within this group. However the dynamical ages estimated for these outflows are an order of magnitude lower than in the case of HD 200775. HD 200775 is very likely one of the oldest intermediate-mass outflows so far studied.

4.3. Around the star

It is natural to think that the star should be located in a clump that is the residual of the core in which it was born. We have not detected any molecular and/or atomic clump at the star position. Although intense HI emission is observed at the star position, the star is not located at a peak in HI column density. In fact it is located at a local minimum, between the two intense HI clumps in the narrow waist of the biconical cavity. Bipolar outflows are thought to have an important role in destroying the parent core. One could think that the outflow has disrupted the initial core and now the star is surrounded by a dense ring perpendicular to the outflow axis. In this scheme, the clumps located in the waist of the cavity are part of this ring. Surprisingly, the star is not found aligned with these clumps. It is displaced 50" East from them. The remnant core has an asymmetric structure with the densest gas located to the west of the star. This structure has also been observed in the interferometric HCO⁺ images of the region (Fuente et al. 1996) and in the recent infrared continuum maps obtained by ISO (Cesarsky et al. 1996). Since the same asymmetric spatial distribution is observed in HI, the dust continuum emission and the molecular lines, we conclude that this asymmetry is not the consequence of a chemical effect. It is the consequence of the anisotropic distribution of the material around the star. This asymmetry is also observed in the dust temperature map reported by Rogers et al. (1995). The dust temperature is higher in the eastern lobe than in the western lobe. This is the expected behavior if the extinction is higher towards the west than towards the east because of the anisotropic distribution of the material around the star. In Sect. 5.1, we will propose this anysotropy to explain also the monopolar nature of the HI emission.

© European Southern Observatory (ESO) 1998

Online publication: October 21, 1998
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