Prompt entrainment in the `wiggling' molecular jet from RNO 15-FIR

How are massive, molecular (CO) outflows driven by collimated jets from young stars? In recent years there has been considerable attention given to this question. Theoretical studies suggest that ambient gas may be accelerated, either in a turbulent mixing layer along the length of the jet, or through the bow shock at the head of the jet. Both models have their advantages: in the turbulent, "steady-state" model, the mixing layer that developes across the interface between the supersonic jet and its surroundings thickens with distance from the source, as jet material and ambient gas is pulled into the layer. Eventually the jet will be completely pinched off and the flow will become fully turbulent and subsonic. Conversely, entrainment in the bow shock or "prompt" model is more localised. Here, the ambient, molecular gas swept up through each bow shock cools rapidly, and although some of the gas may spill down the sides of the jet, a high-velocity "clump" will develop just behind each shock front.

In an effort to distinguish between entrainment models, Chernin & Masson (1995, ApJ 455, 182) recently measured the distribution of momentum in a number of outflows. They point out that, in the turbulent entrainment model, the momentum per unit length should increase with distance from the source, as more and more of the jet momentum is transfered to the swept up molecular gas. Conversely, in the prompt entrainment model, the momentum decreases with distance from the source as the flow "accelerates" into the lower density regions further out from the embedded outflow source. However, their results proved inconclusive in that the momentum profiles along the six outflows considered peaked roughly in the centre of each flow lobe.

Motivated by these ideas, and unswayed by this disappointing result, we have since mapped, at JCMT in CO J=3-2, the outflow driven by RNO 15-FIR. Early CO maps of this region had insufficient resolution to separate the RNO 15-FIR outflow from other, neighbouring systems. However, in a recent H_2 image of the region, Davis et al. (1997, A&A, in press) discovered a sequence of compact line emission features which imply the presence of a highly-collimated, bipolar outflow driven by RNO 15-FIR. The new CO data confirm the existence of this bipolar flow, and furthermore allow us to distinguish between entrainment models. We achieve this by comparing the submillimetre data with the near-IR observations, and by measuring the distribution of momentum along the flow lobes. We also find remarkable evidence of directional variability along this outflow.

Prompt or turbulent entrainment?

In Fig.1 we present an integrated-intensity map showing the blue-shifted and red-shifted CO outflow lobes superimposed onto a near-IR image of the region. The H_2 line-emission features in this image, labelled A, B and C, are thought to be molecular shocks along the flow axis. The high degree of collimation apparent in the RNO 15-FIR CO outflow and the close association between the CO outflow and the H_2 knots (A, B and C) indicate that the outflow is driven by a collimated jet. Indeed, the spatial coincidence between the 3 peaks in the CO map and the 3 molecular shocks traced in H_2 suggest that (1) these H_2 shocks are those which entrain much of the ambient gas to form the CO outflow, and that (2) the prompt entrainment mechanism dominates over turbulent entrainment.

Figure 1: A 2.122 micron image of the RNO 15-FIR outflow region with, overlayed, contours of the high-velocity CO 3-2 emission. The cross marks the IRAS position of the outflow source RNO 15-FIR; the neighbouring source, RNO 15, which also drives a CO outflow (in a northwest-southeast direction) is offset by (108" , -58") from RNO 15-FIR. Note how the red-shifted flow lobe from RNO 15 encroaches on the red lobe from RNO 15-FIR.

In Fig.2 we plot the mass per unit length (dM/dR), momentum per unit length (dP/dR), and mean velocity per unit length (d/dR = [dP/dR]/[dM/dR]) along the outflow axis, integrated across the width of the flow and over the high-velocity blue-shifted and red-shifted line wings. In both the northeastern, red-shifted flow lobe, and the southwestern, blue-shifted flow lobe, the mass and momentum decrease with distance from the source. This decrease is most dramatic in the blue-shifted lobe (at negative offsets in Fig.2). The mean velocity, on the other hand, appears to be relatively constant along both flow lobes.

Figure 2: Distribution of mass (integral T_b.dv -- crosses), momentum (integral T_b.v.dv -- dots+circles), and mean velocity (dots) along the RNO 15-FIR outflow axis. Profiles for the blue-shifted (v < 2.0 km/s) and red-shifted (v > 7.4 km/s) high-velocity gas are plotted, measured at 5" intervals, from the sum of spectra in strips perpendicular to, though centred on, the outflow axis. dM/dR, dP/dR and d/dR are normalised to the maximum measured values. The offsets are in arcseconds from RNO 15-FIR (positive `x' is towards the northeast).

Opacity at low outflow velocities is likely to have the most severe effect on the mass and momentum estimates in Fig.2. However, these errors have little influence on the {emphasise distribution} of mass and momentum, since the mass and momentum profiles have the same overall shape in spite of the fact that the momentum is weighted to higher velocities. Consequently, the profiles in Fig.2 are believed to be an accurate representation of the true distributions. The {emphasise decreasing momentum profiles}, in both the blue-shifted and red-shifted flow lobes, therefore add considerable weight to the idea that {emphasise the prompt entrainment mechanism dominates in RNO 15-FIR}.

Variability in the RNO 15-FIR molecular jet

Curving molecular outflows have been observed in a number of star-forming regions. However, a ``wiggling'' molecular outflow has, to our knowledge, so far not been reported. Variability in jets is seemingly a common occurance so it seems reasonable that jet-driven molecular outflows might also show signs of directional variability.

The high-velocity flow lobes in Fig.1 hint at regular deviations in the outflow direction from the nominal flow axis. To examine this possibility, we have fit a gaussian profile to the integrated intensity contours in Fig.1, at 5" intervals along the RNO 15-FIR outflow axis, each gaussian being perpendicular to the flow axis. A straight line fit through these points leads to a more precise measure of the orientation of the outflow axis, which we find to be 47.2 degrees E of N.

Figure 3: Plot of high-velocity emission centriods (thick dots) along the outflow axis. A sinusoidal least-squares fit is also plotted.

The centroid of each gausian fit is plotted in Fig.3; here, the nominal outflow axis is orientated along the x-axis. The deviation of each point from this x-axis we therefore consider as being due to the wiggling of the molecular flow. The plot in Fig.3 suggests a sinusoidal distribution, so we fit these data with a function of the form y = Asin(2.pi/B)sin(2.pi.x/(lambda - xC), where x is measured (in arcseconds) along the outflow axis. We have excluded from this least-squares fit data points in the range 0" -- 55" , because here the RNO 15-FIR outflow overlaps the red-shifted outflow lobe from another, nearby source, RNO 15 (the end of the red lobe from RNO 15 is evident to the southeast of RNO 15-FIR in Fig.1). To the remaining points we obtain a good fit, which indicates that the outflow is indeed wiggling, with an amplitude A = 2.4" and wavelength lambda = 35.0". We also find that B = 9.0" and C = 0.03, and that the function is point symmetric.

Chris Davis & Tom Ray, Dublin Institute for Advanced Studies

Jochen Eisloffel, Thuringer Landessternwarte, Tautenburg

Tim Jenness, Joint Astronomy Centre