The outflow(s) in LBS17 (NGC 2068)

LBS17 is a dense cloud core which lies close to NGC 2068 in L1630. It was first identified as one of five massive cores (>200 Solar masses) by the CS J=2-1 survey of Lada, Bally & Stark (1991 — LBS). The other four are NGC 2071, HH24-26, NGC 2023 and NGC 2024 — all well-known star-forming complexes. Followup observations of a sample of LBS cores were made using the JCMT in March 1992 initially using CS J=5-4 but later switching to HCO+ J=3-2, because the CS was so faint and we already had HCO+ J=4-3 data on HH24-26 (Gibb & Heaton 1993).

Figure 1.

Our HCO+ map of LBS17 (Gibb et al. 1995) showed that the core is highly elongated - very like the ridges seen in HH24-26 and NGC 2024 - and is composed of at least eight clumps. We originally labelled these clumps B1 to B8 (roughly from north-west to south-east) for purposes of consistency, but labelling them A to H is probably more conventional. Clumps E and H stand out as the most prominent, the latter of which is also the site of a water maser (Haschick et al. 1983). A striking feature of LBS17 is despite its high mass and clumpy appearance, there are no IRAS or near-IR sources associated with the region of molecular line emission (Hodapp 1994, Launhardt et al. 1996).

An outflow in LBS17H?

Closer examination of the HCO+ J=3-2 spectra revealed the presence of spatially-separated blue- and red- shifted wing emission, centred on LBS17H. Fifteen years ago, the reaction to this would have been 'A rotating disc!'; these days the reaction tends to be 'Outflow!'. The latter initially seemed a better choice, especially as the survey by Fukui (1989) revealed a CO outflow in this region (although Fukui's low spatial resolution prevented more positive association).

However, upon calculating the gas parameters and analysing the energetics it became clear that the data could still be interpreted as a rotationally supported disc. Thus (as ever!) further observations were required to try and decipher exactly what was going on. With this in mind, a service proposal was submitted and accepted. The observations were made in 1996 May - a square grid at 15" spacing of the J=3-2 line of CO, with the aim of picking up the CS J=7-6 line in the image sideband.

The four panels in Fig. 1 show the peak Ta*, integrated intensity, equivalent width and velocity centroid over the region mapped in the velocity range 0 to 25 km/s. Black represents lowest values and white the highest. Clearly evident is the spatially bipolar distribution of the wing emission, with blue-shifted gas to the south-east and red-shifted to the north-west. This is in the same sense as the high-velocity features seen in HCO+. The minimum at position (-20,0) is due to the presence of self-absorption at the line centre (at about 11 km/s).

The outflow has a maximum extent of 1 arcmin for each lobe (equivalent to 0.12 pc at a distance of 400 pc). There is no redshifted emission in the blue lobe and vice versa; therefore the outflow is not in the plane of the sky, but neither is it pole-on. The apparent dynamical age is low - only 10(4) years or so. If the inclination is 45 degrees then this is equal to the true age (Shu et al. 1991) indicating that this may be a very young object. The lack of an infrared source supports this interpretation.

The collimation is not very high, except in the map of equivalent width (lower left panel in Fig. 1). Channel maps show some evidence for a laterally-unresolved component to the flow at highest velocities and there is a weak tendency towards a 'Hubble-like' velocity distribution in the red lobe. The compact nature of this source makes it a good target for future interferometric observations.

The dense gas in LBS17H

The search for J=7-6 CS emission produced a total null result - at no position was the line detected (to a 3 sigma level of Ta* = 1 K). For a source LSR velocity of 10 km/s, the CS line should lie at 64.6 km/s. Figure 2 shows the centre spectrum with the CO line at 10 km/s and there is clearly no significant (>3 sigma) emission near this velocity. The J=3-2 HCO+ peaks strongly at the (0,0) position but is elongated with a position angle 135 degrees east of north. For comparison, the outflow has a position angle of approximately 100 degrees. Since the HCO+ map is affected by confusion with outflowing gas, submillimetre continuum maps (using SCUBA) are clearly needed to determine the precise distribution of

dense material in this clump.

Two outflows in LBS17?

Another feature is visible in Fig. 1 near (0,75) which is present across several velocity channels, suggesting that it may represent a second flow, perhaps emanating from LBS17E. Clearly, more observations are necessary to determine this! It is possible that Fukui's NGC 2068-H2O outflow represents the superposition of two or more flows.

Summary

These maps have conclusively revealed that an outflow originates from within LBS17H. The swept-up gas is dense enough to excite the J=3-2 HCO+ line up to 0.1 pc from the source. However, despite answering the main question of this project, the data have given rise to several more! What is the nature of the driving source? What is the real distribution of dense gas surrounding the source? Is the second outflow real? The quest continues....

Figure 2.

References

Fukui Y. (1989) in 'Low mass star formation and pre-main sequence objects' ed. B. Reipurth.

Gibb A.G., Heaton B.D. (1993) A&A 276, 511

Gibb A.G., Little L.T., Heaton B.D., Lehtinen K.K. (1995) MNRAS 277, 341

Haschick A.D., Moran M.J., Rodriguez L.F., Ho P.T.P. (1983) ApJ 265, 281

Hodapp K-W. (1994) ApJS 94, 615

Lada E.A., Bally J., Stark A.A. (1991) ApJ 368, 432 (LBS)

Launhardt R., Mezger P.G., Haslam C.G.T., Kreysa E., Lemke R., Sievers A., Zylka R. (1996) A&A (accepted)

Shu F.H., Ruden S.P., Lada C.J., Lizano S. (1991) ApJ 370, L31

Andy Gibb, University of Kent at Canterbury, UK.

agg@starlink.ukc.ac.uk