MacLeod and Avery (1995, in preparation) have previously observed an unusual outflow source, located at 140 pc distance in Taurus, which appears to be partially hidden behind a strong absorbing screen of very cold, dense gas. The outflow is centred on IRAS 04368+2557, and most of the JCMT CO J=3-2 spectra which define it show very strong self-absorption, even well away from the centre of the source. In order to try to see through this screen and study the emission centred on the IRAS source, we observed this young stellar object in March 1994 at the JCMT with the Max Planck Institut fur Extraterrestrische Physik's new SIS version of Receiver G, known as "FANATIC".

The weather was excellent at the beginning of our JCMT run in March 1994. We started by observing a grid of ten points around IRAS 04368. From the first observation, it was clear that strong J = 6-5 lines of order Tr* = 10K were associated with the IRAS source. The grid map taken with the 7" half-power beamwidth showed that the central source was extended over a region at least 16" x 16". Furthermore, we have detected strong emission at several positions well out in the blue and red lobes of the outflow, notably at hot spots on the CO J = 3-2 map, shown in Figure 1. The blue- shifted emission is in the upper half, while the red-shifted emission is in the lower half of the figure.

Four CO J = 6-5 spectra are shown in Figure 2, superimposed on the CO J = 3-2 JCMT spectra from the same positions. The (Da = 0, Dd = 0) J = 6-5 spectrum at the position of the IRAS source (Fig. 2d) shows a central peak rather than a central dip, indicating that the dip in the J = 3-2 spectrum is due to self-absorption and is not two separate velocity components. This conclusion is supported by 13CO and C18O, in which the central absorption is much less prominent. This is also the case at (-10,10) (Fig. 2b), but the situation is not so clear at (100,30) (Fig 2a), where some hint of a dip in the J = 6-5 spectrum also exists. Fig 2c illustrates that no J = 6-5 emission is detected at the southern tip of the blue- shifted CO J=3-2 hot spot.

In all, some 20 positions were observed in CO J = 6-5 before the weather deteriorated. Of these, 16 were detections and 4 had no line. Each J = 6-5 spectrum shown in Figure 2 has an integration time of only 4 minutes, illustrating the excellent sensitivity of the new SIS receiver.

These observations represent the first time, to our knowledge, that a highly excited CO transition has been detected in the outer regions of a low-luminosity bipolar outflow. The upper energy level of the 6-5 transition of CO is 116K above the ground state, so there is a relatively high excitation temperature in the core and at the hot spots. In particular, the J = 6-5 emission at the CO J = 3-2 hot spots may be arising from shocks occuring at the point where a stellar wind or neutral jet is colliding with dense clumps in the ambient medium.

Figure 1. JCMT map (both greyscale and contours) of the CO J=3-2 bipolar outflow surrounding IRAS 04368+2557 in Taurus. The map center is at RA(1950) = 04h 36m 49.30s, Dec(1950) = +25d 57' 16". The upper half of the figure shows the blue lobe, with emisison integrated over 2 km/s < v(lsr) < 5 km/s. The lower half of the figure shows the red lobe, with emission integrated over 7 km/s < v(lsr) < 10 km/s. The beamwidth was 14".

MacLeod and Avery (1995, in preparation) have also mapped HCO+ J = 4-3 over a 50" x 50" region centred on the IRAS source. Strong HCO+ emission is found in this region, and there is a tendency for this emission to outline the walls of the outflow bubble close to the source. An integration at the peak of the blue hot spot at (100,30), however, detected no HCO+. This could be because the density at this position is too low to excite the HCO+ J = 4-3 line, or perhaps HCO+ may be destroyed in the shock. However, both of these explanations are problematical, in that the presence of strong CO J = 6-5 emission suggests a relatively high density, and the theoretical calculations of Pineau des Forêts et al (1988) indicate that the abundance of HCO+ across a shock should show very little variation. The lack of HCO+ emission at this hot spot is therefore puzzling. We intend to investigate other indicators of the presence of shocked gas, including CH3OH and SiO, in the hot spots of the IRAS 04368 outflow.

What do we know about IRAS 04368+2557 from other observations? It is a relatively cool young stellar object located in the molecular cloud Lynds 1527. Its continuum flux density rises steeply throughout the infrared to a flux density of 71 Jy at 100 microns, and it has a far infrared luminosity of 2.9 L¤. Benson and Myers (1989) have found an ammonia core associated with this object. They calculate the radius of the core to be 0.08 pc, and find that it has a mass of 2.4 M¤ from their NH3 observations. The core has a high visual extinction of ~ 1000 magnitudes. Ladd et al (1991) have used the JCMT to detect 04368 at submillimeter wavelengths, and they mapped it at 450 microns and 800 microns. They found a source size of about 30" diameter. The continuum spectrum suggests that 04368 is a low-mass star at a very early stage of formation.

Figure 2. JCMT spectra at selected points within the outflow. The upper spectrum in each box is CO J=3-2, while the lower spectrum is CO J=6-5. The ordinate Tr* (Jup) is Tr* calculated from the main beam efficiency measured on Jupiter.

We believe that nearby outflow sources in Taurus such as IRAS 04368+2557 provide us with an excellent opportunity to study with high spatial resolution the complex interaction which takes place between outflowing gas and the surrounding interstellar medium during the formation of low mass stars.

References:

Benson, P. J., and Myers, P. C., 1989, Ap. J. Suppl. 71, 89.

Ladd, E. F., Adams, F. C., Casey, S., Davidson, J. A., Fuller, G. A., Harper, D. A., Myers, P. C., and Padman, R., 1991, Ap. J. 382, 555.

Pineau des Forêts, G., Flower, D. R., and Dalgarno, A., 1988, Mon. Not. R. astr. Soc. 235, 621.

John MacLeod & Lorne Avery, Herzberg Institute of Astrophysics,

Andy Harris & Linda Tacconi, Max Planck Institut für Extraterrestrische Physik,

and Karl Schuster, IRAM