Astron. Astrophys. 352, L83-L86 (1999)

3. Two infrared sources: a wide binary

The F160W and F205W filters transmit, besides stellar continuum, the prominent emission lines [FeII] 1.64 µm and H₂ 2.12 µm, which originate in shocks. At the bottom of Fig. 1 we show the entire NIC2 field, with F160W as turquoise and F205W as red. On top we have added the WFPC2 images ([SII] blue and H orange) of Reipurth et al.(1997b). The figure shows the entire optical/infrared jet from the energy source to knot L (43" 20,000 AU at 470 pc) at resolution, with a high-extinction gap in the jet near the source of only .

Fig. 1. A mosaic image of HH 111 based on HST NICMOS images (bottom) and WFPC2 images (top). See text for details

Additionally, our NICMOS images of the HH 111 source region are shown in each filter as contour plots in Fig. 2. The jet and counter jet are very weak in H₂, but bright and well defined in [FeII]. This difference between the two emission lines is further discussed by Reipurth et al. (2000).

Fig. 2. Contour plots of the HH 111 source region based on HST NICMOS images through F160W and F205W filters

The main features of the double lobed reflection nebula have been discussed by Gredel & Reipurth (1994), but the high resolution of the present images offers some new and unexpected insights. The brightest part is the western nebula, which lies in the blue lobe of the outflow. It has a total extent perpendicular to the flow axis of 8", which at 470 pc corresponds to the very large dimension of about 3500 AU. The nebula has a sharp cut-off towards the source region as seen in the F160W image, suggesting the presence of an extended obscuring region. On the opposite side, the eastern nebula has, besides being much fainter, a rather different morphology, with a much narrower opening angle.

For an object as symmetric as the HH 111 jet, and considering that we are viewing the outflow at an angle of only 10^o to the plane of the sky (Reipurth et al. 1992), the large asymmetry of the illuminated cavities in the two outflow lobes is surprising. However, closer examination of the western lobe reveals the presence of a faint star, which we here call star B. The star is seen in both the F160W and F205W images, with fluxes listed in Table 1. It is located precisely at the tip of a narrow tongue of reflection nebula stretching 5" to the south-west, suggesting a close relation between star and ambient material. We speculate that the large brightness and transverse extension of the western nebula is due to the additional illumination by star B. As discussed in Sect. 4, star B is detected at 3.6 cm (VLA 2), demonstrating that the star is young and has outflow activity.

Table 1. Sources detected with NICMOS and VLA

In the F205W image we find an isolated stellar source, here called star A, at the coordinates and with the flux listed in Table 1. The absolute position of star A and VLA 1 coincide to within , comparable to the uncertainty of the IR position, which was derived using the plate solution in the NICMOS image header and hence limited by the accuracy of the Guide Star Catalogue, as well as by any zero point difference between the optical and radio coordinate systems. Note that the separation of stars A and B is in perfect agreement with that of VLA 1 and VLA 2. Thus star A undoubtedly represents a near-IR detection of the energy source.

It is noteworthy that there is a dark ridge exactly perpendicular to the jet axis, producing a gap between the western and eastern reflection nebulae about (250 AU) wide. This could be interpreted as a dense circumstellar disk of the driving source. However, since the blue lobe of the flow is towards the west, if the source were visible at all at this low flow angle, we would expect it to be on the western, and not the eastern, side of the dark ridge. We discuss this point further in Sect. 5.

© European Southern Observatory (ESO) 1999

Online publication: December 2, 1999
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