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Astron. Astrophys. 353, 153-162 (2000)

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4. Discussion

4.1. Origin of the bipolar nebula

The most impressing feature in our images is the bipolar infrared nebula. The main questions are what the origin of this nebula is and how this bipolar structure compares with the observations at optical and millimetre wavelengths. The morphology of BBW 192E in the infrared resembles that of other YSOs with disk-like configurations such as IRAS 04302-2247 (Lucas & Roche 1997) and the infrared nebula in Chamaeleon (Feldt et al. 1998). Monte-Carlo calculations demonstrate the presence of bipolar nebulae up to mid-infrared wavelengths produced by dust particles in the cavities of the (flattened) envelopes seen nearly edge-on (Fischer et al. 1994, 1996; Wood et al. 1998). For other viewing angles, the symmetry would be broken and one scattering lobe would be much brighter than the other one. The less bright band seen between the lobes in the near-infrared images and the N band image (source 25 subtracted) strongly suggests the presence of a disk-like structure.

We already noted that the brightness ratio of the north-eastern and south-western lobes are different at H and K (K´). This would be difficult to reconcile with a fixed orientation of the lobes, which would lead to the same brightness ratios at the different NIR wavelengths because of the same radiation mechanism (scattering; see, e.g., Fischer et al. 1996). For this reason, we may conclude that object [FORMULA] 25 is aligned by chance with the north-eastern lobe. This situation would not be unlikely given the source density in the field. The object would be characterized by an SED strongly rising with wavelength (see Fig. 4a).

However, we found evidence for a strong decrease of extinction from the south-east to the north-west. The orientation of the extinction gradient differs from that of the presumed disk equatorial plane by about 30o (see Fig. 3a, b and a more detailed discussion below). Such a spatial variation of the extinction may greatly modify the appearance of an bipolar nebula (see, e.g., Ageorges et al. 1996, Feldt et al. 1998) and could also explain the deviation from a purely centro-symmetric polarization pattern in the case of our source. The peak of the high-resolution Ks band image in the northern lobe has the PSF of a star and is interpreted as the central energy source of the nebula. This situation resembles the findings by Padgett et al. (1999) for low-mass YSOs obtained with the HST.

[FIGURE] Fig. 3a-d. a  H-K´ colour image: white: H-K´ contours (0.9, 1.4, 1.9, 2.4, 2.9, 3.4 mag), black: optical contours (3, 6, 12, 24, 48 [FORMULA])   b  As before, but without stars and without contours of the optical emission.   c  DSS image and 1.3 mm contours (3, 4, 5, 6, 7, 8 [FORMULA])  d  N-Q colour image (contours: black 2.8, 3.3, white 3.8, 4.3 mag), star: position of object 25. In addition, the error ellipse of IRAS 08513-4201 is shown. 

Here we should note that the extinction in the northern lobe is even higher than in the southern lobe because of the inclination between the lobe axis and the extinction border (see Fig. 3b). Together with the visibility of source [FORMULA] 25, this means that the northern lobe points to the observer. It seems likely that the cavities which give rise to the observed bipolar infrared morphology were produced by a molecular outflow. Evidence for the presence of an outflow comes from line wings in a CO(J=1-0) spectrum obtained by Wouterloot & Brand (1989). Since there is no line map available for this region, the driving source and the morphology of the outflow cannot be identified.

The radius of the disk-like structure can be estimated from the high-resolution Ks band image and has a value of about 2" which corresponds to a linear size of 2400 AU at a distance of 1.2 kpc. In the case of the Herbig Ae/Be star HD 245185, Mannings & Sargent (1997) found an interferometric disk radius of 680 AU. Other disks around YSOs have radii between a few tens of AU and 500 AU (see, e.g., McCaughrean et al. 1999).

The optical nebula becomes visible in a region where the extinction is low as seen in Fig. 3a. The most closely spaced H-K´ contour lines are at the south-eastern border of the infrared nebula and outline the steep increase of the extinction towards the south-east. The absence of an infrared source at the location of the optical peak confirms the suggestion of Petterson & Reipurth (1994) that the optical emission is a reflection nebula. Together with the extinction gradient, the shape of the millimetre emission, and the absence of any other bright external source, this suggests that we see scattered light from stellar source(s) in the infrared nebula. We can exclude that the optical nebula is produced by a source behind the molecular clump. The visual extinction at the position of the millimetre peak can be estimated from the hydrogen column density to be about 130 mag (Massa & Savage 1989, Bessell & Brett 1988). This translates into a K band extinction of 14 mag (Rieke & Lebofsky 1985). The visual extinction is still about 40 mag in the region marked by the lowest 1.3 mm contour. Fig. 3c clearly shows that the major part of the optical nebula must be in front of the dust cloud. Here we should note that the interstellar extinction towards the region is negligible. Neckel & Klare (1980) estimated an interstellar extinction in the visual of 1 magnitude which corresponds to [FORMULA] = 0.1 mag (Rieke & Lebofsky 1985).

Fig. 3d shows an (N-Q) colour index image which indicates a shift of the maximum emission towards the centre of the bipolar infrared nebula with increasing wavelength (see also Fig. 2b). This can be primarily attributed to the lower circumstellar extinction in the Q band. Although the strong mid-infrared excess of the illuminating source of the nebula requires the presence of dust close to the star, the question whether this matter actually resides in a Keplerian disk cannot be decided by direct imaging. Unless high-resolution molecular line mapping reveals the kinematics of the circumstellar matter, we cannot rule out that a dust filament right next to the star mimics the appearance of a disk.

4.2. Spectral energy distribution

From the photometric results, the broad-band spectral energy distribution (SED) of BBW 192E was constructed (see Fig. 4a). We included the H and K´ band flux densities of object 25 and that of the bipolar infrared nebula (without stars), the photometer data, the LRS spectrum of IRAS 08513-4201, a KAO spectrum (Cohen et al. 1989), the IRAS as well as the N, Q band and 1.3 mm continuum flux densities. In addition, a modified black-body curve (35 K and [FORMULA]) is shown. This curve matches the data points from 60 µm to 1.3 mm only. In fact, the SED is much broader than a single black-body curve indicating the presence of an internal luminous heating source. We connected the data points of source 25 and also that of the bipolar infrared nebula without stars to emphasize their contributions, respectively. Cohen et al. (1989) and Jourdain de Muizon et al. (1990) found several features (3.3, 7.7, 8.6 and 11.3 µm) which were attributed to polycyclic aromatic hydrocarbons (PAHs), in KAO and IRAS-LRS spectra of our target region (see Fig. 3b), respectively. From the composite of both spectra (see Fig. 4a) we cannot detect silicate emission or absorption at 9.7 µm. However, it is possible that silicate emission and absorption compensate each other in BBW 192E. A detailed radiative transfer modelling which reproduces both the SED and the observed morphology is beyond the scope of this paper.

[FIGURE] Fig. 4. a Spectral energy distribution of BBW 192E which includes our measurements plus KAO and IRAS spectra. Solid line: KAO/IRAS spectra; Dotted line: modified black body curve of 35 K; Dashed line: connects the flux densities of the nebula without stars; Dash-dotted line: connects the flux densities from object 25.  b Colour-magnitude diagram of stellar objects near BBW 192E. Sources which are located towards the millimetre source are marked by triangles and the remaining sources by crosses. All objects are labeled by a number according to Table 2. The direction of the reddening paths is marked by shaded areas. The dashed line shows the reddened zero-age-main sequence (ZAMS) at a distance of 1.2 kpc with an extinction of 1 mag after Neckel & Klare (1980).

4.3. The stellar content of BBW 192E

With the information provided by the NIR images, we are able to characterize the stellar content of BBW 192E for the first time. For this purpose, we plotted the objects in the K´ vs. (H-K´) colour-magnitude diagram (CMD) shown in Fig. 4b. The dashed line indicates the zero-age main-sequence (ZAMS) adopted from Straizys (1995) for the distance of 1.2 kpc and an interstellar extinction of [FORMULA] = 1 mag (Neckel & Klare 1980). The borders of the shaded areas illustrate the reddening line (see Rieke & Lebofsky 1985). In the discussion, we distinguish those sources close the line of sight to the millimetre emission from the remaining objects.

The objects 16, 18, 25, and 27 belong to the population embedded in the dust cloud. This is also true for source [FORMULA] 20 where no H magnitude could be determined. The large extinction derived from the millimetre map excludes that these objects are background stars.

It has been mentioned already that star [FORMULA] 25 is the most luminous object within BBW 192E. Its position in the CMD suggests the spectral type B0. Similarly, object [FORMULA] 27 is very likely of late spectral type B. According to the models of pre-main-sequence (PMS) evolution (e.g., Palla & Stahler 1999), such intermediate-mass stars immediatly appear on the ZAMS without a pre-main-sequence evolution.

From the CMD alone, we cannot infer whether objects [FORMULA] 14, 16, 17, and 18 are ZAMS or PMS stars. In the first case, these stars would be of late spectral type B and early spectral type A. However, it seems reasonable that these stars belong to a PMS population representing a small embedded cluster. If we assume a pre-main-sequence age of [FORMULA] yr, their luminosity is about one order of magnitude larger than that of the final ZAMS spectral type. Thus, we expect that most of these stars eventually become stars of spectral classes F and G.

The objects [FORMULA] 15, 19, 29, and 31 are located left of the ZAMS which suggests that they are foreground main-sequence stars. Indeed, the spectrum of star [FORMULA] 15 obtained by Pettersson & Reipurth (1994) confirms this view. They classified this object as a late-type non-emission line star which is simply located along the line of sight. However, keeping the photometric accuracies in mind, it seems also possible that the stars [FORMULA] 19 and 29 belong to the cluster but are less embedded.

The other objects (22, 23, 26, 28, 30) can be either PMS stars with moderate reddening due to dust not detected in the millimetre map (detection limit corresponds to [FORMULA] of about 4.5 mag) or background stars.

We finally remark that the distribution of stars in the CMD supports the view that BBW 192E harbors a small cluster of young stellar objects. This is very similar to what has been found for the more evolved HAEBEs (e.g., Testi et al. 1997)

4.4. Conclusions

In this paper, we presented the first observational data of the peculiar nebula BBW 192E, covering a wide spectral range from near-infrared to millimetre wavelengths. This approach allows the determination of the complicated source structure and shows the power of multi-wavelength observation for revealing the nature of embedded young stellar objects.

From the morphology at different wavelengths, the information about the extinction behaviour, and the behaviour of the linear polarization, we derived the following properties of the object.

  • The bipolar NIR/MIR nebula within BBW 192E is embedded within a cold molecular cloud core. From the millimetre emission (151" x 55" in size), we estimated a total mass of 180 [FORMULA], a average hydrogen column density of 4.5[FORMULA]1022 cm-2 and a average number density of 2.6[FORMULA]105 cm-3.

  • The cloud core has an opening towards the northwest from which scattered light emerges. The extinction gradient along the opening is the reason why the optical reflection nebulosity is offset from the bipolar infrared nebula.

  • The luminosity of BBW 192E (1400 [FORMULA]) points to the presence of at least one embedded intermediate-mass young stellar object as energy source. We identified this luminous object with the point source north of the dust lane.

  • Due to the bipolar morphology at near-infrared wavelengths, we propose the existence of a disk-like structure surrounding this intermediate-mass young stellar object.

  • The occurence of stars and their photometric properties suggest the presence of a small cluster of low-mass PMS stars in the region BBW 192E.

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© European Southern Observatory (ESO) 2000

Online publication: December 8, 1999