Astron. Astrophys. 364, 613-624 (2000)

3. Results

3.1. Infrared data

The Fig. 1 and Fig. 2 show the region of IRAS 12326-6245 in the optical, the near-, and the mid-infrared wavelength range. The R-band image indicates the external illumination of the northern rim of the molecular cloud.

Fig. 1a-c. The region of IRAS 12326-6245 seen in a R-band (ESO Survey), b J-band, c H-band, and d -band. The open circles show the positions of the ultracompact H II regions (Walsh et al. 1998). The thick ellipses indicate the IRAS error ellipse.

Fig. 2a-d. The region of IRAS 12326-6245 shown in the a H2 (2.12µm) emission, b H2 (2.12µm) emission (continuum subtracted), c N-band (10µm), and d Q-band (20µm) emission. The open circles label the positions of the ultracompact H II (UC H II ) regions (Walsh et al. 1998) and the crosses give the 1.67 GHz OH maser position by Caswell (1998), whereby the lengths of the cross axes indicate their synthesized beam size. The thick ellipses show the IRAS error ellipse. The dotted ellipses mark the positions of the two MSX sources (see Table 2). The contour levels in the N-band image are 15%(=2), 25%, 40%, and 60% of the peak emission. In the Q-band, the levels indicate 20% (=4), 40%, 60% and 80% of the peak emission.

The near-infrared images clearly reveal the presence of a stellar cluster around the UC H II regions. The radial stellar surface density was derived from the image (Fig. 1) which has the best spatial resolution by applying annuli centered on the southern UC H II region. The resulting distribution (Fig. 3) shows four distinct regions. Beyond the central density enhancement due to the embedded cluster, a region of reduced surface density is obvious, which corresponds to the area of the molecular cloud core. In this region, background stars are diminished because of the extinction by the molecular cloud. The boundary of the cloud is obvious from the increase of the stellar surface density which finally drops because of the finite size of the field. From the stellar surface density, we derive a lower limit for the volume density of 123 stars per pc³ within a sphere of radius 0.43 pc. We should note that most of the cluster members seen at near-infrared wavelengths are located in front of the molecular cloud core or at its surface because the total extinction derived from the millimeter data amounts to 200-600 mag. The extinction over the cluster varies, which implies that multi-object spectroscopy is needed for the derivation of a reliable colour-magnitude diagram.

Fig. 3. Radial surface density of the stars in the image. The annuli were centered on the position of the southern UC H II region. The distribution can be subdivided into four regions: r 80 pixel: radial density profile of the embedded stellar cluster, 80 pixel r 200 pixel: reduced background density due to the extinction of the molecular cloud core, 200 pixel r 230 pixel: "normal" background density, r 230 pixel: decreasing density due to finite field size.

In the 2.2 µm broad- and narrow-band images (see Fig. 1d, Fig. 2a,b), an infrared nebula is present with the apex near the northern UC H II region. Most of the emission is scattered light. The nebula is located at the border of the molecular outflow discussed in Sect. 3.3. It may be the illuminated rim of this flow.

The H₂ image (Fig. 2b) shows three knots north of the brightest star (located in the centre of the image) as well as a bow-shaped feature. The comparison of Fig. 2a with Fig. 2b demonstrates that these structures are not produced by light scattering. They are located in a region which may be related to the north-western part of the molecular outflow. However without further spectroscopic information, we cannot distinguish between shock excitation or fluorescence producing this emission.

In the N-band image (Fig. 2c) four objects have been detected which are not present in the image. Therefore, these sources must be much more deeply embedded and presumably resemble a proto-Trapezium system. With a lower limit for the assumed Q- to N-band flux ratio of 10 (see, e.g., Henning et al. 2000b) for deeply embedded sources, we would expect Q flux values for MIR 3 of 8 Jy and for MIR 4 of 5 Jy, respectively. The flux for MIR 3 is about the 2 detection limit which well fits the fact that this source is just seen at this level in the Q band. MIR 4 could not be detected with the sensitivity of our observations.

The positions of the northern and the strongest southern peak in the N- and Q-band images agree well with those of the unresolved ultracompact H II regions found by Walsh et al. (1998) at 6.67 and 8.64 GHz. The projected distance of these two objects is about 25000 AU which is well within the maximal extent of Trapezium systems with OB primary stars given by Abt (1986). We should note that this is one of the first detections of such a deeply embedded Trapezium system.

The IRAS position uncertainty is rather high and the IRAS source can probably be identified with the southern source although both infrared objects contribute to the IRAS fluxes. The intrinsic fluxes of the individual sources can be better constrained by the MSX data in combination with our MANIAC images. The northern MSX source in the field is coincident with the IRAS source and clearly located at the position of the strongest mid-infrared source MIR 1. Here, we should note that MSX provided a much better positional accuracy and a smaller error ellipse (see Fig. 2). The MSX positions and fluxes are compiled in Table 2 and Table 3. Table 4 summarizes the photometry of the mid-infrared sources. The comparison of the N- and Q-band fluxes of the mid-infrared sources MIR 1 and 2 shows a steep increase of the spectral energy distribution towards longer wavelengths with a similar flux ratio.

Table 2. Coordinates of the sources in the region of IRAS 12326-6245.
Notes:

relax *1) Object can be identified with a stellar source, probably an evolved star with a circumstellar envelope. It is contained in the U.S. Naval Observatory (USNO) catalog.
*2) Positions after Walsh et al. (1998).

Table 3. Fluxes of MSX5C G301.1365-00.2257 which seems to be coincident with IRAS 12326-6245

Table 4. Photometry in the N- and Q-bands

3.2. Structure of the region at millimetre wavelengths

The Fig. 4 and Fig. 5 show the overall structure of the region around IRAS 12326-6245 as seen in the molecular lines as well as in the 1.3 mm continuum. In Fig. 4, the ¹²CO(2-1) and the 1.3 mm continuum measurements are superimposed on the R-band image and the K-band image, respectively. Both maps indicate the presence of a dense cloud core surrounded by a more extended envelope with lower emission. In both cases, we studied a region of roughly 60"60" which covers the cloud center only. A large-scale overview can be obtained by IRAS maps which show a spherical cloud structure with an extension of roughly 5´4.3 These maps are not shown in the paper because they only show the rather symmetric infrared emission.

Fig. 4. a The CO (2-1) measurements of IRAS 12326-6245 are superimposed on the R-band image (Fig. 1 a). The integrated intensity map of the CO line (-47 km s 33.5 km s-1) is shown with thin contour lines (peak 288 K km s-1). The integrated intensities of the red-shifted (-32 km s 11.5 km s thick dashed contour lines) and the blue-shifted CO line wings (-71 km s 47 km s thick solid contour lines) are overlaid. Contour levels are 40 to 90% by steps of 10% of the peak emissions (blue: 62 K km s red: 68 K km s-1). The small crosses indicate the positions of the measurements. b The map of the 1300 µm dust emission (contour levels with a logarithmic intensity scale, peak value = 9.4 Jy/beam) is superimposed on the K-band image (Fig. 1 d). Cuts through the intensity distribution along the major and the minor axis are overplotted with thick lines, whereby the Gaussian beam profile is overlaid with thin lines. The open circles in both figures label the position of the ultracompact H II regions found by Walsh et al. (1998). The beam sizes are indicated by a grey circle in the lower left, and the thick ellipses are the IRAS error ellipse.

Fig. 5a-c. The integrated intensity maps of C18O(2-1) is shown in a , of SO(65-54) in b , and of C34S(3-2) in c as grey scale images. The levels start from 15% of the peak intensities in steps of 7.5%. The solid and dashed contours represent blue- and red-shifted parts of the spectra, respectively. The levels start from 30% of the peak intensities in steps of 15%. The ellipse indicates the IRAS ellipse. Open circles mark UC H II regions (Walsh et al. 1998).

IRAS 12326-6245 is characterized by very strong 1.3 mm continuum emission comparable in strength with the sources discussed by Henning et al. (2000a). The emission peaks at the IRAS position. The Gaussian core profile is barely resolved in the NE-SW direction and remains unresolved in the NW-SE direction. The 1.3 mm peak flux density and the 1 noise value amount to 9.3 Jy/beam and 15 mJy/beam, respectively. The peak flux density agrees well with the earlier On-Off measurement by Osterloh et al. (1997). The total 1.3 mm flux density was obtained by integrating the observed millimeter flux density (surface brightness) of the map within the 3 contour (2.88'). In order to estimate the core flux density the central component was fitted by a 2-dimensional Gaussian and the integrated flux density within this Gaussian was determined separately. The results are = 26.4 Jy and = 12.0 Jy for the total flux and the core flux, respectively.

In Fig. 5, we present the integrated intensity maps of the C¹⁸O(2-1), SO(6₅-5₄), and C³⁴S(3-2) measurements as gray scale images. Whereas the emission in C¹⁸O is somewhat more extended as the image shown, the SO and the C³⁴S measurements roughly cover the whole dense core emission. We should note that the intensity peaks of the C¹⁸O and SO maps are shifted to the eastern side of the southern UC H II region, whereas the peak positions in CO and the 1.3 mm continuum are shifted to the western side (peak separation 12"). The emission of C³⁴S peaks 7" to the south of the southern UC H II region. These shifts are very probably caused by the pointing inaccuracy of the SEST telescope. Summarizing the results, we find that the molecular line emission peaks very close to the position of the southern UC H II region being the most prominent object at mid-infrared wavelengths. The HNCO emission is spatially unresolved with the SEST 23" beam. This is in agreement with the fact that the HNCO molecule traces the densest parts of molecular clouds (see, e.g., Jackson et al. 1984).

In addition, we fitted the line profiles with single Gaussian profiles at the peak positions of each molecular line species. The fits results of the values , , V, and dv are compiled in Table 6.

Table 5. Parameters of the UC H II regions determined from the free-free continuum measured by Walsh et al. (1998) using an electron temperature of = 1 10⁴ K and unresolved beam sizes of 1.5" at 8.64 GHz and 1.9" at 6.67 GHz.

Table 6. Results of molecular line observations for the position of the intensity peaks.
Notes:
*1) The values are given in scale of .
*2) Fit with mask in order to cover the absorption features and the non-Gaussian line wings.

3.3. Outflow

The spectra in CO(2-1), C¹⁸O(2-1), SO(6₅-5₄), ³⁴SO(6₅-5₄), and C³⁴S(3-2) at the peak position are presented in Fig. 6. The CO and SO spectra are obviously characterized by extended wings. The wing component is also present in the C¹⁸O(2-1) line. However, no non-Gaussian wings are seen in the C³⁴S(3-2) spectrum.

Fig. 6. CO(2-1), C18O(2-1), SO(65-54), 34SO(65-54) and C34S(3-2) spectra towards the peak of molecular emission in IRAS 12326-6245. The shaded areas show the blue- and red-shifted parts of the SO(65-54) and C18O(2-1) spectra (see text). For C18O(2-1) and C34S(3-2) single Gaussian fits are shown.

In the Fig. 4a and Fig. 5, we overlaid the integrated intensities of the corresponding red- and blue-shifted line emissions in CO, C¹⁸O, and SO. A more detailed discussion of the properties as well as the estimate of physical parameters of the outflow are given in Sect. 4.3.

© European Southern Observatory (ESO) 2000

Online publication: January 29, 2001
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