Astron. Astrophys. 332, 1035-1043 (1998)

3. Results

3.1. ISO observations

Fig. 2 shows the complete SWS/LWS spectrum for the on- and off-position. The SWS "on-spectrum" and the LWS spectra are rebinned to a resolution of R=200. The SWS observation at the off-source position was performed with the same resolution as the measurement at the on-source position but with a shorter integration time resulting in a lower signal-to-noise ratio. A simple subtraction of the off-source measurement from the on-source data would introduce additional noise into the spectrum and is, therefore, not performed and both spectra are displayed separately (Fig. 2). The SWS "off-spectrum" is averaged over 0.2 µm wide bins.

Fig. 2. In the left part of the figure the SWS spectrum is displayed, in the right part the LWS spectrum. The bold line displays the spectrum observed at the on-position, the thin line stands for the off-position. Where the thin line is missing the data reduction produced "negative" fluxes. The flux scale is the same in both parts of the figure.

The on-spectrum shows a number of relatively strong UIBs, fine-structure lines and Br emission typical for ionized regions. In addition, we see a strong rise of the continuum at wavelengths longer than 15 µm . The data resemble the spectrum of the interface/molecular cloud region of M 17-SW (Cesarsky et al. 1996, Verstraete et al. 1996; see Sect.  4.1). The wavelength region of the spectrum with the distinct UIBs is shown enlarged in Fig. 3. The 6.2 µm UIB is relatively narrow. A weaker feature at 6.7 µm may not be real because of the stronger noise at this wavelength. We see a broad feature between 7.2 and 8.1 µm which has a double-hump structure present also in other ISO spectra (see, e.g., Roelfsema et al. 1996). The 8.6 µm band extends to 8.8 µm with a very low continuum up to the well-known 11.3 µm feature. This behaviour is similar to the behaviour seen in the ISO spectra of compact H II regions (Roelfsema et al., 1996). The depression in the 10 µm region could either be produced by silicate absorption or be just evidence for a very low continuum between molecular emission bands. In addition, we see two more known bands at 12.0 and 12.7 µm . The M 17-North spectrum shows a weak band at 13.5 µm that has been seen rarely in previous observations.

Fig. 3. The range of the UIBs in the SWS spectrum of M 17-North enlarged.

In case of the SWS observation, the background emission is only about 10% compared with the source emission. The lines in the off-source spectrum have about 20 to 30% of the flux of their counterparts in the on-source spectrum if they show up at all. In case of the LWS measurements, the off-source observation detected 25% of the source flux. The fine structure lines in the off-source measurement are all by factors from 2 to 4 less intense except for the 145.5 µm [O I ] line with only 10% of the flux and the 121.9 µm [N II ] line which has the same flux in both spectra within the uncertainties.

The SWS spectra display jumps around 30 µm due to the changing aperture of the SWS. The jump between the SWS and LWS spectrum is also due to the different aperture sizes as M 17-North is quite extended.

3.2. Millimetre continuum observations

The bolometer map of M 17-North (Fig. 4) shows a strong compact emission, 8 in size, elongated in N-S direction. The total flux coming from this compact core is 3.5 Jy. Assuming a gas-to-dust ratio of 150, a dust opacity of 1 cm² /g at 1.3 mm, and a dust temperature of 40 K, the core has a mass of 200 . A large region of emission extends from this compact source to the north with brightness varying between 0.1 and 0.2 Jy/beam emitting a total flux of 14.5 Jy. It covers a region of in size. Assuming a temperature of 20 K and a dust opacity of 0.5 cm² /g for the envelope, it has a mass of 4000 . The dust opacities are taken from Ossenkopf & Henning (1994).

Fig. 4. M 17-North mapped with the bolometer arrays at the IRAM 30m telescope at 1.3 mm wavelength. The contour levels are 0.05, 0.1, 0.2, 0.3,... Jy/beam. The peak flux is 1.25 Jy/beam

The main uncertainties for the mass estimate are the dust opacity and the dust temperature, which may both vary by a factor of 2. For lower temperatures, the derived masses are more sensitive to the assumed temperature values. Therefore, the envelope mass is uncertain by at least a factor of 4.

In addition to the main structure seen in the millimetre map, more cloud fragments can be seen. One fragment is located south-west of the core and emits 2.25 Jy. The other fragments can be found east of the main core and emit 0.93 Jy.

3.3. NIR and MIR observations

The NIR images shown in Fig. 5 reveal many sources in the region of M 17-North . However, only two red objects are located in the cloud core. The northern object has a H magnitude of 14.5 mag and a colour index of =0.9 mag. We call this object M 17N-IRS1 . The colour index of the southern source (M 17N-IRS2) is only =0.1 mag, too low to be deeply embedded, though it is right at the millimetre peak. The fluxes measured in the H- and -band for IRS1 are 1.7 and 2.5 mJy, respectively. This source was also detected in the N-band. We measured a flux of 0.4 Jy in the N-band, while neither IRS2 nor any other 10 µm source were detected in the cloud core. IRS1 is not located at the mm peak flux position. However, its colour and the detection at 10 µm suggests that it is deeply embedded in the cloud core. We cannot exclude that there are even more deeply embedded sources not seen up to 10 µm .

Fig. 5. The symbols mark the stars with flux measurements in the H- and -band. Here the colour index is shown. The asterisks denote the "reddest" stars with positive. The crosses stand for between -1 and 0 mag, and the diamonds for lower than -1 mag. The contours show the 1.3 mm continuum emission (cf. Fig. 4).

© European Southern Observatory (ESO) 1998

Online publication: March 30, 1998
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