Astron. Astrophys. 336, 662-666 (1998)

4. Discussion

The ratio of intensities in the LW3 and LW2 filters (the LW3/LW2 color map) is displayed in Fig. 4. Due to the difference of the point spread functions in the LW2 and LW3 filters (about and respectively) we convolved the image in the LW3 filter with the point spread function of the LW2 filter and vice versa. Actually, due to undersampling of the data the two products of convolution are not exactly the same. We might obtain a better result by downgrading images in both filters to a resolution of 2 pixels () but at the expense of a further loss of reso lution.

Fig. 4. Color map of the N 4 region displaying the F(LW3)/F(LW2) intensity ratio (contours) after smoothing to the same resolution, superimposed on an H image. The coordinates are J2000. The contour levels are rather uncertain, as explained in the text. Note the peaks on the two HII regions N 4A and B corresponding respectively to values 4.5 and 3 of the ratio. The ratio for N 4B is particularly uncertain since the intensity of the source in the LW2 filter is at the limit of the 3 times r.m.s. noise level per pixel of the convolved image. The line through N 4A and N 4B corresponds to the cut displayed in Fig. 5.

The ratio map was then produced after clipping the convolved data to 3 times the r.m.s. noise as measured in empty fields of each convolved map. Due to uncertainties in calibration, in the background level and in the relative positioning of the two maps, the numerical values of the LW3/LW2 intensity ratio are uncertain where gradients exist and should be considered as indicative only. This ratio peaks on N 4A, where it reaches its maximum value of about 5, and also on N 4B (peak value about 3). In the region to the East of N 4A, far from the H II   regions, the ratio is 0.6-0.7.

Another presentation of the color data is given in Fig. 5. It displays the LW3/LW2 flux ratio profile obtained along a direction which passes through the exciting stars of N 4A and N 4B, as shown in Fig. 4. The enhancement of the ratio around these exciting stars is obvious.

Fig. 5. Variations of the F(LW3)/F(LW2) flux ratio along a cut passing through the direction of the exciting stars of N 4A and N 4B (see Fig. 4). The scale of the abscissae is in parsec, and the arrows point to the estimated values of the UV radiation density due to N 4A (at 1600 Å) expressed in units of times the interstellar radiation field in the vicinity of the Sun. These values have not been corrected for extinction and scattering and are upper limits. The values of the F(LW3)/F(LW2) ratios are rather uncertain, as explained in the text. The ratio for N 4B is particularly uncertain since the intensity of the source in the LW2 filter is at the limit of the 3 r.m.s. noise level per pixel of the convolved image. Note the peaks on the two HII regions N 4A and B corresponding respectively to values 4.5 and 3 of this ratio.

We now compare the flux ratios observed in the region of N 4 with those in the Galaxy. In relatively quiet regions of the Galaxy, the LW3/LW2 flux ratio for pure PAHs is between 0.55 and 1.0 as estimated from the CAM-CVF spectra of NGC 7023 (Cesarsky et al.  1996a) and from the filter maps of the Ophiuchi cloud obtained by Abergel et al. (1996). The values obtained to the east of N 4A, of the order of 0.6-0.7, are similar to the galactic values.

It is interesting to note that the ratio tends to be lower in regions submitted to a stronger radiation flux but not so strong as to yield a contribution of VSGs in the LW3 filter. This is for example the case for the brighter region in the map of the Ophiuchi cloud which is illuminated by the B2V star HD 147889 (Abergel et al. 1996). On the other hand, the F(LW3)/F(LW2) ratio increases when the radiation field is very strong, due to the fact that the emission by VSGs becomes important in the LW3 filter. According to Cesarsky et al. (1996b) this occurs when the radiation density which excites the UIB emission is of the order of times the density of the interstellar radiation field in the vicinity of the Sun: fields with such high densities exist inside H II   regions or in their immediate vicinity. It is noticeable that although the outline of the UIB emission is similar to that of the CO one (Fig. 3) there is no correspondence between the CO and UIB emission peaks. Presumably the UIB emission comes from the external skin of the molecular complex whose internal structure is revealed by the CO map. The UIBs on this surface are excited by the general radiation field of this region of the LMC, except of course close to the H II   region.

It is clear that the higher values of the F(LW3)/F(LW2) ratio near the H II   regions N 4A and B are due to the contribution of VSGs to the flux around 15 µm. These grains are heated to temperatures sufficiently high for radiating at this wavelength, requiring a high UV flux as discussed previously. This flux is marginally reached near N 4C, but the flux near N 4B is clearly higher and that near N 4A considerably higher.

In order to be quantitative, we have estimated the far-UV radiation density at 1600 Å (a wavelength efficient for heating dust) around N 4A in the following way. The spectral type of the two exciting stars of N 4A, assumed identical, was determined to be O5V from the emitted flux of Lyman continuum photons (Panagia 1973), itself derived from the radio continuum flux with the relation given in Lequeux et al. (1981), after correction by a factor 2 to take into account the loss of such photons by dust absorption inside the H II   region. The radio continuum data at 6 cm used in this determination can be found in Heydari-Malayeri & Lecavelier des Etangs (1994). Then the flux emitted by the stars at 1600 Å was estimated from the relations of Nandy et al. (1976), and the radiation density was computed as a function of the distance to the stars and normalized to the radiation density near the Sun taken from Gondhalekar et al. (1980). Extinction and scattering of the 1600 Å photons by dust have been neglected in view of our ignorance of the geometry of the dust distribution around N 4A, so that the estimated radiation density is an upper limit. This may not be too inaccurate in view of the fact that the region south of N 4A does not contain much matter, and the cut through the north passes through a minimum in the clumpy distribution of molecules in the interface H II   region/molecular cloud, as can be seen on the ¹²CO(2-1) map of Fig. 8b of Heydari-Malayeri & Lecavelier des Etangs (1994). The normalized radiation density at 1600 Å  is indicated in the abscissae of Fig. 5 in units of . It appears that the 15 to 6.75 µm flux ratio becomes significantly higher than the ratio of 0.6-0.7 for quiet regions when the far-UV radiation density is higher than about times that in the vicinity of the Sun. We cannot assess if this limit is the same as in our Galaxy or is different.

An analysis of N 4B and of its exciting star should allow us to determine the local radiation density and would be a welcome check of the above result. At any rate, it is interesting to note that there is dust (at least VSGs and probably UIB carriers) remaining inside this relatively faint H II   region, which according to Heydari-Malayeri & Lecavelier des Etangs (1994) is older than N 4A.

Both the LW2 and LW3 filter maps show an intensity maximum displaced to the NE, on the edge of the H II   region. This shows that most of the emission comes from the interface between the H II   region and the molecular cloud, where the PAHs and VSGs are concentrated. This interface contains the compact H II   region designated as by Heydari-Malayeri & Lecavelier des Etangs (1994). It coincides with a secondary maximum in the color map, while the main maximum almost coincides with the directions of the exciting stars: this shows that VSGs are probably also present in the H II   region N 4A itself. This situation is very similar to that in the Galactic complex M17 (Cesarsky et al.  1996b), including the existence of a compact H II   region in the interface which presumably represents the last stage of massive star formation in the complex.

© European Southern Observatory (ESO) 1998

Online publication: July 20, 1998
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