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

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3. ISO filter maps of the region of N 4

Fig. 1 shows the LW3 (15 µm) map superimposed on the H[FORMULA] image obtained by Heydari-Malayeri & Lecavelier des Etangs (1994). This image (their Fig. 1a) shows the bright H II   region N 4A. N 4A is presumably ionized by a pair of central stars, not visible in this saturated image. [FORMULA] to the NE is the fainter H II   region N 4B with a conspicuous central star. [FORMULA] to the NW of N 4A is the bright star WOH 53 (Westerlund et al.  1981), a M supergiant belonging to the Large Magellanic Cloud (Rebeirot et al. 1983). The relatively faint, filamentary H II   region N 4C is located [FORMULA] E of N 4A, outside the H[FORMULA] map. A strong emission is seen associated with N 4A but displaced to the NE. There is a fainter emission in the direction of N 4B and of N 4C.

[FIGURE] Fig. 1. Map of the N 4 region in the LW3 filter centered at 15 µm (contours) superimposed on an H[FORMULA] image. The coordinates are J2000. The "source" at [FORMULA]= 04h 52m 20s and [FORMULA]= -660 [FORMULA] is an artefact due to the remanence of the detector. The H II   region N 4A is located at the center of the figure. [FORMULA] to the NNE is the fainter H II   region N 4B with its conspicuous central star. [FORMULA] to the WNW of N 4A is the bright star WOH 53, one of the brightest M supergiants in the LMC (Westerlund et al.  1981). The relatively faint, filamentary H II   region N 4C is located [FORMULA] E of N 4A, outside the H[FORMULA] map. It corresponds to the easternmost peak of the ISO map. Contour levels are in mJy per square arcsecond. The first contour is at 3 r.m.s. noise level.

Fig. 2 shows the LW2 (6.75 µm) map, this time superimposed on the Digital Sky Survey image. The sensitivity is higher in this map, which displays extended structure not seen in the LW3 map. Emission is seen associated with N 4A and N 4C. The emission associated to N 4B is present but fainter than the 3 r.m.s noise level (per pixel): it is not displayed in the figure.

[FIGURE] Fig. 2. Map of the N 4 region in the LW2 filter centered at 6.75 µm (contours) superimposed on the Digitized Sky Survey image. The coordinates are J2000. The "source" at [FORMULA]= 04h 52m 20s and [FORMULA]= -660 [FORMULA] is an artefact due to the remanence of the detector. The emission from the faint H II   region N 4B is present, but below the displayed level. Contour levels are in mJy per square arc second. Compare to Fig. 1.

The outlines of the LW2 map coincide rather well with the 12CO(1-0) map obtained at [FORMULA] resolution (Fig. 8a of Heydari-Malayeri & Lecavelier des Etangs 1994), as it can be seen on Fig. 3. Unfortunately the CO map is not sufficiently extended to see if there is CO coinciding with the PAH emission in the easternmost part of the LW2 map. There is a strong emission in the LW2 filter in the direction of WOH 53; it will be discussed later. Another point source in the LW2 image at [FORMULA](2000)= 04h 52m 02s and [FORMULA](2000)= -660 [FORMULA] is identified with a star in the ESO on-line Digital Sky Survey (DSS). The absence of photometry for this object does not allow us to further investigate on the origin of its 7 µm emission. Both stars have been used to re-center the LW2 map on the DSS, while the LW3 map was centered only on WOH 53; its orientation was assumed to be correct, which is usually the case for such ISO observations. The relative position uncertainty between the two maps is of the order of half a pixel (3 arc seconds).

[FIGURE] Fig. 3. CO(1-0) map of the N 4 region (contours) superimposed on the LW2 (6.75 µm) ISO image (gray scale). The coordinates are J2000. The CO map is from Heydari-Malayeri & Lecavelier des Etangs (1994) and has a [FORMULA] resolution. The contour units are 2 to 8 in steps of 1 K km/s. The CO observations do not extend beyond the last displayed contour to the left of the figure.

For the bright M2 supergiant WOH 53, (Westerlund et al.  1981) we measure a flux density of approximately 0.47 Jy and 0.14 Jy at 6.75 and 15 µm respectively. Together with the optical

agnitudes of the star (B=13.3, V=11.6, R=9.9, I=7.8: Rebeirot et al.  1983; Westerlund et al.  1981), these fluxes indicate an approximate effective temperature of 3000 K, corresponding to a M supergiant with no circumstellar envelope, or to a hotter supergiant with some circumstellar emission. A discussion of the properties of this star requires a deeper optical study.

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© European Southern Observatory (ESO) 1998

Online publication: July 20, 1998
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