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Astron. Astrophys. 339, 759-772 (1998)

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3. Results

In this section we will present the images resulting from our multi-wavelength campaign and the basic parameters that were derived from them. Conclusions drawn from these parameters and comments on specific features visible in the images can be found in in the next section.

The resulting [FORMULA]-band image is shown in Fig. 1. All figures use the star at ([FORMULA],[FORMULA]) as positional reference. The wavefront calibrator is visible at position ([FORMULA],[FORMULA]). G45 itself is at and west of the image centre. The brightest source belonging to G45 is the strong one at position (+3:005,+6) which looks elongated in east-west direction because it actually consists of three close point sources. North of this triple source several unresolved sources are grouped along an arc opening to the south, much like pearls on the chain of a necklace. In Fig. 3, it turns out that part of this arc coincides with the ionization front. Several point sources are also visible above the chain, one of these is especially interesting: The one at position ([FORMULA],+10:005) appears as a very strong mid-infrared source in the corresponding images. Immediately east of that one at position ([FORMULA],+10:008) is a source with an extended structure attached to it. Another source of this kind can be seen to the southeast at position ([FORMULA],+9:008). A preliminary reduced colour version of this image can be found in Stecklum et al. (1995).

Fig. 2 shows the H-band image which is of quite similar structure. Albeit it shows only the southwestern portion of the [FORMULA]-band image, the same structure of pearl-chain, strong source below, and positional reference is visible.

[FIGURE] Fig. 2. H image of G45.

Fig. 3 shows the deconvolved version of the [FORMULA]-band image. Only the portion common to both the [FORMULA]- and H-band images was deconvolved. The point sources are now clearly visible and have been denoted by letters for identification. From the superimposed VLA map by WC89 it becomes clear, that the chain-like structure also follows the ionization front. The strong source below the arc of the chain is now clearly resolved into three single sources labelled l to n.

[FIGURE] Fig. 3. Deconvolved [FORMULA] image of G45.45+0.06. The image from Fig. 1 was subject to 200 iterations of a damped Richardson-Lucy deconvolution. The contours represent a 6 cm VLA image from WC89.

The result of our Br[FORMULA] imaging can be found in Fig. 4. It shows an overview map of the region, taken from the continuum narrow-band image at 2.22 µm. Superimposed are the contours of the continuum subtracted line emission. Since the image was taken with "conventional" NIR imaging, the resolution is much lower than in the [FORMULA] and H images. However, due to the larger field of view we gain valuable information on the overall structure of G45. It can be seen that the whole ionized region has a structure like a horseshoe opening to the southeast with an extra structure pointing north from G45. Taking the weak 3.6 cm emission in the southeastern corner into account, the horseshoe completes itself to a shell-like structure with a diameter of approximately [FORMULA]. The [FORMULA] diameter corresponds to a linear size of 1.3 pc.

[FIGURE] Fig. 4. Br[FORMULA] image of G45. The superimposed contours denote the continuum-subtracted line emission with contour levels set at [FORMULA] mJy/[FORMULA][FORMULA]. The gray scale image is the corresponding continuum image (2.22 µm). Negative contours are dotted, they indicate artifacts from imperfect alignment during continuum subtraction.

The results of our MIR imaging can be seen in Fig. 5. Fig. 5a shows the 3.5 µm image, Fig. 5b the 10 µm image and part c the 12 µm image. The contour lines represent the 6 cm VLA map obtained by WC89, the crosses denote the point sources found in the near-infrared images. Two point sources can be seen in all images, a strong one at position ([FORMULA],+10:005) (This will be referred to as MIR1 ) and another one at position (+3:008,+4:008) (MIR2 ).

[FIGURE] Fig. 5. Mid-infrared images of G45. The contour lines are from a radio map from WC89, the crosses denote point sources from the [FORMULA]-band image. Apart from the topmost cross, all others represent only point-sources denoted with letters in Fig. 3. a  L-band image (3.5 µm). b  Broad band N image (centred at 10.3 µm). c  This image was taken using a narrow band N (11.7 µm) filter.

Table 2 compiles the results of the photometric and positional measurements of the point-like sources visible in the [FORMULA] band image. Brightnesses were measured whenever possible in [FORMULA] and H. The astrometric positions were determined in [FORMULA] by centering an aperture of [FORMULA] diameter on the star and computing the mean of all pixels weighted on their brightness (an analogue to the conventional "centre of mass"). The identification letters (IDs) of the sources are the same as in Fig. 3. Sources inside the VLA map are denoted by characters from a to o, other point sources in our images by roman numbers.


[TABLE]

Table 2. Point sources in G45
Notes:
a) Source labelling is the same as in Fig. 3, except for source MIR1 , which is explained in the text.
b) Not all point sources are detected in H.
c) Photometry not possible in un-deconvolved image.


Fig. 6 shows the spectral energy distribution of G45. The upper line is the integrated flux of the whole region. Our contributions to this curve (marked by the triangle symbols) were measured in an aperture of [FORMULA] diameter. This explains the step between our data and the IRAS data, which were measured in a much larger aperture. The steep rise in the NIR is followed by a decline longwards of 100 µm and the typical flat radio plateau with a spectral index of [FORMULA] longwards of 1 cm. The latter one is caused by free-free emission from the ionized gas. The lower curve represents only the source MIR1 , measured in an 0:005 diameter aperture in [FORMULA], 2:0075 in L and [FORMULA] at 10 µm. This curve shows, that source MIR1 is contributing up to 10% of the total flux of G45 (including MIR1 !) at infrared wavelengths. In a new VLA map of the source taken at 1.3 cm by Hofner et al. (1998), this source also stands out as a single point source and contributes a significant portion of the total radio flux. However, as the VLA observation was pointed at G45.47+0.05, G45 lies only at the rim of the primary beam where the signal-to-noise ratio is not sufficient to measure the flux contribution from the point source MIR1 reliably.

[FIGURE] Fig. 6. Spectral energy distribution of G45. The upper line is the integrated flux from the whole UCHII region, the lower one is source MIR1 alone. The dashed line denotes an obvious gap in the SED where no observations have been made up to now.

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© European Southern Observatory (ESO) 1998

Online publication: October 22, 1998
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