Astron. Astrophys. 357, L33-L36 (2000)

3. Morphology

Figs. 1 and 2 show the MIR images of the M 17-UC1 area at 10.5 and 20.0 µm and Fig. 3 the same region at 1.3 cm. An extended diffuse emission structure is visible in all images. It is referred to as Arc by FJC and FS. Apart from M 17-UC1 at and , which is very faint at NIR wavelengths and clearly detected in the MIR and at 1.3 cm, we find a second compact IR source to the southwest at and , which is very bright at J,K, N and Q but does not have any counterpart in the VLA map (see Table 1). The described IR morphology differs significantly from the results of FS. While the diffuse emission region reported by FS is located approximately southeast of M 17-UC1 our images clearly show a compact source southwest of it. The relative orientation of M 17-UC1 and IRS 5S - our internal designation for the southern source - is indubitable correct and matches an early N-band image in 1995 with TIMMI at the 3.6 m telescope on La Silla and the ISOCAM images published by Crété et al. (1999).

Fig. 1. M 17-UC1 at 10.5 µm. Contours indicate flux densities in steps of , beginning with .

Fig. 2. M 17-UC1 at 20.0 µm. Contours indicate flux densities in steps of , beginning with .

Fig. 3. M 17-UC1 at 1.3 cm. Contours indicate flux densities in steps of , beginning with .

In order to explain this discrepancy we may firstly assume that the diffuse structure to the SE of M 17-UC1 and IRS 5S are the same object and that the difference originates in a proper motion of the object between the epochs 1984 and 1997. With an adopted distance of M 17 of 1.6 kpc (Nielbock et al., in prep.) the displacement would run up to a linear distance of m which implies an unrealistic velocity of km/s. It is very striking that the morphology proposed by FS seems to appear mirrored in our images. The angular distances between the two southern sources and M 17-UC1 as well as their position angles are comparable. This might suggest that the difference could be simulated by an east-west flip of the MIR maps shown by FS. Unfortunately, an intensive discussion with Dr. Felli about this topic could not solve the problem because the original data cannot be accessed any longer.

The distance between M 17-UC1 and IRS 5S is only  AU (Figs. 1 and 2) and a sheer coincidence of their close position seems quite unlikely. We have mapped an area of about in total and did not find any similar strong MIR sources. Furthermore, extinction within M 17 SW of up to mag (Chini & Wargau 1998, hereafter CW) rules out any background stars we might have picked up. Therefore, we suggest that both sources belong to a physically bound binary system of young high mass stars.

© European Southern Observatory (ESO) 2000

Online publication: June 5, 2000
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