Astron. Astrophys. 336, 1024-1028 (1998)

1. Introduction

NGC 6334 F (following the radio nomenclature of Rodríguez et al.  1982) is an ultra compact (UC) HII region totally obscured optically and detected at radio wavelengths (Rodríguez et al.  1982). It is located near the northern end of a chain of star forming complexes detected in the FIR, at the position of component I (roman one, following the FIR nomenclature of McBreen et al.  1979). In the radio continuum it has a cometary morphology (Rodríguez et al.  1982; De Pree et al.  1995, Carral et al.  1997) with a steep brightness gradient toward the northwest, i.e. in the direction facing a high density molecular cloud detected in HC₃N (Bachiller & Cernicharo 1990), NH₃(1,1) (Jackson et al.  1988), and NH₃(3,3) (Kraemer & Jackson 1995). The UC HII region must be excited by a massive star as deduced from the luminosity of the mid-IR source (IRS-I 1) associated with it (Harvey & Gatley 1983) and the number of UV photons required to ionize the nebulosity (Rodríguez et al.  1982). Persi et al.  (1996) have shown with J, H and K images that IRS-I 1 has a complex structure in the near-IR, with at least four very red components all within a circle of in diameter.

The NH₃(1,1) gas shows two distinct spatial components aligned in the NE-SW direction and placed on both sides of the HII region (Jackson et al.  1988). Their velocities are separated by more than 3 km s^-1 and the velocity pattern was interpreted by Jackson et al.  (1988) as evidence for a molecular disk in keplerian rotation around a massive star (presumably IRS-I 1, the ionizing star of the HII region). However, observations of CO(2-1) by Bachiller & Cernicharo (1990) showed a well collimated high-velocity CO bipolar outflow (with terminal velocity of 70 km s^-1) exactly in the same direction. This was in evident contrast with the expected direction of the toroid (i.e. perpendicular to the outflow axis) and led Bachiller & Cernicharo (1990) to re-interpret the NH₃(1,1) observations as the low velocity part of the bipolar outflow (see also De Pree et al.  1995). Subsequently, the bipolar outflow interpretation was further confirmed by the detection of NH₃(3,3) maser emission at the two heads of the CO lobes (Kraemer & Jackson 1995) and of shocked H₂ emission at the same position (Persi et al.  1996).

According to Bachiller & Cernicharo (1990) the bipolar outflow and the ionization of the HII region originate from the same star, namely that associated with IRS-I 1. Alternatively, De Pree et al.  (1995), proposed that IRS-I 1 coincided with the early type star that provides the UV photons ionizing the HII region, and that a second source (IRS-I 2), to the northwest and detected at 20 and 30µm by Harvey & Gatley (1983), could be the stellar source at the origin of the bipolar outflow. In fact, the outflow axis seems to pass slightly to the NW of the sharp ionization front. The hypothesis of two distinct early type stars was further reinforced by the detection of a weak unresolved 7 mm source just beyond the edge of the cometary ultracompact HII region (which is by far the strongest source at 7 mm with a flux density of almost 3 Jy compared to a flux density of 267 mJy for the weak unresolved component), roughly south of IRS-I 2 (Carral et al. 1997). The spectral index of the weak unresolved 7 mm source (actually a lower limit, since the source is not detected at longer wavelengths) indicates that the emission must come from heated dust surrounding a protostar, suggesting the presence ahead of the blister of a protostar in an even earlier evolutionary phase with respect to the UC HII region. However, no near IR source was detected at the position of IRS-I 2 (Persi et al.  1996). The near IR source # 48 (Tapia et al.  1996) located within of the 7 mm clump most probably is a foreground highly reddened B4-A2 star, unrelated to the 7 mm clump.

An H₂O maser is present in this star forming complex. The accurate position of Forster & Caswell (1989) (see also note in the caption to Fig. 1 of Carral et al.  1997) associate it with IRS-I 1, while no H₂O maser is reported from the position of IRS-I 2.

In order to identify the driving source of the molecular outflow and to study in more detail the circumstellar material surrounding IRS-I 1, we have obtained high spatial resolution thermal images of NGC 6334 F in the -band (3.8µm) and at 11.2µm . Our observations show that IRS-I 1 has an extended structure at 11.2µm with a blister morphology similar to that seen in the radio. We find two additional small diameter sources and an extended diffuse emission at the position of the molecular cloud at 11.2µm, but we do not detect any small diameter mid-IR source at the position of IRS-I 2 or of the 7 mm unresolved clump.

© European Southern Observatory (ESO) 1998

Online publication: July 27, 1998
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