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

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3. Discussion

Most of the discussion reported here relates to the analysis of the 11.2µm image, given its higher sensitivity and spatial resolution.

All the mid-IR sources reported in Fig. 2 have been detected in the near-IR by Tapia et al.  (1996), and their identification is given in Table 1, column 5. MIR 1 is immersed in the diffuse emission extending in the direction of the molecular cloud-bipolar outflow. A second point-like source, not reported in Table 1 and identified with the near-IR source #68 (Tapia et al.  1996) and St 11 (Strow et al.  1989), has been detected within the cloud at the limit of the diffuse emission.

The source MIR 3 coincides with IRS-I 3 of Harvey & Gatley (1983), and appears sligthly extended, as is also observed in the K-band image of Persi et al.  (1996). Combining the 2.2 and 11.2µm flux densities we derive for this source a spectral index n = dLog ([FORMULA] F[FORMULA])/ dLog [FORMULA] = - 2.2, typical of very young stellar objects, with an IR luminosity of a B0 ZAMS stars. In fact, its JHK colors show very strong excess at 2µm (Tapia et al.  1996).

We have obtained the integrated flux distribution of MIR 2 (IRS-I 1) combining observations from the near-IR to the radio continuum taken from the literature as well as those presented in this work (Fig. 4, upper panel). This continuum flux density distribution is very similar to those of UC HII regions studied by Wood & Churchwell (1989), in which thermal emission from dust heated by a central star accounts for nearly all the radiation at wavelengths less than [FORMULA]1 mm. A color temperature of T[11.2-20] [FORMULA]130 K has been derived from our observed 11.2µm flux density and the 20µm flux density of Harvey & Gatley (1983). In addition, Fig. 4 shows that the 7 mm and 2 cm emission are due to optically thin free-free emission from the ionized gas surrounding IRS-I 1. This free-free emission could contribute also to the nebulosity observed at 2.2µm by Persi et al.  (1996) as can be seen extrapolating the 7 mm flux density to 2.2µm with a law of the type [FORMULA] [FORMULA] [FORMULA]. The flux distribution has been compared with model "H" of dusty HII regions developed by Natta & Panagia (1976) (dashed line in Fig. 4). This model, computed for a uniform spherical nebula composed of gas and two types of grains and with a central empty cavity surrounding an early type star, fits quite well the flux distribution in the near, mid-IR and radio region of the spectrum. The discrepancy with the far-IR, 400µm and 1 mm spectral points may be due to the fact that these observations have been obtained with a much larger beam compared with the other observations.

[FIGURE] Fig. 4. Infrared flux distribution of IRS-I 1 (upper panel), and IRS-I 2 (lower panel). References for the observed flux densities are: J, H and K, Tapia et al.  (1996); L and M, Persi& Ferrari-Toniolo (1982); from 8.7 to 30µm, Harvey & Gatley (1983) and this work; 42-134µm, Loughran et al.  (1986); 400µm, Gezari (1982); 1 mm, Cheung et al.  (1978); 7 mm, Carral et al.  (1997); 2 and 6 cm, De Pree et al.  (1995). The dashed line represents the model "H" of dusty HII regions of Natta & Panagia (1976).

The morphology of MIR 2 is best seen in the deconvolved map shown in Fig. 3. An arcuate or cometary apparence is present, with two knots of emission. The brightest peak is coincident with IRS1E, the reddest near IR-source of IRS-I 1 (Persi et al.  1996). This source appears resolved with a size of [FORMULA] 3 1016 cm (see Sect. 2), similar to that observed in the radio continuum by Rodríguez et al.  (1982) ([FORMULA] 6 1016 cm). Comparing the 11.2µm image with the radio continuum map at 2 cm obtained with similar resolution by De Pree et al.  (1995), we conclude that the warm dust in NGC 6334 F is either inside or at the outer boundary of the ionization front, as also observed in the cometary UC HII region G29.96-0.02 (Ball et al.  1996). A second knot of emission is present approximately [FORMULA] east and [FORMULA] north of IRS1E.

Assuming that the ionizing star in NGC 6334 F is an O8 ZAMS (L [FORMULA]8 104 [FORMULA] Harvey & Gatley 1983), we have derived a dust temperature Td [FORMULA] 105 K at a distance of 3 1016 cm from the central star applying the model of Churchwell et al.  (1990). This temperature is in agreement with the color temperature derived by the IR spectral points.

MIR 2 (IRS-I 1) coincides with IRAS 17175-3544, whose color-corrected flux density at 12µm is 115 Jy. This value is consistent with that observed at 11.2µm, including the contribution of all the three sources (MIR 1,2,3) and the diffuse emission.

The 11.2µm diffuse emission extends towards the dense core of the molecular cloud, as clearly shown by the overlay of the integrated thermal NH3 (1,1) line emission (Jackson et al.  1988) with the 11.2µm emission of Fig. 5, and could be either due to externally heated dust, or to polycyclic aromatic hydrocarbons emission (PAH's). In fact, spectrophotometric observations of the HII region/neutral interface in M17 obtained with ISOCAM by Cesarsky et al.  (1996), indicate that the extended emission is dominated by PAH's. A similar interpretation has been given by Minchin et al.  (1992) to explain the extended IR emission in the CepB-S155 interface region.

[FIGURE] Fig. 5. The integrated NH3 (1,1) main hyperfine line emission detected by Jackson et al.  (1988) is superposed on the 11.2µm image

As far as IRS-I 2, using our quoted upper limits (for a point source), the 20 and 30µm flux densities taken from the contour maps of Harvey & Gatley (1983) and the 7 mm flux density of Carral et al.  (1997), assuming they arise from the same object, we have derived the flux density distribution shown in Fig. 4, lower pannel. Considering that at a level of 1.2 Jy we do not detect any source at 11.2µm coincident with the 7 mm clump reported by Carral et al.  (1997), it is improbable that this clump, if confirmed, has a hot internal stellar source heating it from the inside. In fact, emission from hot dust around an early type star (e.g. an O9.5 with luminosity [FORMULA] 3 104 [FORMULA], following Harvey & Gatley 1983), with temperature in the range 200-1000 K should be easly detectable at 11.2µm unless the extintion has a very high value, AV [FORMULA] 110-140. Without further observations of this clump at millimeter and submillimeter wavelengths, little can be said about its nature. There is also the possibility that the 20 and and 30µm flux densities of IRS-I 2 (Harvey & Gatley 1983) might be contamined by the extended diffuse emission observed at 11.2µm and associated with the molecular cloud.

With respect to the model presented by Testi et al.  (1998) to put the different objects found in the star forming complex G9.62+0.19 into a consistent evolutionary sequence, MIR 2 (IRS-I 1) is clearly the one in a more evolved phase. The ionized gas is clearly detectable in the radio continuum, the cluster of early type stars embedded in the HII region can be seen in the near-IR and the warm dust emission can be seen at longer wavelengths. The H2O maser is located in between IRS1E and the sharp ionization front. This phase is comparable to component B in G9.62+0.19 or even earlier, given the presence of the H2O maser and the compactness of the HII region. MIR 3 seems to be in an earlier stage in which the hot dust emission around the early type star is well detectable in the near and middle-IR, but the radio continuum emission from a possible HII region is still not detectable, most probably for self-absorption effects in these early stages. This phase is not dissimilar to component F in G9.62+0.19 or even earlier given the lack of H2O maser. Finally, IRS-I 2, not detected in the present observations or in the near-IR but possibly only at 7 mm and (if not confused) at longer IR wavelengths, could represent an even earlier stage, in which only the cold dust emission is detectable and no early type star has been formed yet inside the clump. It would be of extreme interest to know if molecular outflow can be produced in such early phases, but to confirm the association of IRS-I 2 with the molecular outflow (and its true nature), higher resolution observations at 20 - 30 µm as well as improved VLA observations of the 7mm clump are needed.

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© European Southern Observatory (ESO) 1998

Online publication: July 27, 1998