Astron. Astrophys. 335, 243-247 (1998)

3. Discussion

3.1. FIRS 2

NGC 7129 FIRS 2 is neither visible in the optical nor in the near-infrared; in addition, it is point-like at far-infrared and (sub)millimetre wavelengths within the IRAS and JCMT spatial resolutions. Thus, it is most likely the youngest object detected in the region. The observed spectral energy distribution (SED) of FIRS 2 is shown in Fig. 2. The total integrated luminosity is , showing that FIRS 2 is a further intermediate-mass young stellar object in the region, although less massive than LkH 234. The SED can be fitted assuming a grey-body of the form , where and is the Planck function. The best -fit () is achieved for and . There is a small excess at 25 µm which indicates that hotter dust is also present. The large fluxes emitted by FIRS 2 at smm and mm wavelengths in comparison to the total luminosity are remarkable. For instance we may consider the ratio . The millimetre luminosity is given by , where d is the distance to the source, is the observed flux density and the bandpass is = . We estimate for FIRS 2, i.e. of the same order as Class 0 source values (André et al. 1993).

Fig. 2. SED of NGC 7129 FIRS 2. The solid line is a grey body consistent with the data (best -fit) and has values , .

The total (gas + dust) circumstellar mass in the JCMT beam can be estimated using

where symbols take their usual meanings and (Hildebrand 1983). The estimated value (taking as reference ) of the mass is or . Assuming that this mass is distributed in a sphere equal in size to the JCMT beam, then mass and particle densities are and . These are lower limits since the mass is most likely distributed in a disk as suggested by the CO ouflow associated with FIRS 2. The estimated column density is and mag (considering mag). Again, these are lower limits. The estimated figures indicate that FIRS 2 is a very young stellar object. Note that a star with the luminosity of FIRS 2 is expected to be . Since the mass of the dust and gas is of the same order of magnitude, we see that still a large fraction of the mass is in a dusty envelope or, most likely, in a circumstellar disk.

It is interesting to note that FIRS 2 shares its observational properties with those of the very young, low-mass Class 0 protostellar objects: 1. its SED is a single-temperature cool grey-body; 2. most of the object mass is still in a circumstellar disk or envelope; 3. the ratio is among the highest found for Class 0 sources, but much lower that the same ratio for the more evolved Class I sources. Class 0 objects are normally undetected in the 12 µm IRAS band, although some of them are - e.g. L 1448/mm (Bachiller et al. 1991) -; therefore, even if the weak 12 µm source discussed in the previous section was really associated with FIRS 2, the similarity between FIRS 2 and Class 0 objects would remain. As pointed out before, the bolometric and submillimetre luminosities are indirect indicators of the stellar and circumstellar masses, respectively. Our estimates for FIRS 2 give a ratio and the formal boundary for Class 0 and Class I sources is set at (André 1997). A further characteristics of Class 0 objects is their association with highly collimated outflows with dynamical time scales years. In the case of FIRS 2, Edwards & Snell (1983) estimate a dynamical time for its associated outflow of years. However, this estimate is based on the apparent size of the blue outflow, which has a secondary maximum at the position of the Herbig-Haro complex GGD 32/HH 103 field. Optical spectroscopy indicates that this complex of shocked gas is more likely associated with the expansion of the NGC 7129 cavity (Miranda & Eiroa, in preparation); consequently, the size of the FIRS 2 CO outflow and its dynamical time would be considerably smaller. If we estimate the dynamical time using the size of the red lobe, the result is years, which is a value close to those of some Class 0 outflows, e.g. B 335 (Saraceno et al. 1996).

Considering the total luminosity, this parameter is much higher for our source than for any of the known Class 0 objects (see Bachiller 1996). Only, IRAS20050 - a Class 0 object which needs confirmation - has a luminosity of around 60% the luminosity of FIRS 2. This probably reflects the fact that FIRS 2 is a more massive protostellar object.

Concerning the dust characteristics around Class 0 objects, most of them have emissivity index values , similar to the interstellar medium value; e.g. André et al. (1993) and Ward Thompson et al. (1995b) estimate for the prototypical Class 0 objects VLA 1623 and NGC 2264G respectively, although smaller values have also been estimated, e.g. HH 24mms with in the range 0.8 - 1.5 (Ward Thompson et al. 1995a). In the case of FIRS 2, 1.5 provides a very unsatisfactory fit. Dust particles in circumstellar disks around T Tauri stars have values (Beckwith & Sargent 1991), i.e. similar to the value found by us for FIRS 2. This small value has been interpreted as due to grain growth and fractal formation. In this scenario, the grains around FIRS 2, , would be larger than the interstellar solid particles. An alternative, however, has recently been proposed by Chandler et al. (1995). These authors found a value of 0.68 in the disk around HH 24mms and suggested that such low values may be a feature of a high density environment as may also be the case in our source. Summarizing, FIRS 2 is a high luminosity young stellar object which shares the typical properties of the very young, low-mass Class 0 objects and is therefore the highest mass counterpart of these extremely young sources.

3.2. The far-infrared emission in NGC 7129

Several interesting morphological features are revealed by the IRAS maps (Fig. 1). At 12 and 25 µm, the emission is clearly dominated by FIRS 1 surrounded by a diffuse extended emission mainly directed towards the southwest; a very strong gradient of the emission towards the E-NE of FIRS 1 is observed, meanwhile towards the W-SW the emission is smoother. At 60 and 100 µm, the diffuse emission extends to a larger area, although FIRS 1, FIRS 2 and also the strong gradient E-NE of FIRS 1 are also very prominent at these wavelengths. FIRS 1 coincides with the HAeBe star LkH 234; however, a significant contribution can come from the recently discovered mid-infrared companion IRS 6, particularly at the longest wavelengths (Weintraub et al. 1994, Cabrit et al. 1997).

At 12 and 25 µm the extended emission has a conical shape with FIRS 1 at its apex and two protusions surrounding a cavity. The cavity is also remarkable in the static CO (J=1-0) emission gas (Bertout 1987) and in the optical. Fig. 3. shows the 25 µm IRAS image superimposed on a [SII] mosaicing-image of the region, taken at the Calar Alto 3.5 m telescope. The optical nebulosity has a very sharp edge towards the NE of LkH 234, as does the far-IR emission, although the later extends farther towards the NW; both wavelengths reveal the cavity and arms and filaments surrounding it. Even the strongest contours at 12 and 25 µm, i.e. those delineating the FIRS 1 peak, are morphologically similar to the optical appearance of LkH 234 and its immediate surroundings as well as to the near-IR cometary nebulosity detected at this position, which is most likely associated with the infrared companion (Weintraub et al. 1996). All these facts are consistent with the idea that there is a good coupling between the gas and dust in the region and that the dust responsible for the 12 and 25 µm emissions is the same dust that produces the NGC 7129 reflection nebulosity even in the closest surroundings of the LkH 234 field. At the western end, the cavity is closed by diffuse, shocked optical emission in which many HH condensations are embedded, among them GGD 32 and HH 103 (Eiroa et al. 1992, Miranda & Eiroa, in preparation). It is interesting to point out that the 25 µm emission also ends at this position approximately.

Fig. 3. IRAS 25µm image overlaid on a gray-scale [SII] image of NGC 7129

The total integrated IRAS luminosity of the region is 3.1 10³ , and an extrapolation to infinite wavelengths gives 4.5 10³ . This estimate makes reasonable the assumption that the dust in NGC 7129 is heated by LkH 234 and its companion, FIRS 2 and also by other young B stars in the field, as has been previously suggested (e.g. Harvey et al. 1984). Values of the dust colour temperatures and the 100 µm optical depth estimated from the IRAS data are similar to those found by Harvey et al. (1984) and Bechis et al (1978) and so are not included here.

Two alternatives have been proposed to explain the observed cavity in NGC 7129. Bertout (1987) attributes the cavity to the stellar wind from LkH 234 which would have excavated the molecular cloud. Ray et al. (1990) point out that the cometary-like appearance of the NGC 7129 reflection nebula and the optical jet along the cavity axis support this idea. The recent discovery of a mid-IR companion of LkH 234, which could be the driving source of the optical jet, does not contradict this scenario. On the other hand, Bechis et al. (1978) and Mitchell & Matthews (1994) favour the idea that the cavity has been produced by older stars in the region, i.e. BD 1637 and BD 1638; in this case, the argument is that LkH 234 is embedded in a CO molecular ridge sharply bounded to the west by the cavity, suggesting a shell formation event and triggered star formation. In our opinion, the IRAS data do not rule out any of the alternatives, although a sharp gradient towards the west is not observed in the dust emission as it is in the CO static gas. Optical spectroscopic data and proper motions in the GGD 32/HH 103 field are also compatible with both alternatives (Miranda & Eiroa, in preparation).

© European Southern Observatory (ESO) 1998

Online publication: June 12, 1998

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