4. The infrared data
Within the studied area, some regions exhibit a high 5 GHz radio emission without any H counterpart, meanwhile others with weaker radio emission are detected at H wavelength. In order to understand this lack of H detection, we compared the infrared data of the different sources in the field. There are two possible cases to explain the absence of H emission: either the considered regions are deeply embedded within the molecular cloud where they were born, or some absorbing clouds are distributed along the line of sight.
With regard to the first case, pre main-sequence star(s) create an HII region, the hydrogen being ionized by Lyman continuum photons of massive star(s). The HII region emission lines, and the subionizing stellar flux are then absorbed by dust grains. When the source is deeply embedded in dust, the bolometric luminosity of the star(s) inside the HII region should be roughly equal to the far-infrared luminosity (Codella et al. 1994). No H detection should be possible for such embedded sources.
We examine this physical aspect for the radio sources of the studied zone. The far-infrared luminosities are estimated from IRAS calibrated sky flux maps supplied by IPAC. Simulating a circular aperture around each source, we determine the flux in the 12, 25, 60 and 100 µm passbands. The dust color temperature is estimated from the f(60)/f(100) ratio and the total far-infrared luminosity is estimated (Schewring 1989) taking for each region the distance of the associated complex determined in Sect. 5.4. The 5 GHz radio emission (Caswell & Haynes 1987) allows us to estimate the ionizing photon number taking = 10000K (Lequeux 1980). Then the total luminosity of a single star that would be required to ionize the HII region is inferred from Thompson's (1984) conversion table. The infrared measurements, total luminosity and color temperature for each source are given in Table 3.
Table 3. Infrared observations
First we can compare the far-infrared color indexes, estimated from the IRAS fluxes, with the existing criteria used for classifying HII regions as normal (Hughes & Mac Leod 1989) or ultra compact (Wood & Churchwell 1989) (respectively labeled HII and UC in Table 1).
Most of the region studied fulfil both the ultra compact and the classical HII regions criteria. It has been shown that more diffuse HII regions also satisfy the Wood & Churchwell criteria (Codella et al. 1994).
Surprisingly RCW 64 (= G 299.363 - 0.257) does not satisfy any of these criteria. Let us note that the infrared emission of RCW 64 seen on IRAS maps is probably contaminated by a nearby infrared emitting object.
Amid far HII regions detected in H the more extended H emissions (295.76, 296.593) have a high luminosity ratio / . This fact can be linked to the HII region evolution. When an HII region ages, the dust is progressively destroyed and the region spreads. For young embedded objects, a luminosity ratio of 1 is expected. Amongst the 3 regions of the distant complex which have a luminosity ratio below one, two exhibit an H counterpart. It is hard to know whether these values are significant or due to uncertainties in the infrared measurements or to IRAS map calibration effects. The luminosity ratio of the regions not detected at H wavelength is not significantly different from the other ones. Since they are found in the same part of the field, this suggests that the lack of H detection is due to absorbing interstellar cloud rather than to circumstellar matter.
The IRAS map investigation, has exhibited four extended infrared sources without radio nor H counterpart. The coordinates of the center of these sources, measured from IRAS maps, are 12 08 , 11 56 , 12 01 and 11 57 . IRAS sources which are not seen in radio continuum may be BN type objects, molecular cores or dark clouds. We compared far-infrared colors of the 4 IRAS sources to the color criteria determined by Henning et al. (1990), Emerson (1987) and Chini et al. (1986) and found 2 sources fullfilling no criteria and 2 others fullfilling both BN-type object and molecular core criteria. In addition their color temperature (above 20K) and their 100 µm flux (above 500 Jy) suggest that these sources are not associated with dark clouds (Chini et al. 1986). Then their real nature remains to be determined.
© European Southern Observatory (ESO) 1997
Online publication: July 3, 1998