Astron. Astrophys. 339, 159-164 (1998)

4. Discussion

The effect of a ionized bounded region on the line ratios in moving envelopes was first discussed by Simon et al. (1983) who showed that the ratios of infrared lines usually observed from the ground (like Br and Pf) can have a wide range of values depending on the external radius of the region, until an asymptotic ratio is reached for large values of the R parameter. They predicted that if the source had detectable radio emission, the asymptotic value of the line flux ratio is surely reached, since the radio continuum flux is very sensitive to the ionized region dimension and a too small optically thick region would produce negligible radio emission. Both CoD 11721 and MWC1080 have been observed in the radio by Skinner et al. (1993) who did not detect any appreciable 3.6 cm flux for the two stars. In particular CoD 11721 was considered an undetected star with a quite high 3 upper limit due to a bright extended radio source near the star. For MWC1080 a flux was detected but this emission does not coincide with the star optical coordinates and, moreover, is a factor 5 less than the emission predicted for a density bounded ionized envelope (Nisini et al. 1995).

We checked the consistency of our derived parameters by comparing the expected ionizing photon flux from the two stars, given our estimated spectral types, with the assumption of ionized bounded regions. From Felli & Panagia (1981) the number of recombinations per unit time in the total envelope is:

where

and

for a velocity varying as until a velocity is reached and then becoming constant. With our velocity law, the factor so is an excellent estimate of the number of UV photons required to fully ionize the flow to infinity. We estimate 10⁴⁹ s^-1 for CoD 11721 and 10⁵⁰ s^-1 for MWC1080. These values have to be compared with the continuum Balmer luminosity supplied by a B5 or B0 star which are 10⁴⁷ s^-1 and 710⁴⁸ s^-1 respectively (Thompson 1984). Hence, the stars do not have enough UV photons to ionize the flow to infinity, consistently with our estimated small size of the ionized region.

© European Southern Observatory (ESO) 1998

Online publication: September 30, 1998
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