## 3. Comparison with wind models## 3.1. The modelThe observed HI line emission has been compared with a wind model which considers a spherically symmetric and fully ionized envolope where the gas is moving with a constant rate of mass loss (). The gas is assumed to have a constant temperature of K and to be in LTE; the adopted gas velocity law is: where =20 km s An important parameter to be considered is the amount of extinction for which the line fluxes need to be corrected. The adopted extinction law is that of Rieke & Lebofsky (1985). CoD 11721 has an estimate visual extinction () ranging from 5 mag (Mc Gregor et al. 1988) to 7 mag (de Winter & Thé 1990), while for MWC1080 a value of 5.4 mag has been determined (Cohen & Kuhi 1979). To account for the indetermination, we checked that, changing by a factor of two, the observed line ratios in the wavelength range we are considering do not change significantly. We first compared the observed line ratios in a given recombination series (line decrement) with the standard model in which the envelope is completely ionized (density bounded ionized flow). In Fig. 2 we show the behaviour of the Pfund series decrement as a function of the different parameters. It turns out that the decrement does not change very much by varying any of the considered parameters, with the only exception being the mass loss rate; for very low values of all the lines become optically thin and then their ratios approach the Case B values (Hummer & Storey 1987). In any case, the higher lines in each series are more sensitive to variations in the model parameters than the lower lying lines.
The ratios observed in CoD 11721 are much
above the Case B line ratios, which means that optical depth effects
start to play a significant role. They are however also in
disagreement with the wind model predictions, independently of the
choice of the input parameters. We have therefore modified the
assumption of a fully ionized envelope by introducing, as a separate
parameter, the relative physical dimension of the ionized region
(R=), where represents
the radius at which the hydrogen atoms recombine. Fig. 3 shows
how the line decrement significantly changes with this parameter. The
behaviour of the line ratios decrement is essentially due to the
different optical depths in the lines. For very small values of R, the
lines are all emitted from the same optically thick surface and the
emission can be approximated as proportional to
S()R
From the line decrement we can constrain the mass loss rate and the ionized region dimension; the distance to the star (D) is then derived from the absolute line fluxes. Table 2 summarizes the model parameters of the best fits for the two stars; the errors quoted for the three considered parameters are derived by taking into account the spread in the line ratios of the different decrements. In the following sections we discuss separately the results obtained.
## 3.2. CoD 11721CoD 11721 is an emission line star embedded
in a diffuse nebulosity whose physical parameters are rather
uncertain, mainly because its distance is poorly known. Indeed,
distance estimates range between 2600 pc (Brooke et al. 1993) and only
220 pc (Pezzuto et al. 1997); in turn the estimate of the spectral
type ranges between O9 and B8. H emission is
observed towards the source, with a FWHM of 500
km s We have already shown that the line decrements for this star suggest a very compact ionized region (R=12) with a rather high rate of mass loss . In Fig. 4a we show the best fit to the data for the line ratios of Brackett, Pfund and Humphreys series with respect to the Br line and in Fig. 5a the predicted absolute line fluxes are compared with the observed ones. The estimated distance is 500 pc; at this distance the star luminosity is , which indicates a spectral type B4 - B5. These estimates of distance and spectral type are in agreement with those found by Pezzuto et al. (1997) by fitting the continuum emission of the source.
Emission from the Brackett series was observed by McGregor et al. (1988), who estimated from the Br luminosity a mass loss rate , positioning the star at D2000 pc and assuming a fully ionized envelope model. They however do not check for the consistency of their derived parameters with the observed Brackett line decrement; their measured lines in the Brackett series agree with the model we derive from the SWS data (Fig. 4a). ## 3.3. MWC1080MWC1080 shows H emission with strong P-Cygni
profile, indicating a wind with velocity of 400
km s We have assumed an =5.4 mag (Cohen & Kuhi
1979) and a stellar radius of
3.210 © European Southern Observatory (ESO) 1998 Online publication: September 30, 1998 |