No abstract

No abstract

No abstract

We present and discuss observations of the spectra of diffuse nebulae performed in 1969-1989 at the 70 c m telescope of the Fesenkov Astrophysical Institute. Observations of the spectra of nebulae and stars at the 6 m telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences are also discussed. Ten of the objects d i s d are compact H I1 regions, the remaining objects are at later evolutionary stages and differ from classical H I1 regions in their smaller linear sizes and higher electron densities. All the nebulae are related to molecular clouds.we discuss the relative intensities of emission h e s , electron densitieu Ne[sII], e~ssision measures in the H a line and in the radio range, extinction values Av for the nebulae and A: to their exciting stars, and also the spectral class of these stam and the distances to the objects.In all the objects, components with densities exceeding Id cm-3 have been observed. Some of them are secondary star-forming regions.By comparing optical and radio data, we show that the value of Ne[S II] does not always provide a measure of the real electron density. In some nebulae, there are ~S ; O M with Ne > lo5 cm-3 which do not emit in forbidden lines. There are also unionized regions whose emission spectra are due to reflection by dust of emission from bright emission-line regions. The largest values of Ne and Av are observed at the interfaces between ionized gas and various molecular structures.In "dusty" nebulae containing dense molecular gas, emission lines of the hydrogen Balmer series and sometimes also He I lines are suppressed by their blending with absorption lines of the stellar spectra rdected by dust particles. In particular, this results in emom in the estimations of the ratio of hydrogen to forbidden line intensities, especially in nebulae d a t e d with early B stars.By comparing A, and A:, we show that all of the compact H I1 regions have the exciting stars which posses circumetellar envelopes (disks) absorbing the stellar light, sometimes by 10 mag. One may conjecture that the amount of the absorption depends on the spatial orientation of the rotation axis of the disk. This also affects the apparent structure of the nebula whose appearance is largely determined by dense molecular filaments, cloudlets and their shadows.Fkom the analysis of the spectra of nebulae and stars, we derive empirical criteria for an approximate estimation of the spectral class of the exciting star from the observed nebular spectrum. These criteria can be made more accurate provided that a theoretical model for the nebular emit sion is available, which allows for a combined effect of stellar wind and stellar Lc emission on emission line intensities.The comparison of the stellar spectra obtained at different times shows that all of the stars are spectrum variables and reliable identification of their spectral classes should be based on an observational series obtained at high spectral resolution.

No abstract