We have generated models of complete nova energy distributions, including the effects of the nova dust shell emission and absorption. In order to account for the dust, we require optical constants throughout the UV and EUV. Draine & Lee (1984) is the only source of such data for silicates. However, their ‘astronomical silicate’ has a strong resonance in the UV which causes circumnova dust to be heated far above its sublimation temperature in our models of Nova Her 1991. We have therefore recalculated the silicate optical constants, using the Kramers-Kronig relations, including a λ−1.0 far-IR dependence and improved 9.7μm feature, and decreasing the UV and EUV efficiencies of the grains. With these new silicate constants we can reproduce the observed behaviour of the dust shell in Nova Her 1991. Since our optical constants are consistent with the behaviour of Nova Her 1991, an EUV source, they should also be appropriate for the treatment of PN.

The spectrum of R Sct, a star firstly classified as an RV Tau one and located in the highest part of the instability strip, indicates a Na ejected shell at about 1000 R∗ with an expansion velocity close to 50 km/s (Lèbre and Gillet, 1991). This confirms the post AGB character of this star as already mentionned by Jura (1986). So it is interesting to compare the R Sct with some “classical” RV Tauri (AC Her and U Mon) and some “exceptional” miras (R Cen, U CMi and R Nor) with light curves exhibiting a double maximum and with spectral types earlier than those of the other miras with similar periods.

The radio continuum emission of long-period variables (LPVs) in the continuum is considered. For some LPVs (e.g., R Aql and V Hya) weak emission, observed at centimeter wavelengths, can be explained by thermal free-free radiation of the ionized gas behind the shock front in the stellar atmosphere. The ionized layer behind the shock remains optically thick at the wavelengths as short as 1 cm; the brightness temperature found for R Aql at 2 cm by Drake et al. (1987, AJ, 94, 1280), Tb = 18000 K, is consistent with the expected temperature behind the shock.

We have collected all the available data on light curves, OH, H2O and SiO maser observations for a sample of Mira-type variables. We consider in detail the data on two stars, U Ori and R Leo. There is a net correlation between optical and radio line variations for all the three molecular species in these stars. More pronounced maser flares seem to follow brighter-than-average visual maxima of the stars. We discuss also the drastic changes in the type of the OH maser radio emission which happened in these stars some years ago. Implications for the mechanisms of maser pumping and the evolutionary status of these stars (probably undergoing the helium flash) are discussed.

The statistical study of the Long-Period Variables, provided by various authors, show, that there is a relationship between the photometric characteristics and the physical properties. For example, the asymmetry of the light curve may be related with the strength of the shock wave. The presence of the maser-fed line emission characterizes the definite evolutionary stage of the Long-Period Variable.

The main results on the elaboration of the algorithms and programs for the search of the ‘hidden periodicities’ in the ‘unevenly spaced’ data, are discussed. They are mainly based on the application of the cubic spline-functions practically for all purposed of the variable star research — period determination, approximation of the phase curve, search for the possible period changes and/or the shape of the phase curve, detection of the periodic components in the case of the multi-harmonic oscillations, restoration of the smoothed function by removing the ‘apparat function’, numerical integration etc. Contrary to the commonly used splines with the arguments of the basic points coinciding with that of the real observations, we use the smoothing by the spline with the number of the basic points m, which is smaller than the number of the observations n. The main expressions for the corresponding fitting curves and their statistical properties were published by Andronov (1988, 1989), as well as the FORTRAN programs for the fits and the integration (Andronov, 1986, 1987, respectively).

During the past decade much effort has been spent also on observation and theoretical modelling of AGB stars. However, the Semiregulars have been almost neglected. Kerschbaum & Hron (1992) used GCVS- and IRAS-data to define samples of SRa and SRb stars. These were compared with optical Miras with respect to their pulsational properties, effective temperatures, mass loss rates, luminosities, scale heights and galactic volume densities.

The group of short period Mira variables (periods less than 200 days) is thought to be a mixture of metal poor stars and disk stars. We have compiled JHKL photometry for 159 stars in this period range, the great majority of the data coming from new observations. The photometry is used to separate metal poor from disk stars, following Hron (1991, A&A 252, 583), and to define volume limited subsamples. Distances were derived from the K-magnitudes and the period/K-luminosity relationship.

In 1976 Georgelin et al.(1976) combined optical and radio observational data of HII regions to study the structure of our Galaxy. The distance of each HII region can be obtained from its radio continuum flux caused by free-free emission and the flux of the Balmer lines with high accuracy. The results clearly show three arms and are used to draw the spiral structure of our Galaxy. It is commonly accepted now.

We have computed ice column densities for a sample of O-rich late-type stars.

Water ice is observed in the outflows from several late-type stars (e.g. Smith et al. 1988) and it is of some observational and theoretical importance to understand how common ice mantles may be and how their formation depends upon the physical conditions in the envelope, such as the radial distributions of density and temperature. We have developed a simple theory of the gas-grain interaction in circumstellar shells (Jura & Morris 1985; Charnley et al. 1992) and used it to compute ice column densities (Nice) and mantle thicknesses (d) for a sample of well-studied OH/IR stars (Herman & Habing 1985). The models are most sensitive to the dust and gas mass loss rates, as well as to the inner radius of the envelope, Ri, which is poorly determined. Representative results are presented in the Table below where Ri was estimated according to the prescription of Herman et al. (1986).

We present the first results of a long-term monitoring program of observations in the near infrared of a selected sample of OH/IR stars included in the IRAS Point Source Catalogue. The observations have been made using the 1.5m Sanchez Magro Telescope (SMT) at Izaña (Tenerife, Spain) since the beginning of 1991 and are still in progress. They are being complemented with observations made using the 1m ESO photometric telescope (La Silla, Chile). The sample includes 30 OH/IR stars with a variety of infrared and OH maser luminosities, expansion velocities, LRS classes and position in the IRAS two-colour diagram.

We present a study of the distribution of post-AGB stars and planetary nebulae (PNe) in the IRAS two-colour diagram. From the analysis of the distribution of spectral types of post-AGB stars in this diagram we conclude that the position at which a star leaves the infrared sequence of colours observed in AGB stars depends only on the initial mass of the progenitor star. The evolution after the end of the AGB phase takes place, in a first approximation, at a constant value of [25] – [60].

A new evolutionary interpretation of the sequence of colours observed in the IRAS two-colour diagram by AGB and post-AGB stars is given, which is capable of explaining the observational properties of both kind of objects. It is useful to define a parameter λ to define the position of a given star in this “infrared main sequence” (IRMS). Adopting and from the analysis of the expansion velocities, mass loss rates and luminosities observed in a selected sample of non-variable OH/IR stars with no optical counterpart in the Galactic bulge as a function of λ, we conclude that the position in the IRAS two-colour diagram at which a star leaves the IRMS (λmax) only depends on the initial mass Mz of the progenitor star, so that only massive objects can reach the upper end of this sequence. The relation found is:

Expansion velocities increase with the initial mass while every point in the IRMS is found to be associated to a certain value of the mass loss rate. This model also predicts the evolution with time of the mass loss rate during the AGB as a function of the initial mass of the progenitor star, and confirms that most known planetary nebulae are the result of the evolution of considerably massive stars (between 2–3 solar masses) which means that the contribution of processed material to the interstellar medium is considerably higher than what theoretical models predict. Type I PNe are the result of the evolution of 3 — 5 M⊙ progenitors while progenitors with Mi ≤ 1.2 M⊙ probably do not give PNe. The model is also in agreement with the narrow distribution of core masses found in central stars of PNe and white dwarfs and with the usual expansion velocities found in OH/IR stars.

A complete sample of PPN candidates was selected mainly based on the IRAS colors(Hu et al.,1990). For total 62 unknown objects in the sample we have made 1. ground-based infrared astrometry; 2. optical identifications; 3. near infrared and optical photometry; 4. optical spectroscopic observations; 5. radio molecular line observations(OH maser and CO thermal lines); and 6. check the optical variations from historical plates. Some additional observations such as high resolution spectraoscopic, optical/NIR CCD imaging observations for particularly interested objects were also carried out.

FI Lyr=IRAS18401+2854 is listed in the GCVS as type of SRb, the spectral type is M. In the IRAS LRS Catalog it is classified as 41, namely having the SiC feature at 11.3μm from a carbon-rich dust shell. We have obtained optical and near infrared spectra in the same period, but they show different chemical natures. We suggest that FI Lyr is a good candidate of a binary system consisting of carbon-rich and oxygen-rich companions.

Sixteen candidates for proto-planetary nebulae have been observed with low-resolution infrared spectroscopy in the H and K bands, and 6 in the L band, using the United Kingdom Infrared Telescope. In the H band, the objects show hydrogen Brackett lines in absorption. In the K band, absorption bands (del v=2) of CO were observed, and in three cases the CO bands are in emission. The CO spectrum of IRAS 22272+5435 was found to change from emission to absorption over a three-month interval. This CO emission can be interpreted as an indication of some recent episodes of mass loss in these objects. Four of the objects were found to possess an emission feature at 3.3 um, usually associated with PAHs, and two of these show an unusually strong 3.4 um emission feature (Geballe, Tielens, Kwok, & Hrivnak 1992, ApJ, 387, L89).

Several Proto–Planetary Nebulae (PPN) candidates have been observed at the Canada France Hawaii Telescope with the High Resolution Camera (HRCam) (McClure et al. 1989, PASP, 101, 1156). Candidates were selected based on their IRAS [12]–[25] and [25]–[60] colors and their lack of variability. With HRCams' small angular pixel size (0.13″/pixel) and active optics we achieved an average angular resolution of 0.65″ in V and I waveband images.

The ages of PNNe in our sample (Hu et al., 1990) were derived from the inner radius of the dust shell using the radiative transfer model fitting with SED and expansion velocities of envelopes obtained from radio molecular lines observations (if without radio data a Vexp of 15 km/sec was assumed). The objects are classified into the following groups:

1. 1. invisible;

2. 2. M-type;

3. 3. F (or G)-type; and

4. 4. F (or G)-type with Ra emission.

An AGB star evolves into the proto-planetary nebula(PPN) phase, when the stellar surface temperature is hotter than 5000K(Schonberner, 1983). But the remnant of AGB dust/gas envelope is still thick in the early stages of PPN evolution. The observed optical spectrum of PPN is a spectrum of central star(T∗ hotter than 5000K, the spectral type earlier than G5) overlaped with molecular features formed by molecules in the envelope. The features are changing during the envelope turns to optical thin. Based on our systematic observations of IRAS selected PPN candidates(Hu et al. 1990) we found that molecular features in the spectra evolve as following:

1. 1. When object just exposes from dust shell, the column density of molecules in the envelope is very high. The molecular absorption bands are dominant in the spectra, which are resemble to the M supergiant. There are 6 objects from our 62 PPN candidates sample in this stage.

2. 2. When column density of molecules decreases, the spectra mainly show F-(or G-)type supergiant features from photosphere and partly also from envelope (especially the neutral atomic lines). But we still can detect the molecular bands such as C2, CN in the spectra of carbon-rich objects(3 objects in our sample, IRAS23304+6147, 20000+3239 and 19477+2401), TiO of oxygenrich object(one object, IRAS20406+2953), and ZrO and LaO of S-type transit object(one object, IRAS19454+2920).

3. 3. The evelope becomes optical thin and molecules absorb photons from central star then radiate by fluorescence process. In the spectrum apear the molecular emission bands resemble to the cometary spectra. A well known object in which spectrum C2 Swan bands in emission is CRL 2688(Crampton et al. 1975). We found another object, IRAS14429-4539, in the sample showing similar features.

We are engaged in a program of optical spectroscopy of proto–planetary nebulae (PPN). The objects were initially selected from the IRAS database on the basis of their strong infrared excesses, indicating dust temperature of 150–250 K. Spectra have been obtrained at medium resolution for the purpose of obtaining the spectral type, luminosity class, and to search for chemical peculiarities resulting from the post–AGB nature of the objects. We found our PPN candidates to typically display F–G supergiant spectra.