Circumstellar chemistry has been confined largely to analysis of a few dozen molecules in the envelope of one carbon-rich star, IRC+10216. A new generation of large millimeter wavelength telescopes promises to broaden the data base to include many other stars and additional molecules. Carbon-monoxide emission has already been detected from approximately 130 stars and many of these are prime candidates for chemical studies. The detection of HCN emission in a few oxygen-rich stars was quite unexpected and indicates that nonequilibrium chemical processes are important in at least some circumstellar envelopes. New millimeter wavelength interferometers can measure the spatial distribution of various molecules for comparison with predictions of models for photodissociation, freeze-out on grains, self-shielding, and nonspherical outflow.

The different chemical processes responsible for the observed molecular abundances in massive circumstellar shells are examined: freezing-out of chemical equilibrium, radical and ion reactions, grain and shock processes, photochemistry, etc…

Presumably due to a cosmic accident, circumstellar masers are almost exclusively found in oxygen rich stars with [0/C] >1. The exception here is the SiS maser found towards IRC 10216 (Henkel et al. (1985)) but I will ignore this in what follows and concentrate this review upon the circumstellar shells where OH, H2O, and SiO masers are found in profusion. Such objects are thought to be on the asymptotic giant branch (AGB) of the Herzsprung-Russell diagram and show evidence for rapid mass loss. The derived mass loss rates vary from approximately 10−7 solar masses per year for the optically well studied Mira variables in the solar neighbourhood (Distances <1 kpc) to as high as 10−4 solar masses per year for the optically invisible OH-IR stars and for some supergiants. of these objects, the supergiants appear to be more luminous than the rest (105 L rather than 104 L) and their central stars have masses of order 10 M⊙ as compared to most Mira variables whose mass is probably in the range 1–2 M⊙. Most OH-IR stars are likely to be a late stage in the evolution of solar mass type objects where the mass loss rate suddenly increases. As their name indicates, they are characterised by their strong OH-maser (1612 MHz) emission as well as by their radiation at far infrared wavelengths (20–50 μ). They may well be on the way to becoming planetary nebulae (see Pottasch, 1984 for an account of their probable evolution).

The physical and chemical properties of the circumstellar envelopes of evolved stars are strongly affected by the interstellar radiation field. Other sources of UV radiation should be similarly effective, and some examples are nearby stars (including companions), chromospheres, and the central stars of planetary nebulae. We consider the particular case of Alpha Ori, which has a chromosphere and an extended CSE with a small dust to gas ratio. Its properties are dominated by the chromospheric and interstellar radiation fields. The most common species are neutral atoms and first ions, and the electron fraction is high throughout the entire CSE, i.e. at least 10−4. The abundances of neutrals peak in the outer CSE close to where the chromospheric and interstellar radiation fields are equal. An important application is KI, whose density has been measured by scattering. The theory predicts that the slope of the KI density should change from about −1.5 to −3.5 in the outer envelope, the exact values being determined by the temperature distribution. The mass loss rate implied by the KI density is of the order of 4×10−6 M⊙ yr−1.

We have used the Cassegrain-focus Fourier Transform Spectrometer of the Canada-France-Hawaii Telescope to record high-resolution (0.03 cm−1), high signal-to-noise ratio spectra of the extreme carbon stars IRC+10°216 and CIT6 in the 2850–3100 cm−1 region. Upper limits were obtained for the column densities of silicon nitride (2-0 band of the A-X system), ethylene (ν11 fundamental band at ν0 = 2988.7 cm−1), and ethane (ν7 fundamental band at ν0 = 2985.4 cm−1).

The prominent OI(3s 5S°−3p 5P) triplet near 7774Å has been observed in normal and peculiar B and A type stars, and compared with synthetic non LTE profiles.

Wolf-Rayet stars are rare and their basic properties such as mass, effective temperature, and luminosity are similar to those of 09 to B1 stars. the spectra of Wolf-Rayet stars from about 10 μm to 1000 Å consist of rather broad emission lines on a continuous spectrum. the continuous spectrum corresponds to effective temperatures in the range from 25000 to 30000 K (Underhill 1983b). A few shortward displaced absorption troughs appear and a few emission lines are flat topped. There is a strong infrared excess attributed to thermal bremsstrahlung (Cohen, Barlow, and Kuhi 1975), and there is rather strong 6 cm radiation (a fraction of a mJy from stars at distances of the order of 1500 pc), see for instance, Bieging, Abbott, and Churchwell (1982). the radio radiation can be attributed to thermal bremsstrahlung and to magnetic bremsstrahlung (cyclotron radiation), see Underhill (1983a). X rays have been measured from a few Wolf-Rayet stars (Sanders, et al. 1985).

The construction of model atmospheres is an eight-fold pathway. These pathways are described, with special reference to the chemistry of cool stars. A brief review of the present state of the art is presented, highlighting the outstanding problems. A few salient results are mentioned.

The presence of molecules in cool stellar atmospheres can dominate the opacity. Some calculations have been done with a carbon-rich mixture of C/0=2, and the molecules C2, CN, HCN and C2H2 are major sources of opacity due to their large abundances. New calculations with HCN and C2H2 show they can be important.

It is found that the low excitation lines of the first overtone band of CO in the infrared spectra of normal red giant and supergiant stars include excess absorption that cannot be explained by the photospheric absorption alone. the excess absorption is shown to be due to unresolved circumstellar absorption originating from hitherto unrecognized quasi-static molecular envelope around normal red giant and supergiant stars. As contrasted to the previously known expanding circumstellar envelope recognized by Doppler-shifted absorption lines, the newly found static envelope has the following characteristics: (1) excitation temperature determined from CO lines is between 500 and 1000K, indicating that the envelope may be located at a few stellar radii above the photosphere, (2) turbulent velocity may be as high as 5km/s, (3) CO column density increases from 10+19/cm2 at early M giant to 10+20/cm2 at the latest non-Mira M-giant, and (4) molecular envelope and stellar photosphere show slight relative motion in general. Some implications of such a quasi-static molecular envelope on circumstellar chemistry as well as on stellar mass-loss are discussed.

Ultraviolet and visible spectroscopy of comets has identified a large number of species in the coma, most of which appear to be the photodissociation and photoionization products of the “parent” molecules evaporated directly from the cometary nucleus. Analyses of cometary spectra support the icy conglomerate model of the nucleus with H2O as the dominant ice species. Two molecules detected in the ultraviolet, CO and S2, are of particular interest to the study of the cosmogonic evolution of cometary grains. CO appears to be a highly variable constituent from comet to comet, while S2, first observed in comet IRAS-Araki-Alcock in 1983, is found in no other celestial source. Both of these molecules appear to be parent molecules.

There is now evidence that at least some cometary nuclei are dark and red. Cometary ices prepared from combinations of CH4 with H2O and sometimes NH3 were irradiated at 77 K by corona discharge. CH4 - containing ice reddened and darkened at a dose ~1011 erg cm−2 over a period of ~1 hour. Upon evaporation of the now yellowish, irradiated ice, a slightly yellowish colored solid film remains on the walls of the container at room temperature. Transmission measurements of this organic film (called cometary tholin) were made from 0.2 μm to 50 μm wavelength. Strong UV absorption is seen from 0.45 μm to 0.2 μm. Above 0.45 μm, the spectrum remains flat to ~1.3 μm in the near infrared, except for a very small feature near 1.15 μm. A medium sized feature appears centered at 1.4 μm with shoulders at both sides and a nearby weaker feature at 1.52 μm. A strong feature appears at 1.9 μm accompanied by a smaller feature at 1.78 μm. in the region 2.5 μm to 50 μm, the infrared spectrum was taken by dispersing the film in a CsI matrix. Bands are found at 2.92(M), 3.36(S), 3.40(S), 3.46(M), 3.48(M), 5.75(M), 5.99(S), 6.21(M), 6.83(M), 7.30(S), 7.81(W), 8.89(M), 9.26(M), 20.00(W), 22.22(W), and 28.57(W) micrometers, suggesting complex organics including alkane, alkene, aldehyde, and carboxylic acid functional groups. These results are also relevant to UV and cosmic ray processing of interstellar grains, and to icy bodies in the outer solar system.

In the prevailing icy conglomerate model of the cometary nucleus, the outer surface is processed by galactic cosmic rays in the Oort cloud. Most ices composed of CH4, along with H2O and/or NH3, studied in our laboratory redden and darken upon charged particle irradiation from a high frequency corona discharge at 77 K. Spectral reflectance relative to a BaSO4 standard in the wavelength interval 0.39 μm to 0.70 μm was measured for incidence angle, i = 0° and emission angle, ε = −4°. CH4-containing ice at 77 K was modified at a dose ~1011 erg cm−2 over a period of ~1 hour. Progressive darkening followed continued irradiation. The ratio of the red to blue reflectance [R(λ = 0.70 μm)/R(λ = 0.40 μm)] changed from 0.9 to ~1.4. Using techniques of ellipsometric polarimetry, we are investigating the detailed optical properties (complex refractive index) of these ices before and after irradiation. These results may then be compared with the properties of cometary coma particles predicted by recent light scattering models to determine which classes of laboratory-produced materials best match the population of dark, reddish particles which seem to be prevalent in some cometary comae.

We report here the detection of the J 1-0 rotational line at 88.6 GHz of hydrogen cyanide in comet Halley. Six observational runs were made in the Nov. 19-Dec. 3 1985 period with the IRAM 30-m millimetre radio telescope at Pico Veleta (Spain), when the comet was at rh ~ 1.56 AU from the Sun and Δ ~ 0.63 AU from the Earth.

Sublimation (vaporization) of the icy component of a cometary nucleus determines the initial composition of the coma gas as it streams outward and escapes. Photolytic reactions in the inner coma, escape of fast, light species such as atomic and molecular hydrogen, and solar wind interaction in the outer coma alter the chemical composition and the physical nature of the coma gas. Models that describe these interactions must include (1) chemical kinetics, (2) coma energy balance, (3) multifluid flow for the rapidly escaping light components, the heavier bulk fluid, and the plasma with separate temperatures for electrons and the remainder of the gas, (4) transition from a collision dominated inner region to free molecular flow of neutrals in the outer region, (5) pickup of cometary ions by the solar wind, (6) counter and cross streaming of neutrals with respect to the plasma which outside of the contact surface also contains solar wind ions, and (7) magnetic fields carried by the solar wind. Progress on such models is described and results including velocity, temperature, and number density profiles for important chemical species are presented and compared with observations.

It is described a method for evaluating the low energy cosmic ray flux outside the heliosphere. It is based on the chemical modifications induced in cometary nuclei by impinging ions and on the release of synthesized chemical species by comets entering for the first time into the inner solar system.

The Cometary Spectra in the visible and ultraviolet regions are dominated by the strong emission lines of various molecules. These can be explained in terms of the resonance fluorescence process in the solar radiation.

Excitations of cometary NH and S2 molecules are discussed. Photodissociation and dissociative electron recombination processes may be sources of meta-stable nitrogen (2Po, 2Do) atoms in comets.

It is belived that the unprocessed material of the solar nebula may be preserved in comets. Thus the data concerning the chemical composition and the abundance of stable isotopes in these primitive bodies are of some importance in cosmological and cosmogonical context. Although the isotopic abundance in the small bodies of the Solar System are poorly known, owing to the forthcoming Halley fly-by missions, the discussion as whether or not the comets have preserved the cosmic isotopic ratio in their nuclei became more relevant. From this point of view expected data of the cosmologically and cosmogonically significant isotopic ratios of stable isotopes of the light elements in comets are discussed.

Recent analyses show that, although most meteorites are collisional debris of asteroids, three meteorites collected on the Antarctic ice sheet were projected to Earth from the highlands of the Moon, and eight meteorites have chemical and isotopic compositions suggestive of derivation from Mars. Although meteorites are primarily of interest to planetary scientists for the abundance of clues they hold to the materials and processes that formed the Solar system, they have begun to engage the attention of astrochemists because of isotopic and mineralogical indications that they contain interstellar components. Although each individual observation to this effect is inconclusive, the body of evidence is becoming ever more persuasive. This paper reviews the main classes of meteorites and their probable sources, with special emphasis on components that appear to be exotic to the Solar system.