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We have made an analysis of the observed equivalent widths of the lines of Lyman and Werner bands by Smith. The H2 column densities of 3×1019 to 1020 and the excitation temperature of 80 to 150 K satisfy the observations. This temperature refers to the kinetic temperature. We have also discussed the importance of getting the excitation temperature from lines of H2 and other heteronuclear diatomic molecules for the same star and from different regions of space.

We discuss the formation of molecular hydrogen on the surfaces of grains in a hot intercloud medium, by the process of chemisorption of hydrogen atoms on graphite grains. It is suggested that the molecular hydrogen observed towards stars with low reddening, may be located in the intercloud medium towards these stars. A comparison of the observed population distributions of H2 with the theoretical calculations shows that the observations are in the main consistent with a gas kinetic-temperature ~8000 K and densities about 0.1 to 1 cm-3, parameters which are appropriate to the intercloud phase of the two phase model of the interstellar medium.

We have shown that a correlation exists between the enhancement of abundance of heavy nuclei in cosmic rays and their depletion in interstellar space. A correlation also exists between the abundance enhancement and condensation temperature. We suggest that these correlations imply that much of the heavy nuclei content of cosmic rays may come from grains. A possible model is that grains in star-cloud complexes are accelerated to injection energies by radiation pressure in a supernova explosion and, subsequently, the grain debris is accelerated by shocks to cosmic ray energies.

We have computed two phase models of the interstellar medium, with cosmic rays and X-rays assumed to be the main ionizing agents, heating due to photoelectron ejection from the interstellar grains. We show that it is possible to have a hot and tenuous intercloud medium in pressure equilibrium with the interstellar clouds for a wide range of physical conditions, possibly existing in the interstellar space. The atomic and ionic line observations towards ? Sco are shown to be consistent with the origin of these lines in the intercloud medium for a range of values of the ionizing flux. It is suggested that the intercloud medium may be predominantly neutral, with ionization rates consistent with the limits imposed by molecular observations. The mean fractional ionization of the intercloud medium is ~1%.

Grains ejected from stars at velocities of ~ 107 cm s-1 and/or grains accelerated by the pressure of starlight in the intercloud medium to velocities in the range 2×106–107 cm s-1 are slowed to velocities of about 2×105 cm s-1 in a typical interstellar cloud. The interaction of fast grains with gas atoms as they are slowed in clouds could provide (a) the dominant heat source for interstellar clouds; (b) sites for molecule formation; and (c) a mechanism of providing a pressure balance between clouds and the intercloud medium.

A perusal of the observed column densities of different ions towards two stars, ?2 Vel and ? Pup, has indicated the presence of an ionization mechanism, the rate of which depends exponentially on the ionization potential. An examination of two such mechanisms, collisional ionization and ionization by UV-photon with flux depending exponentially on frequency has shown that the collisional ionization is possibly the dominant one in the intercloud medium towards ?2 Vel and ? Pup.

Populations of the rotational levels of vibrational and electronic ground state of interstellar HD and CO have been calculated. Ultraviolet pumping and molecular formation have been found to be very important in the case of HD but not so in CO, the population distribution of which is mainly governed either by 3 K or by collisional excitation. The population inversion has been found to occur in HD but not in CO, the excitation temperatures of which pass through maximum for large ultraviolet flux. The population inversion at large density does not occur in HD, probably due to neglect of multipole collision. The observation of HD and CO in the excited states may be possible for suitable values of density and ultraviolet flux.

The population of the rotational levels of vibrational and electronic ground state of interstellar H2 has been calculated; different possible excitation and de-excitation mechamisms including the ultraviolet pumping and formation and destruction of molecules have been considered. The values of temperature, density and radiation field towards ? Oph, ? Per and ? Pup have been obtained by comparing the theoretical distributions with the observations. The results indicate the presence of a high temperature low density diffuse cloud towards ? Pup and low temperature high density clouds towards ? Oph and ? Per.

It is shown that the 10–13 µ band observed in the spectra of some carbon stars could arise from simple hydrocarbon molecules, especially of the type CnH2n. These molecules could be present in circumstellar dust shells which are responsible for the 1000 K black-body excess observed for these stars, and the emission band is produced by collisional excitation.

Be stars show both emission and absorption profiles in the optical part of the spectrum, and these are found to vary with the time. Some of the shifts in profiles were earlier interpreted as Doppler shifts. However, some of the shifts of features in both emission and absorption line profiles may be artifacts due to superposition of an emission profile arising from an Hii region around a binary compact object, and a profile due to the Be star. This interpretation of shifts in line profiles has the advantage over the conventional Doppler-shift interpretation in so far as it does not need the oscillation of a large mass of circumstellar gas, and leads to the observed correlation between radial velocity (RV) variations and V/R variations.

The behaviour of the hardness ratio in low-mass X-ray binaries (LMXRB) indicates that at some value of mass accretion rate $$\dot M$$ , the X-ray intensity decreases with increasing $$\dot M$$ . It is suggested here that, at some $$\dot M$$ , wind from the accretion disk becomes appreciable and depletes the mass accretion on to the neutron star and thus reduces the X-ray intensity. This explains the behaviour of the hardness ratio in LMXRB. Possible implications of this on quasi-periodic oscillations are discussed.

Positively charged molecular ions, such as H2O+, which have been observed in cometary. comas, may be efficiently produced by the evaporation of positively charged clathrate grains of radii in the range ~10-6–10-5 cm. Such grains may be expelled from nuclei of comets, along with gaseous molecules. Grain charging occurs via interaction with solar ultraviolet photons and/or solar wind protons. Observational data on the total quantities as well as the distributions of H2O and H2O+ in cometary comas are shown to be in accord with detailed model calculations.

The specific angular momentum is found to vary with mass for earlier and later-type Main Sequence stars. Of various plausible causes, the difference in the interior density distribution of earlier and later-type stars is not sufficient enough to explain the difference either in angular momentum or in its gradient between earlier and later-type stars. The non-rigid rotation, however, may account for this difference in specific angular momentum as well as its gradient, if faster angular velocity in the interior for later-type and/or slower angular velocity for earlier-type stars than the surface value is allowed. A few other possibilities have also been briefly considered to understand this difference.