H. C. Bhatt

Interstellar extinction and polarization by spheroidal dust grains

Using the recently available exact computations of the scattering efficiencies of spheroidal particles numerical calculations of the extinction and polarization curves have been made for a distribution of particle sizes, shapes and orientations. The results are presented and compared with the observed interstellar extinction and polarization. Possible models for interstellar dust with nonspherical grains have been discussed.

Search for the near infrared counterparts to the IRAS sources

As part of a program to search for the near infrared counterparts to the far infrared sources discovered by the Infrared Astronomical Satellite (IRAS) we have scanned two areas including the IRAS error boxes for the sources IRAS 0453+444 and IRAS 1717-087 at 2.2 μm down to the flux density limit of ∼1 Jy. Results on the IRAS sources and the detection of two new bright near infrared sources are reported and discussed.

Segregation of dust grains in dark clouds

Gravitational settling of dust grains in dark clouds has been considered. It has been shown that such a process gives rise to a modification of the grain size distribution. Starting with a simple model of uniform spherical cloud and normal interstellar grain size distribution for the dust we derive expressions for the modified grain size distribution function, average grain size and extinction as functions of distance from the clouds center and the age of the cloud. The mean grain size increases towards the center of the cloud as does the extinction. Results of the numerical evaluation of these quantities have been discussed with their implications for the observations of anomalous reddening and polarization within dark clouds and Bok globules.

Near-infrared observations of the peculiar binary system epsilon Aurigae

Near-infrared photometric and low-resolution spectroscopic observations of ε Aurigae at two phases during the current eclipse are presented. The eclipse depths are found to be wavelength-independent in the infrared right up to 2.5 μm. There is no infrared excess at wavelengths shorter than 2.5 μm. The light absorbing particles in the eclipsing body must be larger than 10 μm in size.

The pulse-period distribution of binary X-ray pulsars

The pulse-period distribution of binary X-ray pulsars has been considered. A gap in this distribution, in the period rangeP∼10 s toP∼100s has been explained in terms of the character of mass transfer in the X-ray binary systems. It is shown that this gap arises because the rotating magnetised neutron stars in these systems are slowed down by accretion torques, either toP≲10 s when the mass transfer is by means of Roche-lobe overflow in low mass binaries, or toP≳100 s by stellar winds in massive binaries. The gap is maintained as the slow pulsars (P>100 s) in their spin-up phase cross the gap in a time short compared to their life-time, because of the increase in mass transfer with the evolution of the normal star.