T. Chandrasekhar

A high speed near infrared photometer for lunar occultation studies

A recently developed near infrared high speed photometer intended for lunar oc-cultation studies is described. The primary scientific objective is to reach milli arc second levels of angular resolution so that circumstellar structure of the occulted sources can be resolved. Near infrared sky brightness close to the lunar limb is also studied. Angular diameter derived from the observed occultation of IRC +20169 is presented and system performance discussed.

The excitation mechanism of [Fe XIV] 5303 å line in the inner regions of solar corona

The line intensity of the green coronal line and the continuum intensity are derived from the filter and white light photographs of the solar corona obtained during the 1980 total solar eclipse. Ratio of the line to continuum intensity is plotted against the radial distancer(=R/R0,R0 is the solar radius), in various position angles. A simple model assuming an electron density dependence of the line and continuum intensities suggests a dominant collisional mechanism for the excitation of the line in the innermost regions (∼ 1.4R0). The measured line to continuum ratio tends to a constant value at different radial distances in different position angles. The constancy of the measured line to continuum ratio indicates significant radiative excitation beyond 1.4 R0, in some of the position angles.

Optical Interferometric Observations of a Transient Event of 1986 March 13 in the Coma of Comet Halley

During the recent apparition of Comet Halley in 1985-86 a transient ionic event in the form of a blob of H2 O+ emission was recorded in thecoma at ∼ 0h UT on 1986 March 13. Observations were carried out using a special IHW filter for H2 O+ emission at 7000 å/175 å, a 35 cm telescope, a Fabry-Perot interferometer and an image intensifier camera from Gurushikhar, Mt Abu. (24‡39’N,72‡ 47’E). A Fabry-Perot inter-ferogram in Ha taken a few minutes later at the same location reveals strong hydrogen emission (Hα) associated with the blob. The velocity field in the blob is structured with relative velocities upto ∼ 35km s−1. The event is interpreted as arising due to the sector boundary crossing of the interplanetary magnetic field by the comet

High-resolution studies of [O I] and NH2 line emissions at 6300 Å in Halley's Comet

The forbidden O I line and NH2 (0-8-0) emissions of Comet Halley at about 6300 A were observed during April 1986 using a Fabry-Perot spectrometer with a resolution of about 0.14 A. The forbidden O I line and NH2 rotational lines were spectrally well separated by the interferometer and a NH2/O I ratio of about 0.5 was found. The profile analysis shows the profiles of forbidden O I line and NH2 to be symmetric with linewidths of about 3.5-3 km/s. The cometary Balmer emission of hydrogen at 6563 A was barely detectable. An upper limit for Halpha based on the instrumental sensitivity yields a value of 30 Rayleighs.

Near infrared coronal line emission in Nova Herculis 1991

Near infrared coronal line emission at 1.98 ± 0.02Μm due to [Si VI] detected in the spectrum of Nova Herculis 1991 about 17 days after optical maximum is reported. The early appearance of coronal emission is yet another unusual feature of this fast nova in which early onset of dust formation processes and X-ray detection five days after outburst have already been reported. The coronal line observations reported here are consistent with X-ray detection and support a hot shocked circumstellar envelope at the periphery of the dust formation zone in the nova.

A high speed photometer in the optical region for lunar occultation studies

High speed photometry during the lunar occultation of a stellar system provides an effective means of achieving high angular resolution in one dimension at the sub arc second level which is well suited for resolving close binary projected separations in the range of 10–100 milliarc seconds. An optical fast photometer designed for such a purpose is described and some results from the initial observations taken with the system including the resolution of a projected separation of 55 milli arcsecond in one binary system are detailed.

Observations on near nucleus activity in comet Halley during January 1986

Abstract A large data base of near nucleus images of Comet Halley in white light, Kodak wratten filters and IHW filters was built up using an image intensifier camera and a 35 cm aperture f/11 telescope at Gurushikhar, Mt.Abu., India (24°39′N, 72°43′E, Alt.1700 m) during 1985–1986. The images taken on January 8.590 and 8.632 UT (R∼0.9 AU, Δ ∼ 1.29 AU) show distinct condensation region in the tail direction. From the relative position of the condensation region in two frames, its velocity was inferred as ∼ 37±3 km/s in the anti-sunward direction. The wide field photographic study shows this feature to be the precursor of the disconnection event observed on 10th Jan 1986. Based on the Near Nucleus photographs obtained by us and wide field photographs from other sources, it is shown that there is a strong correlation of nuclear activity with the tail disturbances observed on 31st Dec 1985, 8th and 10th Jan 1986.