Volume 3, Issue 2
June 1982, pages 93-216
pp 93-99 June 1982
The light curves of the totally eclipsing system TT Hya inUBV colours observed by Kulkarni and Abhyankar during 1973–77 have been analysed by Kopal’s frequency domain method with slight modification. We find ys (primary) = 0.104 ± 0.005, yg (secondary) = 0.215 ± 0.008 and i = 89‡ ± 1‡. The value of yg obtained in this study is smaller than that determined earlier by Kulkarni and Abhyankar by the method of Russell and Merrill; this confirms the undersized nature of the secondary component.The ultraviolet colour excess of the secondary is also confirmed.
pp 101-105 June 1982
Photoelectric radial-velocity measurements show that HD 120803 is a spectroscopic binary with a rather eccentric orbit and a period of 700 days. Early DDO photographic observations, published individually here for the first time, fit the orbit well.
pp 107-109 June 1982
Photoelectric radial-velocity measurements, which, in 1971 showed HD 117064 to be a spectroscopic binary, have been continued and now permit the derivation of an eccentric orbit with a period of 6.08 years.
pp 111-123 June 1982
Given the potential, the equation of continuity and Poisson’s equation are solved for the variation perpendicular to the Galactic plane for a thin galaxy in a steady state. Simple expressions are obtained for the joint density function for the mass density and velocity, and for the distribution function for the velocity and its moments.
These results are applied using a potential suggested by Woolley and Stewart (1967) and Whitley (1977), which is Camm’s potential due to an isothermal gas stratified in parallel layers, plus the potential due to the Galactic centre. The resulting velocity distribution is quite close to the normal distribution usually assumed and cannot be distinguished from it by the data. The mass density distribution fits the log (relative density) curves better than the Camm solution, especially at large distances from the Galactic plane.
A formu1a, valid under conditions more genera1 than usual, is found for the total mass density in the neighbourhood of the Sun.
pp 125-144 June 1982
The spiral arms of disk galaxies are very sensitive to various morphological properties, such as, the gas content, the disk-to-bulge ratioetc. Here, the stability of self-gravitating annular disks surrounding the central rigid bulge component has been studied in order to explain the transition from the tight spiral arms in Sa galaxies to rather open patterns in Sc galaxies as the central amorphous component diminishes. Smooth spiral patterns are found associated with the dominant (or the fastest growing) modes of the system. When the disk-to-bulge mass ratio is small, a tight pattern results restricted to the inner regions of the disk. This pattern opens up and occupies larger disk areas as the disk component becomes comparable to the bulge. It is found here that the ‘explosive’ instabilities of the global density waves do not occur in the presence of a massive bulge. The growth-rates of the eigen-modes decrease as the disk-to-bulge mass ratio decreases. It is also found that unstable modes of the annular disk can be suppressed by increasing the thermal pressure sufficiently.
pp 145-150 June 1982
By using the Boller and Chivens spectrograph with a moderate dispersion (59 å mm-1) in the red spectral region, we obtained 65 spectra covering the whole surface of the planetary nebula NGC 2440. Intensities of Hα, [N II] λλ 6548–6584 and [S II] λλ 6717–6731 lines are derived using the IDS system available at the ESO in La Silva (Chile). The nebula is known to be a nitrogen-rich nebula (Peimbert 1978) surrounded by secondary structures (Minkowski 1964). The unusual high value of the [N II]/Hα in the central core (~ 30) is certainly due to the nitrogen overabundance occurring in that part of the nebula. Its variations from scale ionization structure (Capriotti, Cromwell and Williams 1971). The observations show clearly an outward increase of both [NII]/Hα andI(6717)/I(6713) ratios.
pp 151-159 June 1982
The time structure of solar radio decametre Type III bursts occurring during the periods of enhanced emission is investigated. It is found that the time profiles can take a variety of forms of which three distinct types are the following: (1) profiles where the intensity rises to a small but steady value before the onset of the main burst, (2) the intensity of the main burst reduces to a finite level and remains steady before it decays to the base level, (3) the steady state is present during the rise as well as the decay phase of the main burst.
It is shown that these profiles are not due to random superposition of bursts with varying amplitudes. They are also probably not manifestations of fundamental-harmonic pairs. Some of the observed time profiles can be due to superposition ot bursts caused by ordered electron beams ejected with a constant time delay at the base of the corona.
pp 161-171 June 1982
We have reinvestigated the reported tendency for the extended radio structures associated with bright elliptical galaxies to be oriented preferentially along the optical minor axes. It is found that such a tendency exists only for those galaxies in which the compact radio cores coincident with their nuclei are quite prominent. If the galaxies are divided into two groups according to whether their cores account for less than or greater than 10 per cent of the total flux density at 2.7 GHz, the angle Φ (between the radio axis and the optical minor axis) appears to be uniformly distributed between 0‡ and 90‡ for the former, but is nearly always < 30‡ for the latter group. One possible explanation is that the radio emission from compact cores suffers thermal absorption by ionized gas that is distributed differently in the two groups.
pp 173-187 June 1982
VLA observations at 6 cm have been made of 16 distant luminous 3C sources that appeared to be unresolved or slightly resolved in Cambridge maps but which were known from VLBI observations to contain significant fine structure on the scale of about 1 kpc in their lobes. The general morphology of these sources is very similar to that of their nearby powerful counterparts; most of the lobes contain low brightness tails which are often directed from the hot spots towards the associated optical object. The hot spots are generally unresolved by the VLA observations; in 3C 254, 3C 268.4 and 3C 352, one of the lobes contains two hot spots.
pp 189-192 June 1982
CTA 21 has long been the outstanding example of a strong, compact radio source lacking an optical identification. We report the discovery at infrared wavelengths of the counterpart of CTA 21, and show that its spectral shape is unusual.
pp 193-206 June 1982
In this paper we have considered the structure of a thick perfect fluid disk of constant density rotating around a Schwarzschild black hole and its stability under axisymmetric perturbation. The inner edge of such disk cannot lie within 4m. The critical γc for neutral stability is found to be much less than 4/3 indicating that the disks are generally stable
pp 207-216 June 1982
We have observed the extended supernova remnants HB 9 (G 160.5 + 2.8) and IC 443 (G 189.1 + 2.9) at 34.5 MHz with a resolution of 26 arcmin × 40 arcmin. A map of HB 9 is presented. The integrated flux density of HB 9 at 34.5 MHz is 750 ± 150 Jy. The spectral index in the frequency range from 34.5 MHz to 2700 MHz is found to be constant (- 0.58 ± 0.06) without any spectral break such as was reported earlier by Willis (1973). There is no significant variation of the spectral index across the remnant. The integrated flux density of IC 443 at 34.5 MHz is 440 ± 88 Jy. The spectral index in the frequency range from 20 MHz to 10700 MHz is - 0.36 ± 0.04. The reduction in flux at very low frequencies (10 MHz) is attributable to free-free absorption in the interstellar medium and/or in the H II region S 249.
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