Showing papers in "Monthly Notices of the Royal Astronomical Society in 1973"

Near Infra-Red Magnitudes of 248 Early-Type Emission-Line Stars and Related Objects

Abstract: Nearly 250 early-type emission-line stars have been observed in the near infrared; approximately two thirds of these are found to exhibit excess radiation at these wavelengths. Two distinct mechanisms may contribute to the infra-red excesses: thermal radiation from circumstellar dust, and electron bremsstrahlung originating in a shell of ionized gas. It is not currently possible to distinguish between these two alternatives using infra-red photometry alone.

Models of extended radio sources

Abstract: Some of the basic requirements of viable models of double radio sources are examined. Some models require large mass densities in the most intense radio components and can now be excluded because they predict observable optical emission from the radio components which is not observed. The process of transporting energy from the central object to the radio components is studied, with a view to defining the conditions in which energy can be convcyed without prohibitive losses by adiabatic expansion. It appears likely that energy, and also mass, is brought into the radio components over a time scale comparable with the age of the source. A basic difficulty in accumulating mass in a radio component is outlined. (auth)

Neutral Hydrogen in Virgo Cluster Galaxies

Abstract: Twenty-one centimetre wavelength measurements, using the Mark I and Mark II radio telescopes, were made of the neutral hydrogen content of 25 galaxies in the Virgo cluster. Spectra of some of the brighter galaxies are shown, and the neuiral hydrogen parameters of each galaxy are given in tabular form. A comparison of the neutral hydrogen mass-to-luminosity ratio and the neutral hydrogen surface density for the Virgo cluster galaxies with field galaxies indicates that on average the cluster galaxies are deficient in neutral hydrogen by a factor of approximates 0.63. Calculations of galactic collision times suggest that collisions are likely to be responsible for the deficiency of gas in the Virgo cluster galaxies at the present epoch. (auth)

The Spotted Flare Stars BY Dra and CC Eri: A model for the spots and some Astrophysical Implications

Abstract: The photometric behavior of the flare stars BY Dra and CC Eri outside flare activity is discussed. Excellent agreement with the photometric lightcurves is possible if they are interpreted as due to the rotation of a star having a cool, dark spot on its surface. The orbital solution of the BY Dra system is presented. The system has a very high eccentricity and is presumably young. The inclination of the orbital plane places strong restrictions on the location of the spots. A model for the spotted areas is developed, incorporating four free parameters: longitude extent, latitude extent, limb darkening, and temperature. It is shown that the spots must be large, covering up to 20 per cent of the stellar hemisphere and with effective temperatures near 2000 deg K. In addition, the spots must extend to high latitudes (up to 60 deg ). The model effectively explains the general lack of (B-V) variations over the photometric cycle: the spot is black at these wavelengths relative to the stellar photosphere. Estimates of the magnetic field necessary for the storage of part or all of the energy dammed back by the spots are in the range of a few thousand gauss. An energymore » balance argument suggests that the essence of the flare phenomenon is the release through magnetic field collapse of energy temporarily stored in the field. (auth)« less

Solar Tidal Friction and Satellite Loss

Abstract: This paper treats the dynamical evolution of a satellite orbiting a primary that is being despun by solar tidal friction. The results are then applied to Mercury and Venus. It is concluded that a wide variety of satellites would not have survived the solar tidal braking of these planets. Hence, the absence of satellites around Mercury and Venus is not a compelling reason for thinking that this has always been the situation. Satellites are lost by orbital decay and impact with the primary. It is likely that Mercury would retain a record of such an event on its surface.

Circular Repolarization in Compact Radio Sources

Abstract: Generation of circular polarization in the process of transfer of radiation within a compact source, due to the presence of relativistic electrons, is discussed. The interpretation of the observations of polarization of compact radio sources in terms of this mechanism (called circular repolarization'') yields the values of 1 to 10 for the lower limit of the ratio of relativistic to cold electron densities. (auth)