Optical Identification of 3c 48, 3c 196, and 3c 286 with Stellar Objects.

Abstract

Radio positions of the three sources have been determined with the two 90-foot antennas working as an interferometer with an r.m.s. accuracy in both co-ordinates better than 10 seconds of arc. Direct photographs show that a starlike object exists within the error rectangle at each of the source positions. Exceedingly faint wisps of nebulosity are associated with the stars in 3C 48 and 3C 196. The observations are incomplete for 3C 286 in this regard. Photoelectric photometry of the stars shows each to have quite peculiar color indices, most closely resembling the colors of old novae or possibly white dwarfs, but we are not suggesting identification with these types of stars. Photometry of 3C 48 through 13 months shows the star to be variable by at least AV = 0W4. The radio flux appears to be constant. Optical spectra for 3C 48 show several very broad emission features, the most intense at X 3832 being unidentified. Spectra by Schmidt of 3C 196 and 3C 286 show other unusual features. The radio structure of the three radio stars is similar in that each has an unresolved core of <1" diameter. However, 3C 196 and 3C 286 show halos of 12" and 20", respectively, while no radio halo has been detected for 3C 48. it is shown that the radiant flux in the optical region can be computed from the radio-flux data and the theory of synchrotron radiation for 3C 48 and 3C 196, but not for 3C 286. This, together with other arguments, suggests that the optical as well as the radio flux could be due to the synchrotron mechanism, but the arguments are not conclusive. We have used the assumption of minimum total energy to compute the energy in relativistic particles and magnetic field required by the synchrotron mechanism to explain the observed emission. The magnetic field in each of the core components is near 0.1 gauss and depends mainly on the assumed angular size of the emitting region. The total energy in the core components is near 10 ergs. The rate of radiation is such that the energy in relativistic electrons must be replaced in a time scale of a few years if the value of the magnetic field determined in this way is correct. These calculations are based on a distance of 100pcs. The frequency of occurrence of radio stars is examined, and they are estimated to comprise from 5 to 10 per cent of sources in the 3C catalogue. The percentage is likely to be less for fainter sources. Rough limits have been estimated for the mean distances of these radio stars. A mean distance of approximately 100 pc is suggested if these objects are in the Galaxy. Evidence obtained since this paper was written suggests that 3C 48 has a large redshift of z = 0.3675 (Greenstein and Matthews 1963); thus these objects may be associated with a distant galaxy. The absolute magnitude of the starlike objects is M = -24.3, which is much brighter than any other known galaxy. As a radio source, 3C 48 is not very different from other identified sources. The emitted flux is the same as 3C 295 and Cygnus A, but the emitting volume is much less. The faint nebulosity does not resemble a galaxy, and it also is brighter than a normal galaxy. if caused by an explosion in the past and expanding at the velocity of light, its age would be > 1.8 X 10 years. The synchrotron lifetime calculated in the normal manner is much shorter than that inferred from the extent of the faint nebulosity. Thus either the magnetic field must be much lower than calculated, or high-energy electrons must be supplied continuously.