THE ORION RADIO ZOO REVISITED - SOURCE VARIABILITY

The core of the Orion nebula has been monitored with the VLA in the A and B configurations at 5 and 15 GHz over a period of 7 months. A region of 0.2 pc centered on the Trapezium cluster was imaged about every two weeks at both frequencies. The goal was to study possible temporal variations of flux density and spectral index of a large number of very compact radio sources near the Trapezium cluster. The binary system theta1Ori A is the most variable source in the sample. Although some modulation of emission with orbital phase might be present, the main source of variability appears to be intrinsic and not related to orbital phase; possibly flaring activity on the surface of the T Tauri companion. All of the other radio sources are classified as thermal or nonthermal depending on their variability, spectral index, angular size, and optical identification. The thermal sources are clustered close to theta1Ori C (with the exception of BN and IRc2). We suggest that the thermal sources near theta1Ori C that are coincident with visible stars may be photoionized envelopes (due to UV radiation from theta1Ori C) of accretion disks around low mass stars. In the spirit of Garay (1987), we refer to these objects as EIDERS=Externally Ionized (accretion) Disks in the Environs of Radiation Sources which O'Dell et al. (1993) call PROPLYDS. The thermal radio sources that are not coincident with visible stars are probably neutral condensations whose outer envelopes are ionized by UV radiation from theta1Ori C. The latter are sometimes referred to as PIGs (partially ionized globules). The nonthermal sources are scattered over a larger area. Premain sequence flaring activity is proposed as the source of radio emission. For the brightest nonthermal sources a correlation between spectral index and flux density is found, which can be explained qualitatively by synchrotron emission in a magnetic loop.