Search for Synchrotron Emission from Secondary Leptons in Dense Cold Starless Cores

We report radio continuum observations with the Australia Telescope Compact Array of two molecular clouds. The impetus for these observations is a search for synchrotron radiation by cosmic-ray secondary electrons/positrons in a region of enhanced density and possibly high magnetic field. We present modelling which shows that there should be an appreciable flux of synchrotron above the more diffuse, galactic synchrotron background. The starless core G333.125-0.562 and infrared source IRAS15596-5301 were observed at 1384 and 2368 MHz. For G333.125-0.562, we find no significant levels of radio emission from this source at either frequency, nor any appreciable polarisation: we place an upper limit on the radio continuum flux from this source of 0.5 mJy beam(-1) at both 1384 and 2368 MHz. Due to the higher than expected flux density limits, we also obtained archival ATCA data at 8640 MHz for this cloud and place an upper limit on the flux density of 50 mu Jy beam(-1). Assuming the cosmic ray spectrum is similar to that near the Sun and given the cloud's molecular density and mass, we place an upper limit on the magnetic field of 500 mu G. IRAS 15596-5301, with an RMS of 50 mu Jy beam(-1) at 1384 MHz, shows an HII region consistent with optically thin free-free emission already detected at 4800 MHz. We use the same prescription as G333 to constrain the magnetic field from this cloud to be less than 500 mu G. We find that these values are not inconsistent with the view that magnetic field values scale with the average density of the molecular cloud.