Extending the radio spectrum of magnetic chemically peculiar stars to the mm range^*

¹ INAF - Osservatorio Astrofisico di Catania, Via S. Sofia n. 78, 95123 Catania, Italy e-mail: fleone@ct.astro.it
² Istituto di Radioastronomia del C.N.R., sezione di Noto, PO Box 141, 96017 Noto, Italy e-mail: [c.trigilio;g.umana]@ira.cnr.it
³ Institut de Radioastronomie Millimètrique, 300 rue de la Piscine, 38406 Saint Martin d'Hères, France e-mail: neri@iram.fr

Received: 24 October 2003
Accepted: 14 May 2004

Abstract

Magnetic chemically peculiar (MCP) stars can present radio emission at centimetre wavelengths. The steep decrement of the dominant dipolar component of the photospheric magnetic field results in each radio frequency being mainly emitted in a well localised shell of the circumstellar region. To explore the most internal regions of the magnetosphere, observations of a sample of eleven MCP stars known to be radio sources in the  GHz range were carried out at 87.7 GHz with the IRAM interferometer. Millimeter emission, with a flux density at about  the sensitivity limit of our observations, was detected towards two of the stars: HD 35298 and HD 124224. Combining our mm-observations with previous cm-observations, it appears that MCP stars with a relatively weak magnetic field present a radio spectrum that increases with frequency up to 22.5 GHz and then decreases towards the mm range. In presence of strong fields, the radio spectrum is always decreasing with frequency. A comparison of the observed cm-mm spectrum of HD 124224 with results of numerical simulations of the gyrosynchrotron emission suggests that circumstellar regions emitting in the mm-range cannot present magnetic fields larger than  kG.