A Radio and Optical Study of the Active Young F Star HR1817 (=HD35850)

This paper presents the results of a multiwavelength observational study of the active young F-type star HR1817. The star was monitored at 4.80 and 8.64GHz over 2×12h allocations with the Australia Telescope Compact Array on 8 and 9 December, 2000. The Anglo-Australian Telescope was used for simultaneous optical spectropolarimetry during a 2h period on 9 December. The low levels of observed radio emission have characteristics that are similar to those seen in other active stars, and a gyrosynchrotron mechanism is proposed to explain them; this is supported by the relatively low fractions of circular polarisation measured in HR1817. Comparison of the emissions from 4.80 and 8.64GHz shows a very strong cross-correlation peak, indicative of a common origin, although the shift of this peak indicates that 8.64GHz variations tend to precede those at 4.80GHz by, typically, ~20min. The optical spectropolarimetry reveals polarisation signals characteristic of surface magnetic fields, with profile changes indicating a complex dynamo-type magnetic topology is present on the star. This result makes HR1817 the star with the earliest spectral type on which dynamo magnetic fields have been detected directly up to now.