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Astron. Astrophys. 342, 524-530 (1999)


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HR 1817: the EUV properties of an active F dwarf

M. Mathioudakis 1 and D.J. Mullan 2

1 Department of Pure and Applied Physics, Queens University of Belfast, Belfast BT7 1NN, Ireland (e-mail: M.Mathioudakis@queens-belfast.ac.uk)
2 Bartol Research Institute, University of Delaware, Newark, DE 19716, USA (e-mail: mullan@bartol.udel.edu)

Received 19 June 1998 / Accepted 28 September 1998

Abstract

We examine the coronal properties of the active F dwarf HR 1817. Photometric observations with EUVE show that the source is in a near-continuous state of flare-like activity. Using IUE and EUVE spectroscopic observations we construct the emission measure distribution in the 104-[FORMULA] K temperature range. These observations reveal a hot corona and activity levels similar to those of RS CVn binaries. Based on Fe XXI line ratios we derive an upper limit of [FORMULA] cm-3 for the coronal density of HR 1817 and a magnetic field strength of B[FORMULA] 160 Gauss. A comparison of the EUVE spectroscopic observations with synthetic spectra derived from ASCA and ROSAT fits, shows that optimal agreement is obtained for fits with sub-solar metal abundances. The reduced metal abundances increase the radiative losses significantly at temperatures above [FORMULA] K where Fe is no longer the dominant radiative cooling agent.

Key words: X-rays: stars – ultraviolet: stars – stars: late-type – stars: individual: HR 1817 – stars: coronae – stars: activity

Send offprint requests to: M. Mathioudakis

© European Southern Observatory (ESO) 1999

Online publication: February 22, 1999

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