IUE observations of HD 36705.

Abstract

HD 36705 is one of the most rapidly rotating (0.514-day) possibly single late-type (K0/2) stars known. The IUE was used to obtain ten LWR high resolution and five SWP low-resolution spectra in different phases of the rotational period. The FES light curve indicates a rather uniform coverage by spots during the observations. The Mg II emission lines have identical shapes for all phases but their strengths seem to anticorrelate with brightness variations. The phase-related variability of the transition-region lines is at a level only slightly above the measurement accuracy. The mean relative fluxes f(line)/f(bol) for the Mg II and integrated (Nv, Si IV, C IV) transition-region lines are 0.00015 and 0.00003, respectively. These relative fluxes are similar to those observed in other active stars, but different from FK Com or more typical giants crossing the Hertzsprung gap. The relative flux for the He II 1640 A line is very large (0.000011) confirming its possible relation to the coronal X-ray flux in HD 36705. It is impossible to assign the star either to the RS CVn or BY Dra type on the basis of the IUE spectrum. The available data are discussed with the aim of determining the luminosity and distance of HD 36705, but the results are indeterminate with acceptable absolute magnitude in a considerable range between 3 and 5 M(V) i.e., about 1-3 mag above the MS. The evolutionary status of the star is unknown at present.