Paper detail

Determination of the position angle of stellar spin axes

Measuring the stellar position angle provides valuable information on binary stellar formation or stellar spin axis evolution. We aim to develop a method for determining the absolute stellar position angle using spectro-astrometric analysis of high resolution long-slit spectra. The method has been designed in particular for slowly rotating stars. We investigate its applicability to existing dispersive long-slit spectrographs, identified here by their plate scale, and the size of the resulting stellar sample. The stellar rotation induces a tilt in the stellar lines whose angle depends on the stellar position angle and the orientation of the slit. We developed a rotation model to calculate and reproduce the effects of stellar rotation on unreduced high resolution stellar spectra. Then we retrieved the tilt amplitude using a spectro-astrometric extraction of the position of the photocentre of the spectrum. Finally we present two methods for analysing the position spectrum using either direct measurement of the tilt or a cross-correlation analysis. For stars with large apparent diameter and using a spectrograph with a small plate scale, we show that it is possible to determine the stellar position angle directly within 10deg with a signal-to-noise ratio of the order of 6. Under less favourable conditions, i.e. larger plate scale or smaller stellar diameter, the cross-correlation method yields comparable results. We show that with the currently existing instruments, it is possible to determine the stellar position angle of at least 50 stars precisely, mostly K-type giants with apparent diameter down to 5 milliarcseconds. If we consider errors of around 10deg still acceptable, we may include stars with apparent diameter down to 2 mas in the sample that then comprises also some main sequence stars.