FIRST-PL: on-sky retrieval of Humu’s (Altair) spin axis using a photonic lantern on Subaru Telescope

Abstract

The FIRST-PL instrument on the Subaru Telescope enables a new observational capability: measuring stellar spin orientations for indirect constraints on exoplanet orbital geometry. We describe the instrument and demonstrate data reduction techniques to measure the spin orientation of Humu (Altair), whose apparent size is over five times smaller than the telescope diffraction limit. FIRST-PL uses a spectrally dispersed (R 3000, 630-790 nm) 19-port photonic lantern fed by SCExAO, a dual-stage cascading extreme adaptive optics system. Using wavelength dependent centroid shifts (i.e. spectro-astrometry) enabled by synchronous focal-plane images, we recover the spin orientation of Humu from 100 seconds of on-sky integration. The measured position angle is −61.8° ± 2.8° and inclination angle is 44.9° ± 4.1°, consistent with existing literature. This method will extend to a higher resolution instrument targeting slower rotating stars.