PARALLACTIC MOTION FOR COMPANION DISCOVERY: AN M-DWARF ORBITING ALCOR

Abstract

The A5V star Alcor has an M3-M4 dwarf companion, as evidenced by a novel\nastrometric technique. Imaging spectroscopy combined with adaptive optics\ncoronagraphy allowed for the detection and spectrophotometric characterization\nof the point source at a contrast of ~6 J- and H-band magnitudes and separation\nof 1" from the primary star. The use of an astrometric pupil plane grid allowed\nus to determine the projected separations between the companion and the\ncoronagraphically occulted primary star to <=3 milliarcsecond precision at two\nobservation epochs. Our measurements demonstrate common parallactic and proper\nmotion over the course of 103 days, significantly shorter than the period of\ntime needed for most companion confirmations through proper motion measurements\nalone. This common parallax method is potentially more rigorous than common\nproper motion, ensuring that the neighboring bodies lie at the same distance,\nrather than relying on the statistical improbability that two objects in close\nproximity to each other on the sky move in the same direction. The discovery of\na low-mass (~0.25M_sun) companion around a bright (V = 4.0), nearby (d = 25 pc)\nstar highlights a region of binary star parameter space that to date has not\nbeen fully probed.\n