Parallactic Motion for Companion Discovery: An M-Dwarf Orbiting Alcor

TL;DR

This paper introduces a novel astrometric technique using parallactic motion to confirm a low-mass companion orbiting Alcor, demonstrating a more rigorous method for companion discovery than traditional proper motion approaches.

Contribution

The study presents a new astrometric method leveraging parallactic motion for companion confirmation, improving accuracy over proper motion techniques.

Findings

Detected and characterized an M-dwarf companion to Alcor.

Achieved <=3 milliarcsecond separation measurement precision.

Confirmed common parallax and proper motion over 103 days.

Abstract

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