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Published February 1, 2010 | Published
Journal Article Open

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


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 mas 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 (~0.25 M_☉) companion around a bright (V = 4.0 mag), nearby (d= 25 pc) star highlights a region of binary star parameter space that to date has not been fully probed.

Additional Information

© 2010 American Astronomical Society. Print publication: Issue 2 (2010 February 1); received 2009 November 11; accepted for publication 2009 December 10; published 2010 January 6. The authors wish to express our appreciation toward Jean Mueller, Kajsa Peffer, Karl Dunscombe, and the Mountain Crew at Palomar Observatory. We thank Adam Burgasser and Emily Rice for sharing comparison spectra to aid in our classification of the companion star. We are also indebted to our anonymous referee and the Scientific Editor of the Journal, Steven Kawaler, for expediting our review process. Project 1640 is funded by National Science Foundation grants AST-0520822, AST-0804417, and AST-0908484. A portion of the research in this paper was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration (NASA) and was funded by Internal Research and Technology Development funds. In addition, part of this work was performed under a contract with the California Institute of Technology (Caltech) funded by NASA through the Sagan Fellowship Program. The members of the Project 1640 team are also grateful for support from the Cordelia Corporation, Hilary and Ethel Lipsitz, the Vincent Astor Fund, Judy Vale, Andrew Goodwin, and an anonymous donor. Facilities: Hale (PALAO, Project 1640)

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