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Through the combination of high-order Adaptive Optics and coronagraphy, wereport the discovery of a faint stellar companion to the A3V star zetaVirginis. This companion is ~7 magnitudes fainter than its host star in theH-band, and infrared imaging spanning 4.75 years over five epochs indicatesthis companion has common proper motion with its host star. Using evolutionarymodels, we estimate its mass to be 0.168+/-.016 solar masses, giving a massratio for this system q = 0.082. Assuming the two objects are coeval, this masssuggests a M4V-M7V spectral type for the companion, which is confirmed throughintegral field spectroscopic measurements. We see clear evidence for orbitalmotion from this companion and are able to constrain the semi-major axis to begreater than 24.9 AU, the period > 124$ yrs, and eccentricity > 0.16.Multiplicity studies of higher mass stars are relatively rare, and binarycompanions such as this one at the extreme low end of the mass ratiodistribution are useful additions to surveys incomplete at such a low massratio. Moreover, the frequency of binary companions can help to discriminatebetween binary formation scenarios that predict an abundance of low-masscompanions forming from the early fragmentation of a massive circumstellardisk. A system such as this may provide insight into the anomalous X-rayemission from A stars, hypothesized to be from unseen late-type stellarcompanions. Indeed, we calculate that the presence of this M-dwarf companioneasily accounts for the X-ray emission from this star detected by ROSAT.

Discovery and Characterization of a Faint Stellar Companion to the A3V Star Zeta Virginis