Mid-Infrared Observations of van Maanen 2: No Substellar Companion

Abstract

The results of a comprehensive infrared imaging search for the putative 0.06 M_solar astrometric companion to the 4.4 pc white dwarf van Mannen 2 are reported. Adaptive optics images acquired at 3.8 μm reveal a diffraction-limited core of 0.09" and no direct evidence of a secondary. Models predict that at 5 Gyr, a 50M_J brown dwarf would be only 1 mag fainter than van Maanen 2 at this wavelength, and the astrometric analysis suggested a separation of 0.2". In the case of a chance alignment along the line of sight, a 0.4 mag excess should be measured. An independent photometric observation at the same wavelength reveals no excess. In addition, there exist published Infrared Space Observatory observations of van Maanen 2 at 6.8 and 15.0 μm that are consistent with the photospheric flux of a 6750 K white dwarf. If recent brown dwarf models are correct, there is no substellar companion with T_eff>~500 K.