Cool Customers in the Stellar Graveyard. II. Limits to Substellar Objects around Nearby DAZ White Dwarfs

Abstract

Results from a concerted Hubble Space Telescope (HST) survey of nearby white dwarfs for substellar objects is presented. A total of 7 DAZ white dwarfs with distances of $<$ 50 pc had high contrast and high spatial resolution NICMOS coronagraphic images taken to search for candidate substellar objects at separations $\ltorder$10\arcsec away. Limits to unresolved companions are derived through analysis of 2MASS photometry of the white dwarfs compared to expected fluxes based on the WDs effective temperature, distance, and gravity. From our survey with HST we find several candidate companions, which have been or will be followed up with second epoch observations. We find that for four of the white dwarfs we are sensitive to planetary companions $\gtorder$10 M$_{Jup}$, and sensitive to companions $>$ 18 M$_{Jup}$ for all of the targets. The lack of significant near infrared excesses for our targets limits any kind of unresolved companions present to be substellar. In light of these results we make several comments on the possibility of determining the origin of white dwarfs with metals in their atmospheres.