An HST/WFPC2 Search for Low-Mass Companions in the Alpha Centauri System

T.J. Royle, J. Caldwell, B.E. Hesman (York University), D.J. Schroeder (Beloit College), D.A. Golimowski, H.C. Ford, A. Uomoto, R.A. Brukardt (Johns Hopkins University), C. Burrows (STScI)

The search for low-mass companions most reasonably begins with the nearest stars. As part of an HST/GTO program conceived and developed by W.G.Fastie (JHU) and D.J.Schroeder, we have been using the Planetary Camera (WFPC2) to search the triple system Alpha Centauri for such companions.

This search has utilized seventeen visits from 1995 to 1997, seven filters and various exposure times. Diffraction, column saturation from the PSF core and on-chip scatter limit the effective regions of the image. As we learned about these limitations, our program evolved from red and very near IR filters (547,555,675,814 and 850 nm) to longer wavelengths (953 and 1042 nm). This change reflected a realization that the system would have a much higher probability of detecting a brown dwarf than a giant planet.

A series of IDL programs are used to search systematically the area surrounding the three components of Alpha Centauri. Candidate features are tracked through subsequent images and various filters. Numerous real objects have been detected through May 1997, but the very large proper motion of Alpha Centauri (3.7 arcsec/yr) permits us to classify all of them as background stars. At the time of writing, additional 1042 nm observations have been obtained but not reduced. We expect also to report on the newest data at the meeting.