A search for faint companions to nearby stars using the Wide Field Planetary Camera 2

We have completed a direct-imaging search for faint companions (FCs) to 23 stars within 13 pc of the Sun using the Hubble Space Telescope Planetary Ca mera. The strategy of this search changed considerably from that reported i n 1996. To maximize the image contrast between potential FCs and a target s tar's point-spread function, we adopted the F1042M filter (lambda(c) simila r to 1.02 mu m, Delta lambda similar to 0.04 mu m) as the primary bandpass of our search. Although our sensitivity to FCs varied with the brightness o f and separation from our target stars, an ultimate 10 sigma detection limi t of m(1024) approximate to 18 within 17 " of the fainter targets was achie ved. As the end of the main sequence occurs at M-1024 approximate to 12, th is detection limit makes our search for FCs to nearby stars the most sensit ive yet published. Despite this great sensitivity, no previously undetected FCs were found. Our survey would have detected all stellar companions with in 17 " of our target stars, except for any lowest mass companions lying wi thin 0 ".5-1 " of the brightest (V < 1.5) targets. Applying recent models o f brown dwarf spectra and evolution, we find that our search was sensitive to young (0.5 Gyr), low-mass (less than 10 M-J) brown dwarf companions to t he fainter targets within 5 pc. A brown dwarf with mass 40 M-J and age 5 Gy r would have been detected at separations greater than 5 " from Gl 559A (al pha Centauri A). Our search was not sensitive to 1 Gyr-old brown dwarfs wit h masses less than or similar to 5 M-J, nor was it sensitive to 5 Gyr-old b rown dwarfs with masses less than or similar to 10 M-J. On the positive sid e, we show the first direct image of the triple system GJ 1245ABC in which all three components are well resolved. Multiband photometry of GJ 1245C su ggests for that object a spectral type later than M7. The measured position s of GJ 1245AC are discordant with those expected from published photocentr ic-orbit elements. The observed positions of four other multiple systems (G l 65AB, Gl 244AB, Gl 280AB, and Gl 866ABC) also differ from those expected from published orbital elements. The discrepancies noted for Gl 280AB are s ufficient to resolve the long-standing inconsistency between Procyon's obse rved luminosity and that derived from the theoretical mass-luminosity relat ion. F1042M images of the astrometric binary Gl 105A do not reveal the pres ence of a fourth component, as has been proposed to reconcile the differenc es between the observed location of the M7 V companion Gl 105C and the pred icted separations of the perturbing body from two independent astrometric s tudies.