Mid- and Far-Infrared ISO Limits on Dust Disks around Millisecond Pulsars

We report 60 and 90 micron observations of 7 millisecond pulsars with ISO. The pulsar PSR B1257+12 is orbited by three planets, and other millisecond pulsars may be orbited by dust disks that represent planets that failed to form or their residue. We do not detect any infrared emission from the 7 pulsars in our sample, and typical upper limits are 100 mJy. Using a simple model, we constrain the typical dust disk mass to be less than of order 100 M_Earth, assuming that the heating of any putative dust disk would be coupled only weakly to the pulsar's emission. If the planets around PSR B1257+12 are composed largely of metals, our limits are probably an order of magnitude above plausible values for the disk mass in metals. Future observations with the Spitzer Space Telescope should be capable of probing into the range of disk masses that could plausibly give rise to planets.