Limits from theHubble Space Telescopeon a Point Source in SN 1987A

We observed supernova 1987A (SN 1987A) with the Space Telescope ImagingSpectrograph (STIS) on the Hubble Space Telescope (HST) in 1999 September, andagain with the Advanced Camera for Surveys (ACS) on the HST in 2003 November.No point source is observed in the remnant. We obtain a limiting flux of F_opt< 1.6 x 10^{-14} ergs/s/cm^2 in the wavelength range 2900-9650 Angstroms forany continuum emitter at the center of the supernova remnant (SNR). It islikely that the SNR contains opaque dust that absorbs UV and optical emission,resulting in an attenuation of ~35% due to dust absorption in the SNR. Takinginto account dust absorption in the remnant, we find a limit of L_opt < 8 x10^{33} ergs/s. We compare this upper bound with empirical evidence from pointsources in other supernova remnants, and with theoretical models for possiblecompact sources. Bright young pulsars such as Kes 75 or the Crab pulsar areexcluded by optical and X-ray limits on SN 1987A. Of the young pulsars known tobe associated with SNRs, those with ages < 5000 years are all too bright inX-rays to be compatible with the limits on SN 1987A. Examining theoreticalmodels for accretion onto a compact object, we find that spherical accretiononto a neutron star is firmly ruled out, and that spherical accretion onto ablack hole is possible only if there is a larger amount of dust absorption inthe remnant than predicted. In the case of thin-disk accretion, our flux limitrequires a small disk, no larger than 10^{10} cm, with an accretion rate nomore than 0.3 times the Eddington accretion rate. Possible ways to hide asurviving compact object include the removal of all surrounding material atearly times by a photon-driven wind, a small accretion disk, or very highlevels of dust absorption in the remnant.Comment: 40 pages, 5 figures. AAStex. Accepted, ApJ 04/28/200