THE PRESUPERNOVA H-II REGION AROUND SN-1987A

The slow expansion of the bright ring around SN 1987A and the large si ze estimated for the radio source at an age of similar to 1500 days su ggest a low mass-loss rate, M, from the blue supergiant progenitor sta r; we adopt M = 7.5 x 10(-8) M(.) yr(-1) and a wind velocity upsilon(w ) = 450 km s(-1) as standard values. This wind density would not give substantial radio free-free absorption during the first few days when low-frequency absorption was observed, but the turnover can be attribu ted to synchrotron self-absorption. The X-ray and radio emission obser ved since 1990 require that the supernova shock be interacting with de nser gas than that in the free wind. We propose that it is interacting with an H II region created by the B3 Ia progenitor star in the swept -up red supergiant wind. The H II region extends from a radius r(II) a pproximate to 3 x 10(17) cm to ring radius R = 6 x 10(17) cm in the eq uatorial plane and has a hydrogen density similar to 10(2) cm(-3). The shock front will not reach the dense ring until the year 2005 +/- 3. The X-ray luminosity should slowly increase until that time. The equat orial ionization front is trapped in the swept-up gas; the dense ring and its associated bipolar nebula are the initially neutral part of th e shell. In the polar direction, the ionization front broke out of the swept-up gas, and there is no dense neutral shell, consistent with th e observed lack of a thin shell in that direction.