STScI Preprint #1319

Ultraviolet Observations of SN 1987A

After a summary of the many interesting results obtained from UV observations of SN 1987A, we focus on two aspects: (1) the detection and the study of a UV echo from SN 1987A, and (2) the determination of the distance to SN 1987A based on UV and optical observations of SN 1987A made with orbiting telescopes, i.e., IUE and HST.

(1) Ultraviolet light emitted by SN 1987A at the moment of the shock breakout, and lost to direct observations because the SN was discovered about 24 hours after the explosion, has been detected in a light echo produced in a dust cloud a few hundred parsec in front of the supernova. The spectrum of the echo shows a hot continuum and a wide P Cyg-like feature centered around 1500 е which if interpreted as CIV 1500 е implies an expansion velocity at the time of the shock breakout of -40,000 km s^-1 (in agreement with the first "direct" IUE spectrum, taken 24 hours after the explosion when the UV light was already fading  very fast, which showed a MgII line with a terminal velocity of -35,000 km s^-1).

(2) Combining the results from the IUE emission line light curves and an extensive set of HST images of SN 1987A we have derived an accurate determination of the distance to the supernova. In particular, we have inferred an absolute size of the ring R_abs = (6.23 ± 0.08) x 10¹⁷ cm and an angular size R" = 808 ± 17 mas at the time of the maximum UV line emission.  A comparison of these values leads to a distance to the supernova d(SN 1987A) = 51.4 ± 1.2 kpc and a distance modulus m - M(SN 1987A) = 18.55 ± 0.05. Allowing for a displacement of SN 1987A position relative to the LMC center, the distance to the barycenter of the Large Magellanic Cloud is also estimated to be d(LMC) = 52.0 ± 1.3 kpc, which corresponds to a distance modulus of m - M(LMC) = 18.58 ± 0.05.  Revising accordingly the zero point of the Cepheid distance scale, and using the SNIa measured by Sandage and collaborators, one finds a value of the Hubble constant H₀ = 59 ± 6 km s^-1 Mpc^-1.