Properties of Star 2, One of the SN 1987A Companions, Derived from Hubble Space Telescope--Faint Object Spectrograph Observations

Abstract

We present and discuss the Hubble Space Telescope Faint Object Spectrograph observations of Star 2, one of the two companions of SN 1987A, in the range 1200-8000 A. Our analysis, based on a comparison of the observed spectrum with spectra of O and B type stars and with model atmospheres, confirms that Star 2 is a B2III star. The extinction toward Star 2 is mostly of LMC origin and amounts to an E(B - V) = 0.19 +/- 0.02. The detailed extinction curve has a 2200 A bump less pronounced than the average LMC extinction law. Star 2 has an effective temperature of T_eff_ = 22000 +/- 2000 K and a luminosity of log L/L_sun_ = 4.38 +/- 0.11. Also, Star 2 appears to rotate with an equatorial velocity of ν sin i = 250+/-50 km s^-1^ and is presently undergoing mass loss at a range of log M ~ -7.0 /-+ 0.5 M_sun_ yr^-1^. Judging from its position in the H-R diagram and its present parameters, Star 2 had an original mass of about 13 M_sun_ and was born less than 13 Myr ago, which makes Star 2 coeval to the SN 1987A progenitor Sk 69^deg^202.