Hubble Space TelescopeSpectrum of SN 1987A at an Age of 8 Years: Radioactive Luminescence of Cool Gas

Abstract

The Hubble Space Telescope 2000-8000 A spectrum of SN 1987A observed on 1995 January 7 (7.87 yr after the explosion) is dominated by Hα and UV lines, including Mg II-Mg I λ2825 (equal to Hα in luminosity), Fe II UV 2 (two-thirds the intensity of Hα), Fe II UV 3 (one-half the intensity of Hα), and a 3730 A emission feature identified with a blend of [O II] λ3727 and Fe I emission lines. [O I] λ6300 and lines of [Ca II] and Na I, as well as some Fe II optical forbidden and permitted lines are present at visual wavelengths. Also present are a number of weak emission features, which are presumably metal lines produced by photon degradation as a result of reprocessing of UV radiation into metal lines. Modeling the Mg II-Mg I lines provides the velocity of the outer visible radius of the envelope, 9000 ± 500 km s-1 in the Mg II λ2800 line, which is consistent with the earlier direct HST imaging at near-UV wavelengths. The UV/optical emission lines originate from the radioactive luminescence of the cool gas (T ≈ 130-160 K). The metal lines reflect the instantaneous reprocessing of the energy deposited from 44Ti radioactive decays through collisions with fast electrons, while the Hα emission primarily comes from the recombination of previously ionized hydrogen. The overall luminosity of the Fe II emission lines, ~1035 ergs s-1, can be explained if the bulk of the positrons from a mass (1-2) × 10-4 M☉ of 44Ti release their energy in the iron-rich material, which suggests the presence of a magnetic field B > 5 × 10-13 G prohibiting the escape of positrons into oxygen and hydrogen gas. The ionized fraction in the iron-rich material is small (0.2-0.3), and the total UV/optical emission from Fe I should be comparable to that from Fe II. Most of the 1036 ergs s-1 deposited by the 44Ti positrons should be emitted in the Fe II 26 μm line. The observed Hα luminosity decrease, by 5 orders of magnitude between the ages of 1 to 8 yr, is reproduced in a time-dependent model of ionization and cooling with the standard amount of radioactive nuclides. However, an additional source of energy at the present epoch with a deposition rate 30 ergs s-1 g-1 (≈ 1036 ergs s-1 in the whole envelope) is not ruled out. The present average temperature in the hydrogen envelope predicted by the time-dependent model is 130 K, which is lower than the value T ≈ 350 K obtained from the observed Balmer continuum shape. However, the shape is affected by a possible contribution of metal lines to the Balmer continuum. The luminosity of the [O I] λ6300 doublet is consistent with that expected for the deposited energy of γ-rays from (1-2) × 10-4 M☉ of 44Ti for an assumed 1.5-2 M☉ of oxygen. If the oxygen mass does not exceed 2 M☉, 1 × 10-4 M☉ is a lower limit for the amount of 44Ti in SN 1987A. The maximum fraction of the 44Ti positron energy deposited into oxygen-rich material does not exceed 5%, which is consistent with positron trapping in Fe-rich material. The [O I] λ6300 line intensity rules out the presence of a central source of γ-radiation (hν > 100 keV) with a luminosity Lγ > 4 × 1036 ergs s-1.