HUBBLE SPACE TELESCOPE SPECTRUM OF SN 1987A AT AN AGE OF 8 YEARS: RADIOACTIVE LUMINESCENCE OF COOL GAS

摘要

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 12825 (equal to Hα in luminosity), Fe II UV 2 (two-thirds the intensity of H?), Fe II UV 3 (one-half the intensity of Hα), and a 3730 A emission feature identified with a blend of [O II] 13727 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 ~(44)Ti 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, ～10~(35) ergs s~(-1), can be explained if the bulk of the positrons from a mass (1-2) x 10~(-4) solar mass of ~(44)Ti release their energy in the iron-rich material, which suggests the presence of a magnetic field B > 5 x 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 10~(36) ergs s~(-1) deposited by the ~(44)Ti 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) (≈ 10~(36) 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) x 10~(-4) solar mass of ~(44)Ti for an assumed 1.5-2 solar mass of oxygen. If the oxygen mass does not exceed 2 solar mass, 1 x 10~(-4) solar mass is a lower limit for the amount of ~(44)Ti in SN 1987A. The maximum fraction of the ~(44)Ti 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 L_γ > 4 x 10~(36) ergs s~(-1).