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

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

Jul 1997

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997apj...483..925c&link_type=abstract

Astrophysical Journal v.483, p.925

Astronomy and Astrophysics

Astronomy

26

Line: Formation, Nuclear Reactions, Nucleosynthesis, Abundances, Stars: Supernovae: Individual Alphanumeric: Sn 1987A

Scientific paper

The Hubble Space Telescope 2000--8000 Angstroms spectrum of SN 1987A observed on 1995 January 7 (7.87 yr after the explosion) is dominated by H alpha and UV lines, including Mg II--Mg I lambda 2825 (equal to H alpha in luminosity), Fe II UV 2 (two-thirds the intensity of H alpha ), Fe II UV 3 (one-half the intensity of H alpha ), and a 3730 Angstroms emission feature identified with a blend of [O II] lambda 3727 and Fe I emission lines. [O I] lambda 6300 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 lambda 2800 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 H alpha 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) x 10-4 M&sun; of 44Ti 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 1036 ergs s-1 deposited by the 44Ti positrons should be emitted in the Fe II 26 mu m line. The observed H alpha 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] lambda 6300 doublet is consistent with that expected for the deposited energy of gamma -rays from (1--2) x 10-4 M&sun; of 44Ti for an assumed 1.5--2 M&sun; of oxygen. If the oxygen mass does not exceed 2 M&sun;, 1 x 10-4 M&sun; 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] lambda 6300 line intensity rules out the presence of a central source of gamma -radiation (h nu > 100 keV) with a luminosity L gamma > 4 x 1036 ergs s-1.

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