Astronomy and Astrophysics – Astronomy
Scientific paper
Aug 1998
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998apj...503..857d&link_type=abstract
Astrophysical Journal v.503, p.857
Astronomy and Astrophysics
Astronomy
11
Stars: Abundances, Line: Formation, Stars: Supernovae: Individual Alphanumeric: Sn 1987A
Scientific paper
We model the heating and cooling processes in the hydrogen- and helium-rich zones of the envelope of SN 1987A from t = 200 to 1200 days after outburst and use these results to calculate the light curves of the most prominent emission lines. For the first 600 days, heating and cooling processes are in equilibrium. The main heating mechanism is direct heat deposition by nonthermal electrons, and the main cooling mechanism is collisional excitation of trace elements such as Ca II, Fe II, and C I, followed by the emission of a line photon. After 600 days adiabatic cooling becomes important, and the cooling and heating rates are no longer in equilibrium. Dust, formed in the Fe/Co/Ni zone after t ~ 400 days, plays an important role in the formation of the emission lines. It both modifies the internal UV radiation field that excites the ions and reduces the escaping line fluxes by extinction. The pseudocontinuum opacity in the envelope due to the many absorption lines of metals, which we model crudely by a simple power law, is also important for the emerging spectrum. Our results for the temperature evolution do not depend strongly on our assumptions. We find that the temperatures of the hydrogen and helium zones evolve from T ~ 6000 K at t = 200 days to T ~ 1000 K at t = 1200 days. The ionized fraction of hydrogen evolves from xH ~ 6 x 10-3 at t = 200 days to xH ~ 3 x 10-4 at t = 1200 days. With abundances determined from observations of the circumstellar ring, the model can account for the light curves of most strong emission lines of H I, He I, Ca II, and Fe II, but some discrepancies remain. Especially interesting is the H beta light curve, which exhibits a clear plateau when H beta is still optically thick, but Pa alpha is already optically thin. In all our models this phase appears to occur later than in the observations. For t >~ 800 days, the infrared emission lines of Fe II are produced mainly by primordial iron in the H/He envelope, not by newly synthesized iron. The fluxes of C I and O I lines that our model predicts are much higher than observed, and they may require a significant adjustment in abundance or mass of the different composition zones to make them agree with observations. Our models also indicate that the total helium mass in the core of the remnant (v < 2500 km s-1) must lie in the range 2-5 M&sun;. The hydrogen mass in the core is less well constrained, because the hydrogen line strength does not vary much as long as most of the nonthermal energy is deposited in hydrogen. The ratio of the fluxes of the Br gamma and the He I 2.058 mu m lines is slightly more sensitive, and it indicates a helium mass-to-hydrogen mass ratio ~ 1:2.
de Kool Martijn
Li Hongwei
McCray Richard
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