Hubble Space Telescope Spectroscopic Observations of the Ejecta of SN 1987A at 2000 Days

Abstract

We have used the Faint Object Spectrograph on the Hubble Space Telescope (HST) to observe the spectra of SN 1987A over the wavelength range 2000-8000 Angstrom on dates 1862 and 2210 days after the supernova outburst. Even these pre-COSTAR observations avoid much of the contamination from the bright stars nearby and provide a very useful set of line strengths and shapes for analysis. The spectrum is formed in an unusual physical setting: cold gas that is excited and ionized by energetic electrons from the radioactive debris of the supernova explosion. The spectra of SN 1987A at this phase are surprisingly similar to those of the nova shells of CP Puppis and T Pyxidis decades after outburst. SN 1987A and the novae are characterized by emission from material with electron temperatures of only a few hundred kelvins and show narrow Balmer continuum emission and strong emission Lines from O+. The Balmer continuum shape requires the electron temperature in the supernova ejecta to be as low as 500 K on day 1862 and 400 K on day 2210 after outburst. The [O II] lambda lambda 3726, 3728 doublet is surprisingly strong and is plausibly powered by collisional ionization of neutral oxygen to excited states of O+.The line intensity ratio of the [O I] lambda lambda 6300, 6364 doublet obtained from Gaussian fits of the line profiles is 1.8 +/- 0.2, contrary to the optically thin limit of 3. This ratio is not due to an optical depth effect but, rather, is an artifact of assuming a Gaussian profile to fit the [O I] lambda lambda 6300, 6364 doublet profile. Specifying the line ratio R = F([O I] lambda 6300)/F([O I] lambda 6364) = 3 is consistent with the data and allows a calculation of the decomposed line profile.All the observed strong lines are found to be blueshifted by a similar amount of 400 km s(-1). The line profiles are quite similar for lines arising from different chemical elements. The profiles are all asymmetric, showing redshifted extended tails with velocities up to 10,000 km s(-1) in some strong lines. The blueshift of the line peaks is attributed to dust that condensed from the SN 1987A ejecta that is still distributed in dense opaque clumps. The strongest ultraviolet lines are those of Mg I lambda 2852 and Mg II lambda lambda 2795, 2802. The Mg I lambda 2852 line is significantly broader than most lines in the optical, which provides a natural explanation for the size differences in the optical and ultraviolet of the SN 1987A ejecta derived from HST direct images.