AIRBORNE SPECTROPHOTOMETRY OF SN-1987A FROM 1.7 TO 12.6 MICRONS - TIME HISTORY OF THE DUST CONTINUUM AND LINE EMISSION

Spectrophotometric observations (1.7-12.6 mum) of SN 1987A from the Ku iper Airborne Observatory are presented for five epochs at 60, 260, 41 5, 615, and 775 days after the explosion. A variety of emission lines is seen, including members of the hydrogen Humphreys, Pfund, Brackett, and Paschen series, fine-structure lines of metals (including [Ni II] 6.634 mum, [Ni I] 7.507 mum, [Ar II] 6.985 mum, and [Co II] 10.521 mu m), and CO and SiO molecular bands. The temporal evolution of the seve n strongest H lines follows case C recombination theory and yields lar ge values Of tau(Halpha) at 260 and 415 days. A mass of approximately 2 x 10(-3) M. is derived for stable nickel, and the ratio of the [Ni I ] 7.507 mum and [Ni II] 6.634 mum line intensities yields a high ioniz ation fraction of 0.9 in the nickel zone. Dust condensation is clearly detected at 615 days for the first time in a Type II supernova. At no time is there a 9.7 mum emission feature characteristic of interstell ar astronomical silicates in the spectra of SN 1987A, nor are the 6.2 or 7.7 mum emission features attributed to polycyclic aromatic hydroca rbons seen. These airbome data are combined with other airbome and gro und-based measurements taken at (or near) the same time to form five c omposite spectra of SN 1987A with wavelength coverage from approximate ly 3200 angstrom to 100 mum. The IR continuum emission between approxi mately 2 and 100 mum is compared with a three-component model-(1) hot photospheric continuum, (2) free-free and free-bound H emission, and ( 3) dust continuum-with the best fit determined using a nonlinear chi2 method. The dust continuum component is well characterized by a single -temperature graybody emission spectrum, i.e., by the radiation from g ray grains or dust in optically thick clumps. At early times (less tha n 400 days after core collapse), the dust emission tracks the bolometr ic luminosity at about the 2% level. By 615 days, the fraction of the total luminosity contributed by the IR dust continuum increases dramat ically to 0.45, and then to 0.83 at 775 days. We suggest that this dic hotomy in the temporal evolution of the dust emission arises from dust with different origins. Circumstellar dust present before the superno va and then heated by it may account for the early emission. Newly con densed dust in the ejecta accounts for the later emission. A lower lim it to the dust mass at 775 days is approximately 10(-4) M., but much m ore dust could be present. Since the emission is well fitted by a gray body, no information on the dust composition can be directly discerned from our data.