Astronomy and Astrophysics – Astronomy
Scientific paper
Dec 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992aas...18110705t&link_type=abstract
American Astronomical Society, 181st AAS Meeting, #107.05; Bulletin of the American Astronomical Society, Vol. 24, p.1293
Astronomy and Astrophysics
Astronomy
Scientific paper
We report the detection of a sharp absorption edge at 2.7 microns in the near-IR spectrum of SN 1987A on day 53 after the progenitor star's core collapsed. We assign this feature to structural OH in silicate minerals in circumstellar dust ejected from the progenitor star during its red supergiant phase. We derive a mass column density of 4.3x10(-4) g cm(-2) (+/-7%) for the absorbing material in the line of sight towards SN 1987A by fitting optical depths of serpentine and chlorite, calculated via Mie scattering theory, to the observed spectrum of SN 1987A. To be consistent with the interstellar extinction of A_{B-V} = 0.31 towards SN 1987A we find that the radii of the particles must not be greater than 0.04 microns. Moreover, interlayer H_2O is also present in these circumstellar silicates inferred from a region of broad absorption extending from the long wavelength side of the absorption edge at 2.7 microns. Using a simple chemical model we demonstrate that the vapor phase hydration of silicate grains in the atmospheres of red supergiants is not necessarily kinetically inhibited. Although there is an even more conspicuous SiO band near 10 microns in laboratory spectra of hydrated silicates, no such feature has been observed at this wavelength in spectra of SN 1987A at any time. We explain the absence of this feature with a model of an IR echo emitted from circumstellar dust around SN 1987A. Certain combinations of the mass of the circumstellar shell or ring, its radius, and the particle radii can be found so that the 10-microns feature appears neither in absorption nor in emission. Observations in the wavelength range from 20 to 50 microns suggest that the radii of the dust particles are equal or smaller than 0.01 microns and that the circumstellar dust is located rather close to SN 1987A. However, preexisting circumstellar silicates could not have survived the UV burst of SN 1987A at distances closer than approximately 10(18) cm to the supernova. The total mass of circumstellar dust that is required to produce such an IR echo is about 0.12 M_&sun;.
Larson Harold P.
Timmermann Ralf
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