The condensation of dust around η Carinae

• Published in: Monthly notices of the Royal Astronomical Society 1978-12, Vol.185 (4), p.771-788
• Main Authors: Andriesse, C. D., Donn, B. D., Viotti, R.
• Format: Article
• Language: English
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1093/mnras/185.4.771

A study is made of the present condensation process of solid material around η Car. We show that this high luminosity (5 × 106 L⊙) star with a mass of 160 M⊙ is losing mass since 1840 at the rate $\dot M=7.5\times10^{-2}M_\odot/\text{yr}$. To drive this mass flow, a power is required of 8 × 106 L⊙, which may account for the intrinsic fading since 1840 by 1 mag. Since 1856 dust has been condensing from the circumstellar gas at distances in excess of Ri = 3.7 × 1014 m from the star, initially at the rate $\dot M_\text d=1\times10^{-4}\enspace M_\odot/\text{yr}$ and presently at a somewhat higher rate. This follows from an interpretation of the light curve with a model of the time evolution of the dust envelope, which accounts for the wavelength-dependent extent of the infrared source. The average density at Ri of about 1 × 1012 m−3 falls short by orders of magnitude to explain the condensation of solid material. The possibility is discussed of strong inhomogeneities in the density, which are stabilized against turbulent perturbations by a stellar magnetic field. Such a field would have a strength that is normal for Ap and Am stars. We furthermore give chemical arguments on the nature of the condensate and conclude that it consists in part of disordered silicate clusters. Using the pertinent optical properties and a model of the temperature stratification of the dust envelope, we fit the observed infrared spectrum for clusters with a size of about 1 μm. Such large grains give an almost grey circumstellar extinction of 3−4 mag in the visual and ultraviolet, which fits well in an independent interpretation of the extinction of η Car.