Characteristics and interpretation of the photometric variability of eta carinae and its nebula

Abstract

A short review is given on the history of the peculiar variable object η Car and on a number of relevant references describing and discussing its physical characteristics and behaviour, based on different types of observational techniques. The star is known to be variable since the 17th century. The excessive mass loss to which it was subject during the 19th century is now visible as an ellipsoidal reflection nebula of 15″ diameter: the so-called ‘homunculus’. The remainder of the paper is spent on different kinds of problems partly based on the results of a decade of photometric monitoring in the VBLUW photometric system of Walraven. Foreground reddening and reddening by dust in the homunculus are determined and amount to E(B - V) _J = 0$_{ .}^m$50 and <$_{ .}^m$6, respectively. Scanning of the homunculus revealed an estimate for the photometric characteristics of the central object, which presumably consists of a massive hot star surrounded by a cooler gas envelope. The total luminosity is derived using fluxes of various sources in the wavelength region 0.15 < λ < 175 μn resulting in M _bol = - 12$_{ .}^m$3 ± 0$_{ .}^m$2. The total observed flux corrected for foreground extinction corresponds to a star with R ∼ 96 R _⊙ if T _eff ∼ 30 000 K. The mass may be near 150 M _⊙. The excess luminosity in 1843, when the star was presumably bolometrically at least ∽ 2$_{ .}^m$5 brighter than at present, may have been caused by envelope-energized pulsations when the star's luminosity was close to its Eddington limit. The temperature should then have been ∼ 50 000 K. The mass loss rate, during the excess luminosity phase lasting ∼ 30 yr, is estimated to amount to M ∼ 4 × 10^-3 M _⊙ yr^-1. At present the mass loss may be M ∼ 10^-4M _⊙ yr^-1. Since the homunculus is mainly built up from material expelled in the 30 yr interval (from ∼ 1830 to 1860), its total mass amounts to M _hom ∼ 0.15 M _⊙. The historical observations of the colours of η Car and a comparison with the characteristics of S Dor type stars, suggest that η Car was subject to a number of S Dor type phases similar to those of P Cyg (in the 17th century), S Dor and others. A satisfactory explanation is found for the complete historical light curve. The decrease in light after the 1843 maximum by ∼9^m, presumably consists of a fading of the luminosity excess and the S Dor effect by ∽ 2$_{ .}^m$5 and ∽ 3^m, respectively, and a ∼ 3$_{ .}^m$5 extinction by circumstellar dust. The small amplitude light variations which η Car showed during the last decade, were studied with the aid of the variations of the Balmer jump. They are presumably caused by temperature variations of the central star.

Percy and Welch (1983) (Publ. Astron. Soc. Pacific 95, 491) have observed P Cyg on a number of nights in 1982 and found for the photometric variations a time scale of 30 to 50 days and an amplitude of ∼0 ^m_. 15.