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Distinguishing circumstellar from stellar photometric variability in Eta Carinae
摘要： The interacting binary Eta Carinae remains one of the most enigmatic massive stars in our Galaxy despite over four centuries of observations. In this work, its light curve from the ultraviolet to the near-infrared is analysed using spatially resolved HST observations and intense monitoring at the La Plata Observatory, combined with previously published photometry. We have developed a method to separate the central stellar object in the ground-based images using HST photometry and applying it to the more numerous ground-based data, which supports the hypothesis that the central source is brightening faster than the almost-constant Homunculus. After detrending from long-term brightening, the light curve shows periodic orbital modulation (?V ～ 0.6 mag) attributed to the wind-wind collision cavity as it sweeps around the primary star and it shows variable projected area to our line-of-sight. Two quasi-periodic components with time scales of 2-3 and 8-10 yr and low amplitude, ?V < 0.2 mag, are superimposed on the brightening light-curve, being the only stellar component of variability found, which indicates minimal stellar instability. Moreover, the light curve analysis shows no evidence of 'shell ejections' at periastron. We propose that the long-term brightening of the stellar core is due to the dissipation of a dusty clump in front of the central star, which works like a natural coronagraph. Thus, the central stars appear to be more stable than previously thought since the dominant variability originates from a changing circumstellar medium. We predict that the brightening phase, due mainly to dust dissipation, will be completed around 2032 ± 4 yr, when the star will be brighter than in the 1600's by up to ?V ～ 1 mag.
关键词： stars: individual (η Carinae),stars: winds, outflows,(stars:) binaries: general,stars: evolution,(ISM:) dust, extinction
更新于2025-09-19 17:15:36
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Near-infrared evidence for a sudden temperature increase in Eta Carinae
摘要： Aims. Eta Car’s ultra-violet, optical, and X-ray light curves and its spectrum suggest a physical change in its stellar wind over the last decade. It has been proposed that the mass-loss rate has decreased by a factor of about 2 over the last 15 years. We complement these recent results by investigating the past evolution and the current state of η Car in the near-infrared (IR). Methods. We present JHKL photometry of η Car obtained at SAAO Sutherland from 2004–2013 with the Mk II photometer at the 0.75 m telescope and JHKs photometry with SIRIUS at the 1.4 m IRSF telescope from 2012–2013. The near-IR light curves since 1972 are analyzed. Results. The long-term brightening trends in η Car’s JHKL light curves were discontinuous around the 1998 periastron passage. After 1998, the star shows excess emission above the extrapolated trend from earlier dates, especially in J and H, and the blueward, cyclical progression in its near-IR colors is accelerated. The near-IR color evolution is strongly correlated with the periastron passages. After correcting for the secular trend we ?nd that the color evolution matches an apparent increase in blackbody temperature of an optically thick near-IR emitting plasma component from about 3500 K to 6000 K over the last 20 years. Conclusions. We suggest that the changing near-IR emission may be caused by variability in optically thick bremsstrahlung emission. Periastron passages play an important role in the observed excess near-IR emission after 1998 and the long-term color evolution. We thus propose the hypothesis that angular momentum transfer (via tidal acceleration) during periastron passages leads to sudden changes in η Car’s atmosphere resulting in a long-term decrease in the mass-loss rate.
关键词： stars: individual: η Carinae,stars: winds, out?ows,stars: mass-loss,stars: variables: S Doradus,stars: massive
更新于2025-09-10 09:29:36