η Carinae is one of the most massive binary stars in the Milky Way1,2. It became the second-brightest star in our sky during its mid-nineteenth-century ‘Great Eruption’, but then faded from view (with only naked-eye estimates of brightness3,4). Its eruption is unique in that it exceeded the Eddington luminosity limit for ten years. Because it is only 2.3 kiloparsecs away, spatially resolved studies of the nebula have constrained the ejected mass and velocity, indicating that during its nineteenth-century eruption, η Car ejected more than ten solar masses in an event that released ten per cent of the energy of a typical core-collapse supernova5,6, without destroying the star. Here we report observations of light echoes of η Carinae from the 1838–1858 Great Eruption. Spectra of these light echoes show only absorption lines, which are blueshifted by −210 km s−1, in good agreement with predicted expansion speeds6. The light-echo spectra correlate best with those of G2-to-G5 supergiants, which have effective temperatures of around 5,000 kelvin. In contrast to the class of extragalactic outbursts assumed to be analogues of the Great Eruption of η Carinae7,8,9,10,11,12, the effective temperature of its outburst is significantly lower than that allowed by standard opaque wind models13. This indicates that other physical mechanisms such as an energetic blast wave may have triggered and influenced the eruption.
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We thank R. Humphreys, K. Davidson and J. Vink for comments and discussions. We thank S. Blondin for help with the continuum subtraction. The Blanco 4-m telescope is a facility of the Cerro Tololo Inter-American Observatory, National Optical Astronomy Observatory, which is operated by the Association of Universities for Research in Astronomy, under contract with the National Science Foundation. We use data from the UVES Paranal Observatory Project. The computations in this paper were run on the Odyssey cluster supported by the FAS Science Division Research Computing Group at Harvard University. Observations were obtained at LCOGT, and F.B.B. and D.A.H. acknowledge support from LCOGT. J.L.P. is a Hubble Carnegie-Princeton Fellow. R.J.F. is a Clay Fellow.
The authors declare no competing financial interests.
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Rest, A., Prieto, J., Walborn, N. et al. Light echoes reveal an unexpectedly cool η Carinae during its nineteenth-century Great Eruption. Nature 482, 375–378 (2012) doi:10.1038/nature10775
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