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Light Echoes Reveal an Unexpectedly Cool Η Carinae During Its Nineteenth

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Light Echoes Reveal an Unexpectedly Cool Η Carinae During Its Nineteenth LETTER doi:10.1038/nature10775 Light echoes reveal an unexpectedly cool g Carinae during its nineteenth-century Great Eruption A. Rest1, J. L. Prieto2,3, N. R. Walborn1, N. Smith4, F. B. Bianco5,6, R. Chornock7, D. L. Welch8, D. A. Howell5,6, M. E. Huber9, R. J. Foley7, W. Fong7, B. Sinnott8, H. E. Bond1, R. C. Smith10, I. Toledo11, D. Minniti12 & K. Mandel7,13 g Carinae is one of the most massive binary stars in the Milky itself, but in any case it must be weak, if present. By cross-correlating Way1,2. It became the second-brightest star in our sky during its each of our g Car echo spectra with the Ultraviolet and Visual Echelle mid-nineteenth-century ‘Great Eruption’, but then faded from view Spectrograph (UVES) spectral library14 (see Supplementary Figs 2 (with only naked-eye estimates of brightness3,4). Its eruption is and 3), we find the best agreement with supergiant spectral types in unique in that it exceeded the Eddington luminosity limit for ten the range of G2 to G5, with an effective temperature of around 5,000 K. years. Because it is only 2.3 kiloparsecs away, spatially resolved Spectral types of F7 or earlier are ruled out by our analysis (see studies of the nebula have constrained the ejected mass and velocity, Supplementary Information for more details). indicating that during its nineteenth-century eruption, g Car Doppler shifts of absorption features in the echo spectra provide ejected more than ten solar masses in an event that released ten direct information about the outflow speeds during the eruption. The 5,6 per cent of the energy of a typical core-collapse supernova , Ca II infrared triplet absorption features in the spectrum are noticeably without destroying the star. Here we report observations of light blueshifted (see Supplementary Fig. 2). By cross-correlation with echoes of g Carinae from the 1838–1858 Great Eruption. Spectra of G-type15 templates, we determine velocities of –202 6 9, –210 6 14 these light echoes show only absorption lines, which are blueshifted and –237 6 17 km s21 (errors are standard deviation) for our three by 2210 km s21, in good agreement with predicted expansion individual spectra and an average velocity of –210 6 30 km s21, which speeds6. The light-echo spectra correlate best with those of G2- includes an uncertainty for the dust sheet velocity. to-G5 supergiants, which have effective temperatures of around The bipolar nature of the Homunculus nebula shows that the g Car 5,000 kelvin. In contrast to the class of extragalactic outbursts Great Eruption was strongly aspherical. It has been predicted that the assumed to be analogues of the Great Eruption of g Carinae7–12, outflow speeds that one would derive from the spectra of g Car in the effective temperature of its outburst is significantly lower than outburst, looking at the poles and equator of the double lobes, would that allowed by standard opaque wind models13. This indicates that be about 2650 km s21 and 240 to 2100 km s21, (note that the speeds other physical mechanisms such as an energetic blast wave may have are negative because they are blueshifted, that is, the outflow is moving triggered and influenced the eruption. towards us) respectively16 (outflow speeds near the equator have a g-Car-like giant eruptions of luminous blue variable stars are steep latitude dependence). The light echo we investigate here arises characterized by significant mass-loss and an increase in luminosity from latitudes near the equator of g Car (see Supplementary Fig. 1), by several magnitudes8,9. It has been thought that this increase in and the measured blueshifted velocity of 2210 6 30 km s21 is in good luminosity drives a dense wind, producing an optically thick, cooler agreement with expansion speeds within 620u of the equatorial plane. pseudo-photosphere with a minimum effective temperature of 7,000 K We also find a strong asymmetry in the Ca II infrared triplet, extending and an F-type spectrum13. Within this model, g Car has been consid- to a velocity of 2850 km s21—well below the speed of the fastest polar ered the prototype of these ‘‘supernova imposters’’7–12. ejecta found previously6, but in good agreement with speeds observed We obtained images in proximity to g Car (Fig. 1) that, when in the blast wave at lower latitudes6. Future observations of light echoes differenced, show a rich set of light echoes. The largest interval between viewing the g Car eruption from different directions, in particular our images is eight years. We have also found similar echo candidates from the poles, has the potential to observe these very-high-velocity at other positions, which we are currently monitoring. The large ejecta and other asymmetries. brightening and long duration point to the Great Eruption as the A characteristic of luminous-blue-variable outbursts is their trans- source of the light echoes. We have also obtained a composite light ition from a hot quiescent state to a cooler outburst state, although this curve in the Sloan Digital Sky Survey (SDSS) i filter of the light echoes feature is less well observed for the giant eruptions (see Fig. 4). Two (see Fig. 2), showing a slow decline of several tenths of a magnitude potential models for luminous-blue-variable outbursts involve either over half a year. This light curve is most consistent with the historical an opaque stellar wind driven by an increase in luminosity, or a observations4 of a peak in 1843, part of the 1838–1858 Great Eruption, hydrodynamic explosion. The traditional mechanism for g-Car-like although further observations are necessary to be certain (see the giant eruptions has been that an unexplained increase in luminosity Supplementary Information). drives a denser wind, so that an optically thick pseudo-photosphere Spectra of the light echoes (see Fig. 3) show only absorption lines forms at a layer much larger and cooler than the hydrostatic stellar characteristic of cool stellar photospheres, but no evidence of emission surface13. This model predicts a minimum effective temperature of lines. In particular, the Ca II infrared triplet is only observed as absorp- 7,000 K, resembling A- or F-type supergiants8,17, because the wind tion lines in the spectrum. Because of bright ambient nebular emission, opacity depends on the temperature (see Fig. 4). A giant eruption it is difficult to determine whether there is any Ha emission from g Car evidently occurs as a massive star attempts to evolve redward and 1Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, Maryland 21218, USA. 2Carnegie Observatories, 813 Santa Barbara Street, Pasadena, California 91101, USA. 3Department of Astrophysical Sciences, Princeton University, Peyton Hall, Princeton, New Jersey 08544, USA. 4Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, Arizona 85721, USA. 5Las Cumbres Observatory Global Telescope Network, Goleta, California 93117, USA. 6Department of Physics, University of California, Santa Barbara, California 93106, USA. 7Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, Massachusetts 02138, USA. 8Department of Physics and Astronomy, McMaster University, Hamilton, Ontario, L8S 4M1, Canada. 9Department of Physics and Astronomy, Johns Hopkins University, Baltimore, 3400 North Charles Street, Maryland 21218, USA. 10Cerro Tololo Inter-American Observatory, National Optical Astronomy Observatory, Colina el Pino S/N, La Serena, Chile. 11ALMA, KM 121 CH 23, San Pedro de Atacama, II Region, Chile. 12Department of Astronomy and Astrophysics, Pontificia Universidad Catolica de Chile, Santiago 22, Chile. 13Imperial College London, Blackett Laboratory, Prince Consort Road, London SW7 2AZ, UK. 16 FEBRUARY 2012 | VOL 482 | NATURE | 375 ©2012 Macmillan Publishers Limited. All rights reserved RESEARCH LETTER 10 March 2003 (A) Difference image C – A η Carinae ′′ ′′ 120 120 10 May 2010 (B) Difference image C – B ′′ ′′ 120 120 6 February 2011 (C) C – A Difference image zoom 8.5′ × 8.5′ N ′ 15 ′′ ′′ E 30 120 Figure 1 | g Car light echoes. The left panel shows the positions of the star blue line. For all panels north is up and east is to the left. Applying the vector g Carinae and our images (white box), plotted on an image in the light of three method that previously allowed us to identify the source of the light echoes different emission lines: oxygen (blue), hydrogen (green) and sulphur (red). from the supernovae that produced the supernova remnants SNR 0509–67.5, (Photo taken by N.S.) The middle panels show the images obtained with the Cas A and Tycho24,25, we find that a dramatic brightening of g Car must be the CTIO 4-m Blanco telescope of a region about 0.5u to the south of g Car at 10 origin. In these echoes, unlike those of Galactic supernovae26, there is still March 2003 (epoch A), 10 May 2010 (epoch B), and 6 February 2011 (epoch C), significant spatial overlap even at separations of one light-year, suggesting that from top to bottom. The right panels show the difference images ‘C minus A’ the duration of the event causing them must be significantly longer than one and ‘C minus B’ at the top and middle, respectively. Example light-echo year. We also see brightening of two magnitudes or more within eight years. positions are indicated with blue (epoch A) and red (epochs B and C) arrows. Thus, the Lesser Eruption from 1887 to 1896, which brightened by only a The bottom right panel shows a zoom of the spectrograph slit, indicated with a magnitude, is excluded as the source. Historic visual light curve –1.0 Light echo light curve, year minus 174.20 13 Light echo light curve, encounters the Humphreys–Davidson limit , beyond which no stable –0.5 year minus 167.95 Light echo light curve, stars are observed.
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