GeminiFocus October 2018 - Page 10

complex. A 2012 study of such “light echoes” reported observations of Eta Car’s pre- 1845 luminosity spikes illumi- nating a group of background clouds. Now, the same team has published two new pa- pers dissecting light echoes reflected by another cloud at a lesser distance from the star. Based on the geometry, the team believes the light is as- sociated with the enormous mass loss that occurred dur- ing the plateau phase of the Great Eruption in the 1850s. Figure 1 shows spectra of the light echoes taken with the Gemini Multi-Object Spec- trograph (GMOS) at Gemini South and the Inamori-Ma- gellan Areal Camera and Spectrograph (IMACS) at Ma- gellan Observatory. Figure 1. Gemini/GMOS and Magellan/IMACS spectra centered on the Hα line of the light echo believed to correspond to the latter part (circa 1855) of Eta Carinae’s Great Eruption. The spectra show very broad Hα line wings extending to at least ±10,000 km/s, indicating outflow velocities typically seen in supernovae. The blue curve represents a blackbody of temperature 6,000 K. (Figure reproduced from Smith, Rest, Andrews, et al., 2018.) 8 Eta Car is now known to be a binary star with an orbital period of 5.5 years, a distance of 2.3 kiloparsecs (kpc), and a combined mass of at least 250 solar masses. The pre-1845 lu- minosity spikes appear to coincide with pe- riastra of the binary orbit when mass transfer would be most efficient, while the plateau phase of the Great Eruption may have been the effect of a hydrodynamic explosion of uncertain origin and its consequent shock plowing through the circumstellar material. What is clear is that the eruption involved enormous mass loss: the bipolar “Homuncu- lus Nebula” contains at least 15 solar masses of material expanding away from the star at about 600 km/s and dates from this event. Although the Great Eruption concluded 160 years ago, it is possible to observe the light from that event reflected off cold clouds on the far side of the extended Carinae Nebula GeminiFocus The H-alpha emission lines in the new spec- tra have wings that reach –10,000 km/s to the blue and at least +12,000 km/s to the red. The team argues that the wings span the range of mass outflow speeds during the plateau phase of the Great Eruption; such speeds on stellar scales have only been seen previously in supernova ejecta and outflows from accreting compact stellar remnants. The broad wings are absent in the previ- ously studied echoes of the earlier phases of the eruption. The extremely fast material constitutes only a small fraction of the total ejecta, the majority of which is expanding at about 600 km/s. However, it provides strong evidence in favor of the explosive outflow explanation of the Great Eruption. The new papers have been accepted for publication in the Monthly Notices of the Royal Astronom- ical Society. October 2018