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