June2006
by Nathan Smith
Phoenix Gives a New View of Eta Carinae Figure 1.
An example of the long-slit spectra obtained with Phoenix on the Gemini South telescope. The vertical axis is the spatial position along a slit, aligned with the polar axis of the nebula (rotated about 50 degrees clockwise from vertical in the image in Figure 2). The horizontal axis is the Doppler-shifted velocity, which translates directly to a cross section of the 3-D shape along our line of sight. In this representation, red is emission from molecular hydrogen at 2.122 microns, and blue is emission from singly-ionized iron at 1.644 microns.
N
ew spectra obtained with the high-
infrared by dust formed in the nebula. This makes
resolution infrared Phoenix spectrograph
Eta Carinae the brightest 10-micron object in the
on Gemini South provide striking insight
sky outside the solar system. The outburst ejected a
to the structure of the so-called “Homunculus”
huge amount of mass—more than ten times that of
nebula around the massive southern hemisphere star
our Sun—and almost 1050 ergs of kinetic energy, as
Eta Carinae. In addition, they help produce the first
inferred from observations in 2003 of emission from
definitive three-dimensional picture of the nebula’s
its dust. (That is about a billion times more energy
geometry.
than our Sun gives off in a year). The cause of the event and the mechanism that launched such a huge
This bipolar nebula (Figures 1 and 2) was ejected by
amount of mass off the star (while allowing the star
the massive star at its heart in the mid-nineteenth
to survive) remains unknown, but the structure of
century. During that outburst, Eta Carinae was
the fossil nebula it created holds important clues.
briefly the second-brightest star in the night sky,
38
despite its distance of 2.3 kiloparsecs (about 7,500
The new Phoenix spectra in Figure 1 reveal that the
light-years). The star faded after 15 - 20 years, but
Homunculus is composed of an intricate double
today its luminosity is re-radiated in the thermal
shell structure, with a thin outer shell seen in
Gemini Observatory