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Eta Carinae: A Spectacular Show From A Convulsing Massive Star

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Although the Homunculus Nebula around the massive star Eta Carinae has been the subject of intense study for many years, it has always been reluctant to divulge its innermost secrets. However, an important chapter in the recent evolution of this unique star was revealed when Nathan Smith (University of Colorado) used the high-resolution infrared spectrograph PHOENIX on the Gemini South telescope to observe the bipolar nebula surrounding Eta Carinae.

Multi-slit spectroscopy (see Figure 1) allowed Smith to reconstruct both the geometry and the velocity structure of the expanding gas in the nebula based on the behavior of the molecular line of hydrogen H2 at 2.1218 microns and the atomic line of ionized iron [Fe II] at 1.6435 microns (Figure 2).

Figure 1. One of the five PHOENIX spectrograph long-slit aperture positions superposed on a 2-micron HST/NICMOS image of Eta Carinae.

Analysis of the PHOENIX spectrum shows a very well-defined shell structure expanding ballistically at about 500 kilometers per second (Figure 2). A “thick,” warm inner dust shell traced by [Fe II] emission is surrounded by a cooler and denser outer shell that is traced by strong H2 emission. Even though the outer H2 skin is remarkably thin and uniform it contains about 11 solar masses of gas and dust ejected over a period of less than five years. The Gemini spectra show that the density in the outer shell may reach 107 particles per cm3.

The spatio-kinematic structure of H2 emission at the pinched waist of the nebula helps explain the unusual and complex structures seen in other high-resolution images.  The current shape of the Homunculus nebula is of two well-defined polar lobes outlined by an outer massive shell of gas and dust (Figure 3). Smith states that these Gemini/PHOENIX data indicate that most of the mass lost during the Great Eruption of the mid-nineteenth century was limited to the high latitudes of the star, with almost all of the mechanical energy escaping between 45 degrees and the pole.

"The mass distribution in the nebula indicates that its shape is a direct result of an aspherical explosion from the star itself, instead of being pinched at the waist by the surrounding circumstellar material," said Smith.

For more details read “The Structure of the Homunculus: I. Shape and Latitutude Dependence from H2 and [Fe II] velocity Maps of Eta Carinae,” by Nathan Smith, The Astrophysical Journal, in press or at astro-ph/0602464.

Figure 2. Long-slit composite spectrum from Gemini/PHOENIX showing [Fe II] in blue and molecular hydrogen H2 in red, simultaneously. The bright blue feature in the center, which appears stretched horizontally here, is a younger and smaller bipolar nebula called the Little Homunculus.  It was ejected about 50 years after the major eruption that created the larger nebula.

Figure 3. Model shape (thin line) plotted over the H2 (2.1218-micron) emission from the Homunculus, showing the thin bipolar lobes. The bright streak running diagonally across the image is residual continuum emission from the bright central star (saturated). Spatial scales are converted to astronomical units assuming an age of 160 years and a distance of 2,350 parsecs.

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