Observations of a massive, unstable star called Eta Carinae, made with the Wide Field and Planetary Camera (WF/PC) aboard NASA's Hubble Space Telescope HST, reveal surprising and complex new structure in a dusty nebula surrounding the star. New details include a jet of material flowing away from the star and a regularly spaced set of wave-like features. The results are being reported on May 17, 1991 at the Hubble Space Telescope Science Writer's Workshop in Baltimore Md.
The opportunity to study such an unusual star at a time when it is highly unstable makes Eta Carinae a unique and important "Rosetta Stone” for understanding the evolution and physics of the massive stars that play a fundamental role in the structure of the galaxy.
Astronomers have long known that Eta Carinae is an extremely variable star. During an outburst in 1843 Eta Carinae reached a visual magnitude of - 1, making it the second brightest star in the night sky, behind only Sirius (it has since faded to sixth magnitude, the limit of naked-eye visibility). During that outburst, Eta Carinae also expelled a large amount of gas into the surrounding interstellar medium. This material can be seen from the ground today as a small oblong nebula which is expanding away from the star. This bright inner nebula, dubbed the homunculus (or "little man" because it has appendages that vaguely resemble a head and feet) by Enrique Gaviola in 1950, is an expanding shell of dusty stellar ejecta about two thirds of a light year across along its long axis.
Until recently astronomers thought the star itself had exploded in 1843. However, infrared light images taken in 1969 show that Eta Carinae is the brightest object in the sky behind only the Sun and the Moon. This means the star is still there, but it is now hidden inside a dense dusty cloak of its own making. (Much as yellow or red light can better be seen through the fog than blue light, infrared light is better able to penetrate clouds of interstellar dust than is visible light).
The new Eta Carinae image shows that the homunculus is clumpy down to the limit of HST’s resolution, which reveals individual clumps only about ten times the size of our Solar System. The fact that the homunculus is clumpy on such small scales, and has such a well defined edge, suggests that it is a very thin and well defined shell (rather than a filled volume) of material. Such a dusty shell could either have been ejected in a single burst from the star, or swept up from material surrounding the star. In either case, the new image shows that the shell has become fragmented as a result of the combination of radiation pressure and stellar wind which drove it away from the star.
The ridge of emission to the southwest (lower right) of the homunculus is due to material which was ejected from the star at velocities in excess of 5 million kilometers (3 million miles) per hour. As this ejecta slams into slower moving gas (which was perhaps expelled by Eta Carinae at some earlier time), shock waves heat the gas and cause it to glow. Previous studies of the spectrum of the light emitted from the southwest ridge show that this gas is enriched in elements such as nitrogen which are formed in the interiors of massive stars. The new image clearly shows the small knots and filaments which trace the locations of the shock fronts. The new image also shows that this ridge is in fact part of a "cap" of material located to the southwest and behind the star.
The most remarkable discovery resulting from the new data is that there is a well collimated jet of material flowing away from Eta Carinae. This jet can be seen as two parallel lines pointing to the northeast (upper left) away from the star. These two lines mark the edges of the jet, which appears to be a narrow tube-like structure. The jet terminates in an inverted U-shaped feature, which is the bow shock (much like the bow wave of a boat) that the jet is driving into ambient material surrounding the star.
Stellar jets and their associated bow shocks are often seen in association with star formation. However the existence of a jet emanating from Eta Carinae came as a complete surprise. In other stars, jets are thought to occur because the disks of material from which stars form confine the flow away from the star into two thin beams. Such a disk might account for the elongation of the homunculus, but the previously proposed disk is at a right angle to the disk required to explain the newly discovered jet.
The new image also reveals the presence of a fascinating "ladder-like" structure associated with the jet. While this feature is not understood in detail, it is clear that the "rungs" of the ladder represent some kind of wave phenomenon in the flow of material away from the star. One possibility is that these are standing waves, much like sound waves inside an organ pipe. Another possibility is that they are ripples in the flow of material along the jet's bow shock, much like the ripples seen in the water moving away from the point where the stream from a faucet hits the bottom of a sink.
Prior to HST's observation the homunculus was thought to be a "bipolar outflow" of material moving to the northwest and southeast (upper right and lower left) away from Eta Carinae. This previous picture is put into question in light of HST's discoveries: the jet to the northeast of the star, the cap of material to the southwest, and the thinness and dumpiness of the shell, as all revealed by the HST, suggests that the axis of the Eta Carinae system is instead northeast to southwest (upper left to lower right). In this new model the jet and the cap represent material blown out the poles of the system, while the homunculus is an oblate equatorial shell.
Astronomers currently estimate that Eta Carinae has a mass over 100 times that of our Sun, and a luminosity of 4 million times that of the Sun, making it one of the most massive and luminous stars in the Milky Way Galaxy. Located at a distance of around 9,000 light years from the Earth, Eta Carinae is also relatively nearby.
HST's new detailed view of Eta Carinae may provide important clues relating to the evolution of another class of high-mass stars called "Wolf-Rayet stars." At least one such star is surrounded by a similar (but much larger) oblate shell with uneven polar blowouts. This object (called NGC 6888) may be the more evolved version of a shell much like that surrounding Eta Carinae.
Based on the new image a paper with these conclusions is to be published in the August edition of The Astronomical Journal.
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