Theatre in the Rectangle
The great sculptor gravity ensures that larger, denser objects in the universe -- like stars, planets and moons -- take on the familiar spherical shape. Occasionally, though, an interplay of mysterious cosmic forces leaves objects outside the round, those with more geometrically curious forms. One such object, discovered in 1975, is the Red Rectangle.

Now astronomers Dr. Raghvendra Sahai of NASA's Jet Propulsion Laboratory, Pasadena, Calif., and Dr. Carmen Sanchez Contreras of the California Institute of Technology, also in Pasadena, have uncovered a physical mechanism that might support this bizarre red rectangular structure. They report their findings in a paper titled "A Very Young, Fast, Bipolar Outflow at the Center of the Red Rectangle" at the 2003 summer meeting of the American Astronomical Society, being held this week in Nashville, Tennessee.

A protoplanetary nebula forms when a red giant star ejects most of its outer layers, and is observable when dust within the nebula reflects light from the central star. Subsequently, as the very hot core (six or more times hotter than the Sun) gets further exposed, the cloud of ejected material becomes bathed with ionizing ultraviolet light, which makes it glow. At this later stage these objects are known as planetary nebulas.

The Red Rectangle, or HD44179, represents the crimson light from a protoplanetary nebula that lurks about 1,000 light years from Earth. Images taken with the sharp eyes of the Hubble Space Telescope show that two bright diagonals intersect its roughly rectangular shape; the diagonals appear connected by straight lines parallel to the short side of the rectangle.

An image of the Red Rectangle and its bipolar outflows may be seen on the Web:

Coming up with a physical mechanism to produce these oddly shaped nebulas, and the Red Rectangle in particular, has been a serious challenge for astronomers. Many recent studies of such objects with the Hubble have led to a new model by Sahai in which very fast jet-like outflows carve out these diverse shapes.

The jets are thought to operate for a very short period of time -- only a few hundred or so years -- during the protoplanetary phase, making studies of protoplanetary nebulas very important for understanding the nature and origin of the jets. Finding direct evidence for these jet-like outflows has been generally very difficult, because they are compact, occasionally dormant, and difficult to see against the bright nebular background. Fortunately, the Hubble's imaging spectrograph provides a way to discriminate these outflows by separating the nebular light into its constituent colors (like a prism revealing the rainbow of colors in sunlight).

The violent interaction of these outflows with the surrounding dense nebular gas causes hydrogen atoms in the outflow (but not the general nebula) to emit light at a specific wavelength (called H-alpha radiation) that is detected by the imaging spectrograph. A small deviation, or shift, in the measured wavelength from the laboratory value allows researchers to find out how fast, and in which direction (i.e. towards or away from the observer), the material in the high-speed outflow moves.

After examining archival data on the Red Rectangle, Sahai and Sanchez Contreras have found evidence of H-alpha radiation emitted by a fast outflow expanding with a speed of about 65 kilometers per second (40 miles per second). This is the first time this inner, bipolar outflow has been seen in this object.

"Discovering this outflow in the famous, yet mysterious Red Rectangle is very exciting because it provides us an unprecedented opportunity to study the fast outflows believed to shape such objects," said Sahai. "The outflow is very young and its innermost parts are only a few years old, so we will be able to directly observe if, and how, it changes with time."

One theory about the formation of such fast outflows requires the central star to have a close companion object. Strong evidence exists that the central star in the Red Rectangle has such a companion.

A full understanding of this outflow and how it supports the intriguing shape of the Red Rectangle will require further observations and detailed computer modeling.

The Space Telescope Imaging Spectrograph is managed by NASA's Goddard Space Flight Center in Greenbelt, Maryland. The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency.

Written by Paul Morledge

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