Background on Planetary Nebulae and OH231.8+4.2:

The object OH231.8+4.2 is a star entering the final stages of life. It is going through spectacular death throes as it evolves into what astronomers call a "planetary nebula." A nebula is a visibly diffuse region composed of gas and dust. The term planetary nebula came about not because of any real association with planets, but because in early telescopes objects of this type often appeared planet like to the astronomer – even glowing with a bright green color. We now know that green color to be from very hot oxygen atoms in the gas surrounding the central star.

After a star has depleted the majority of its nuclear fuel, profound changes occur as it enters a poorly understood phase of evolution. First, a combination of stellar pulsations and radiation pressure drives the atmosphere outward, forming an extended envelope around the star. The envelope can be so large that, if such a star were our Sun, the gas and dust expelled from the star might extend many times farther out than the average distance of the planet Pluto to the Sun (this will in fact be the Sun's fate). During this period the star loses material at very high rates; a star several times the mass of the Sun might shed an amount equal to the total mass of the Sun in less than 10,000 years. The wind that propels the envelope has speeds in excess of 43,000 miles per hour.

The gas in the circumstellar envelope is mostly made up of simple molecules such as molecular hydrogen and carbon monoxide, combined with several other gases such as cyanide, sodium chloride, and possibly water vapor. Complex hydrocarbon molecules are also known to be present in circumstellar envelopes. Most importantly, the material cast off during this phase of stellar evolution includes a large abundance of the key elements to the origin of terrestrial life – carbon, nitrogen, and oxygen. These elements are created through nuclear fusion of hydrogen and helium in the stellar core.

While the envelope is being formed and ejected into the interstellar medium, the central star of the young planetary nebula continues to evolve. Its surface heats to temperatures in excess of 360,000^o F. The increase in ultraviolet radiation as the star heats first dissociates the molecules in the envelope and then ionizes the constituent atoms. This transition phase is very short – perhaps less than 1000 years.

See also this very good tutorial on the late stages of stellar evolution here.

HST and NICMOS Imaging; The Infrared View of OH231.8+4.2:

In February 1997, following a spectacular nighttime launch, astronauts aboard STS-82, the 22nd mission of the Space Shuttle Discovery, installed a new set of instruments in the Hubble Space Telescope. One of those instruments, the Near-Infrared Camera and Multiobject Spectrometer (NICMOS) is designed to observe at wavelengths outside our normal view – the infrared – giving HST new eyes on the Universe.

The object shown in this NASA/ESA Hubble Space Telescope image is a remarkable example of a star going through death throes just as it dramatically transforms itself from a normal red giant star into a planetary nebula. This process happens so quickly that such objects are quite rare, even though astronomers believe that most stars like the Sun will eventually go through such a phase.

This star, with the prosaic name of OH231.8+4.2, is seen in this infrared picture blowing out gas and dust in two opposite directions. So much dust has been cast off and now surrounds the star it cannot be seen directly, only its starlight that is reflected off the dust. The flow of gas is very fast, with a velocity up to 450,000 miles/hour (700,000 km/hour). With extreme clarity, this Hubble Near Infrared Camera and Multi-Object Spectrometer image reveals that the fast-moving gas and dust are being collimated into several thin streamers and a jet-like structure, which can be seen extending away from center of the image. Wisps of material in jet-like streamers appear to strike some dense blobs of gas. This interaction must produce strong shock waves in the gas.

Observations by radio astronomers have found many unusual molecules in the gas around this star, including many containing sulfur, such as hydrogen sulfide and sulfur dioxide. These sulfur compounds are believed to be produced in the shock waves passing through the gas. Because of the large amount of sulfur compounds, this object has earned the nickname "The Rotten Egg” Nebula.

These NICMOS data pose a serious challenge to astrophysical theorists: How can a star generate such tightly collimated streams of gas and dust and accelerate them to such very high velocities? William B. Latter from California Institute of Technology and his group are using these data to obtain a better understanding of the detailed structure in the outflowing material, look for evidence for the origin of the thin streamers and jets, and learn more about the star itself. This information will give astronomers a more complete understanding of the final stages in the life of stars like our Sun.

These HST data were acquired as part of an HST/NICMOS study of compact planetary nebulae and proto-planetary nebulae (Objects with central stars still too cool for it to ionize the core region.). This program is funded by grants from the National Aeronautics and Space Administration.
 

Investigators on this program are:

William B. Latter (SIRTF Science Center/Caltech)
John H. Bieging (Steward Observatory)
Aditya Dayal (IPAC/JPL)
Douglas M. Kelly (Steward Observatory)
Joseph L. Hora (Harvard/Smithsonian Center for Astrophysics)
Casey Meakin (Steward Observatory)
A.G.G.M. Tielens (Kapteyn Astronomical Institute)
 

"If I have seen farther than others, it is because I was standing on the shoulders of giants."

                                                                                           – Isaac Newton

For Rick.

Factoids:
                          Object name: OH231.8+4.2
                          Common name: The Rotten Egg Nebula
                          Object type: Galactic proto-planetary nebula
                          R.A.:   7h 42m 16.8s (J2000)
                          Dec.:  -14deg 42min 52sec (J2000)
                          Spectral type: M9 III
                          Approximate distance: 1500 parsecs
                          Constellation: Puppis
                          Instruments: HST and NICMOS

The color composite image is made data taken in these NICMOS filters:

                          Red: K-band (F205W), or 2.05 microns
                          Green: Hydrogen (F187N+F190N), or 1.87 and 1.90 microns
                          Blue: H-band (F160W), or 1.60 microns

The color-mapped (reddish) image is  K-band (F205W), or 2.05 microns.

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