The Beautiful Death: the Halos of Planetary Nebulae Revealed

Only a hot stellar core remains in the form of a white dwarf; the rest of the star is dispersed into the interstellar medium, enriching it with chemically processed elements, such as carbon, that is found in all living organisms on Earth.

These elements were cooked in the stellar furnace during a stellar life span covering billions of years. The high-energy radiation from the hot white dwarf makes the blown gas to shine for a short period of time, and the result is one of the most colourful and beautiful astronomical objects: a planetary nebula.

The complex history of mass loss

The events which lead to the formation of a planetary nebula develop in two phases that finally induce a structure composed of a denser, inner region –the planetary nebula itself– and an external fainter halo, that consists of the ionized stellar wind. All together, the blowing of this material is performed in a relatively short time, in astronomical terms, and the planetary nebula is visible only during a few thousand years. For this reason there are not many of these objects available for study.

External halos of planetary nebulae are faint and difficult to study, but they can provide a wealth of information on the physical properties of the final mass loss stage of the dying star. Although there is progress in understanding both stellar evolution and mass loss theoretically, observational details of, in particular, the last phase of the mass loss process have remained obscure. Classical astronomical spectrographs and other instruments are able to study only a few points of such faint and extended objects, making the analysis of these halos an extremely cumbersome, or even impossible task.

Integral field spectroscopy to the rescue

Through the new technique of integral field spectroscopy it is possible to obtain hundreds of spectra across a relatively large area of the sky, and this opens new prospects for the analysis of extended objects, such as planetary nebulae. Calar Alto Observatory has one of the world's best integral field spectrographs, PMAS (Potsdam Multi-Aperture Spectrophotometer), attached to its 3.5 m telescope.

In a research article, that was just published in the journal Astronomy and Astrophysics, a research team from the Astrophysical Institute in Potsdam, lead by C. Sandin, has used PMAS to study the two-dimensional structure of a selected set of five planetary nebulae in our Galaxy: the Blue Snowball Nebula (NGC 7662), M2-2, IC 3568, the Blinking Planetary Nebula (NGC 6826) and the Owl Nebula (NGC 3587).

The halos of planetary nebulae revealed

For four of these objects the research team derived a temperature structure, which extended all the way from the central star and out into the halo, and found, in three cases, that the temperature increases steeply in the inner halo. According to Sandin, “The appearance of such hot halos can be readily explained as a transient phenomenon which occurs when the halo is being ionized.” Another remarkable result of this study is that it has been possible, for the first time, to measure the mass loss history of the final evolution of the stars which produced the planetary nebulae.

Sandin says that “In comparison to other methods which measure mass loss rates, our estimates are made directly on the gas component of the stellar wind.” The results allow important insights on how mass is lost in time, and the researchers found that “the mass loss rate increases by a factor of about 4-7 during the final, say, 10 000 years of mass loss.”

The research team plans to continue with this study of the final evolutionary phases of low mass stars, and have observed planetary nebulae in the Magellanic Clouds. As the authors argue “on the theoretical side the results of our studies should provide a challenging basis for further improvement of models of stellar winds.”