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By analyzing the available optical spectroscopic data, astronomers have investigated a nearby planetary nebula designated IC 418, also known as the Spirograph Nebula. Results of the study, August 20 in The Astrophysical Journal Letters, shed more light on the evolution of this nebula.

Expanding shells of gas and dust that have been ejected from a star during the process of its evolution from a main sequence (MS) star into a red giant or white dwarf (WD) are known as planetary nebulae (PNe). These objects are relatively rare, but important for astronomers studying the chemical evolution of stars and galaxies.

The Spirograph Nebula is a well-known, high-surface brightness, compact, mildly elliptical PN in the constellation of Lepus, at a distance of some 4,400 light years. It has a radius of about 0.15 light years and its photo-dominated region has a mass of 0.5 solar masses. The central star of this PN (CSPN) is HD 35914—a bright O-type star with a spectral type of O7fp, and an effective temperature of 37,000 K.

Previous observations of the Spirograph Nebula have found that it has an expansion age of approximately 1,200 years, which makes it one of the youngest PNe known to date. Now, a new study by a team of astronomers led by Albert A. Zijlstra of the University of Manchester, UK, has provided more insights into the evolutionary state of this nebula.

The new study found a secular, linear increase in the strength of oxygen emission lines relative to hydrogen-beta emission lines over a period of 130 years. This increase was by a factor of 2.5 and turned out to be caused by the rising temperature of the central star.

The average heating rate of the central star of the Spirograph Nebula over the entire time interval since the star left the asymptotic giant branch (AGB) was estimated to be about 20,000–30,000 K per year. The astronomers noted that such a rapid heating rate may cause measurable ionization structure evolution of the photo-ionized nebula over decadal timescales.

In general, the collected data indicate that the Spirograph Nebula is a carbon-rich PN and its central star has evolved from an AGB carbon star. The mass of the central star was calculated to be approximately 0.57 solar masses, while the mass of its progenitor was estimated to be 1.25–1.55 solar masses.

Summing up the results, the authors of the paper underlined what implications their study has on our understanding of the evolution of post-AGB stars.

"In conclusion, we have, for the first time, shown robust, direct, secular evolution of a CSPN over an unprecedented 130-year time period. This provides an important new tracer for the evolution of post-AGB stars. Implications are that at solar metallicity the lower mass cutoff for carbon-star formation may need further consideration," the researchers conclude.

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