Astronomers have gained deeper insights into the final evolutionary stages of stars like our Sun, thanks to careful modeling of dust behavior and the thermal and ionization structures in planetary nebulae. Leveraging data from the Vainu Bappu Telescope in Kavalur, this research sheds light on the enigmatic processes that shape hydrogen-deficient stars.

Planetary nebulae, consisting of gas and dust shells, are expelled by stars after they deplete their hydrogen and helium fuel—a fate our Sun is expected to face approximately 5 billion years from now. As these stars' cores contract and heat up due to a lack of nuclear energy generation, they emit intense far-ultraviolet radiation. Historically, these nebulae appeared planet-like to early astronomers using small telescopes, giving rise to their misleading name.

While most aging stars develop cores enveloped by residual hydrogen, about 25% exhibit hydrogen deficiency, revealing helium-rich surfaces instead. Some of these stars also display significant mass loss and emission lines of ionized helium, carbon, and oxygen—features characteristic of Wolf-Rayet (WR) stars. Among these rare celestial phenomena is Planetary Nebula PN IC 2003, housing a hydrogen-deficient central remnant star of the Wolf-Rayet type.

Despite substantial progress in understanding the evolution of typical central stars in planetary nebulae, the mechanisms by which some become hydrogen-poor remain elusive. Clues to these processes lie in the surrounding nebular gas, necessitating detailed studies of their physical and chemical structures.

To address this mystery, researchers from the Indian Institute of Astrophysics (IIA), an autonomous institute under the Department of Science and Technology, observed PN IC 2003 using the optical medium-resolution spectrograph (OMR) attached to the 2.3-meter Vainu Bappu Telescope at the Vainu Bappu Observatory in Tamil Nadu.

“We also utilized ultraviolet spectra from the IUE satellite and broadband infrared fluxes from the IRAS satellite archives,” explained K. Khushbu, the study’s lead author and a Ph.D. student at IIA. These combined observations allowed the team to delineate the relative roles of gas and dust in shaping the thermal structure of the nebula, ultimately enabling them to derive precise parameters for the central star.

The models revealed significant deviations in the derived parameters of the nebula and its ionizing source, including mass and temperature, compared to dust-free models. “This study underscores the critical role of dust grains in maintaining the thermal balance of ionized gas and explains the temperature variations needed to resolve abundance discrepancies observed in astrophysical nebulae,” noted Prof. C. Muthumariappan, the study’s supervisor and co-author.

Using the one-dimensional dusty photo-ionization code CLOUDY17.3, the researchers simulated ultraviolet, optical, and infrared data. By accurately modeling the photo-electric heating of the nebula by dust grains, they reproduced the thermal structure observed in PN IC 2003. “We were even able to replicate the large temperature gradient commonly seen in nebulae with Wolf-Rayet stars. Our determinations of elemental abundances—such as helium, nitrogen, and oxygen—differed significantly from empirically obtained values,” Khushbu added.

Additionally, the team derived accurate grain size distributions within the nebula and emphasized the importance of photo-electric heating in explaining temperature variations. The study also determined the luminosity, temperature, and mass of the central star, estimating its initial mass to be 3.26 times that of the Sun, indicative of a higher-mass progenitor.

This research not only advances our understanding of planetary nebulae but also provides a window into the complex end-of-life processes of stars similar to our Sun, paving the way for future explorations into the cosmos.