The stars of the brightest planetary nebulae

A recent study led by researchers from the Instituto de Astrofísica de Canarias (IAC) resolves an old debate about which are the progenitor stars of the brightest planetary nebulae. The main author of the article, which has just been published in the journal Astronomy and Astrophysicsis Rebeca Galera Rosillo, a doctoral student at the IAC who passed away in 2020 while completing this work.

The first and most important thing to understand what the Universe is like is to know how big it is, to measure the distances to the galaxies. Just as in the Renaissance people began to decipher how big the world was, where the seas and continents were, today the Universe is mapped using distance scales that have been determined little by little, star by star, galaxy by galaxy.

Only a century ago it was not even known that galaxies were star systems, billions of them, and it has been advances in technology, ever larger telescopes and ever more sensitive instruments, that have made it possible to study the galaxies and start isolating their individual stars. Even today normal stars, such as the Sun, outside our galaxy cannot be studied, but they can when, as they evolve, they become planetary nebulae.

planetary nebulae

A planetary nebula is the gaseous envelope expelled by a star after becoming a red giant, that is, when it is in a critical phase in which the star is no longer able to support the weight of its own mass: it has run out of “fuel”. » best and most abundant he had, hydrogen, and starts using his helium reserve. It is then that the inner core is exposed and, due to its extremely high temperature (in a few thousand years, it goes from about 3,000 °C to 100,000 °C or more), it emits almost all its light in the ultraviolet, violently heating the gas layers expelled until ionized.

“What is fascinating about the case is that these envelopes, which we call planetary nebulae, convert the immense amount of ultraviolet energy generated by the star into visible light and, mainly, into an emission line that falls just where the human eye is most sensitive, into the yellow-green part of the spectrumexplains Antonio Mampaso, IAC researcher and co-author of the article. It is the emission line of the doubly ionized oxygen atom. [OIII] 5007 Angstroms”clarifies.

Planetary nebulae are key to understanding the continued chemical enrichment of the Universe

According to Romano Corradi, director of the Gran Telescopio de Canarias (GTC or Grantecan) and co-author of the article, “Planetary nebulae are key to understanding the continuous chemical enrichment of the Universe, the inexorable tick-tock that marks an arrow of chemical advance towards the future”.

And adds: “But it has also been shown that they can be used as beacons to find out the distances to galaxies, due to the fact that in any type of galaxy (spiral, elliptical, young, old…) all the brightest planetary nebulae reach the same intrinsic brightness. on the emission line [OIII] 5007 Angstrom, and they don’t go beyond that “.

This constancy is such a robust property that it is used to measure distances to galaxies out to about 70 million light-years and beyond. But researchers don’t know why the brightest planetary nebulae cluster precisely around a certain value. “magical” of luminosity, considering the various physical processes involved.

Ephemeral, but splendid

Standard theoretical models predict that the maximum luminosity of a planetary nebula should be different depending on the type of galaxy and, furthermore, that such bright nebulae should not exist in highly evolved systems, because their progenitors are expected to be relatively massive stars, close to twice the mass of the Sun or more, which should already have disappeared in such ancient systems. The observation contradicts both.

A team of eight astronomers led by the IAC and including Jorge García Rojas and David Jones, postdoctoral researchers at the IAC, have tackled this enigma by determining the physical and chemical parameters of the brightest planetary nebulae and their stars as accurately as possible progenitors in the disk of the nearest spiral galaxy, Andromeda, M31.

For this, very deep spectra of a sample of planetary nebulae from M31 have been obtained with the Grantecan, located at the Roque de los Muchachos Observatory (Garafía, La Palma). The result is that the brightest planetaries are normal nebulae, with a slightly higher density than average and with progenitor stars of masses close to 1.5 times that of the Sun.

Most up-to-date evolutionary models

“Recent theoretical work, using the most up-to-date evolutionary models, suggested that stars with these masses could generate, for at least a thousand years, such luminous planetaries”highlights Mampaso. “The results obtained indicate, therefore, that to understand the brightest nebulae, massive stars are not necessary, even if there are plenty of them in a galaxy like M31″it states.

This work was led by Rebeca Galera Rosillo, an IAC doctoral student who passed away in 2020 while completing the recent research. A native of Puebla de Don Fadrique, in Granada, Rebeca was the only female astronomer in the history of her municipality.

After completing his studies at the University of Granada, where he stood out as a student, he joined the IAC in 2014 through a predoctoral contract for research staff in training under the supervision of Antonio Mampaso and Romano Corradi. During her last years, she worked as an astronomer at the Isaac Newton Group of Telescopes (ING) in La Palma, while finishing his doctoral thesis.

“Rebeca used to say that planetary nebulae are cosmic fireflies: they don’t live long, they shine a lot and they are beautiful”remember its co-directors, Corradi and Mampaso. “In her work as an astrophysicist, she reached the frontier of knowledge, as far as it can be today,” they point out. But in addition to her passion for Astronomy, she also left us her joy and a vision of the world where music and art, solidarity with the most disadvantaged and science can come together to make a better and fairer world ».

Font: IAC, DICYT,

Reference article: https://www.dicyt.com/news/the-enigma-of-the-brightest-planetary-nebulae