The stars of the brightest planetary nebulae

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A recent study by researchers at the Instituto de Astrofísica de Canarias (IAC) resolves an old debate about 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, PhD student at the IAC who died in 2020 while completing this work.

The first and most important thing to understand what the Universe looks like is knowing how big it is, measuring distances to galaxies. Just as in the Renaissance we began to decipher the extent of the world, where the seas and continents were located, 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, we didn’t even know that galaxies were star systems, billions of them, and it was advances in technology, ever larger telescopes and ever more sensitive instruments, that made it possible to ‘study galaxies and begin to isolate their individuality. stars. Even today, normal stars, like the Sun, outside our galaxy cannot be studied, but they can when they evolve into planetary nebulae.

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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 where the star is no longer able to support the weight of its own mass: it has run out of “fuel”. the best and most abundant he had, hydrogen, and began to tap into his supply of helium. This is when the inner core is exposed and, due to its extremely high temperature (in a few thousand years it rises from around 3,000°C to 100,000°C or more), it emits almost all of its light in the ultraviolet, violently heating the expelled gas layers until they are ionized.

“What is fascinating about this 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 which falls right where the human eye is most sensitive, in the yellow-green part of the spectrumexplains Antonio Mampaso, researcher at the IAC and co-author of the article. It is the emission line of the doubly ionized oxygen atom. [OIII] 5007 Angstroms »precise.

Planetary nebulae are key to understanding the continual 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 continual chemical enrichment of the Universe, the inexorable ticking that marks an arrow of chemical advance into the future”.

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

This constancy is such a robust property that it is used to measure distances to galaxies up 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 taking into account the different 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, moreover, that such bright nebulae should not exist in highly evolved systems, since their progenitors should be stars relatively massive, close to twice the mass of the Sun or more, which should have already disappeared in these old 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, tackled this conundrum by determining the physical and chemical parameters of the brightest planetary nebulae and their stars. as precisely as possible the progenitors of the IAC. disk of the nearest spiral galaxy, Andromeda, M31.

For this, very deep spectra of a sample of planetary nebulae from M31 were 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 density slightly above average and with progenitor stars of masses close to 1.5 times that of the Sun.

Latest Evolutionary Models

“Recent theoretical work, using the most recent evolutionary models, has suggested that stars of these masses could generate, for at least a thousand years, such luminous planetaries”Mampaso points out. “The results obtained therefore indicate that to understand the brightest nebulae, massive stars are not necessary, even if there are many of them in a galaxy like M31”it is said.

This work was led by Rebeca Galera Rosillo, an IAC PhD student who died in 2020 while completing recent research. Originally from 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 personnel in training under the supervision of ‘Antonio Mampaso and Romano Corradi. In her later years, she worked as an astronomer at Isaac Newton Telescope Group (ING) in La Palma, while completing 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 she can be today,” they point out. But in addition to his passion for astronomy, he also left us his joy and a vision of the world where music and art, solidarity with the poorest and science can combine to make a better and fairer world..

Character font: IAC, DICYT,

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

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