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New views on the origins of carbon

Every carbon atom in the Universe was created by stars, through the fusion of three helium nuclei, but which types of stars are the primary source is the subject of an ongoing debate among astrophysicists.

Some favour low-mass stars that blew off their envelopes in stellar winds and became white dwarfs, while others favour massive stars that eventually exploded as supernovae.

Now, writing in the journal Nature Astronomy, an international team led by Paola Marigo from Italy’s University of Padova has thrown up a new talking point or two.

Using data from Hawaii’s WM Keck Observatory, they analysed white dwarfs belonging to the Milky Way’s open star clusters – groups of up to a few thousand stars held together by mutual gravitational attraction.

They measured their masses and, using the theory of stellar evolution, calculated their masses at birth. The “initial-to-final mass relation” is a fundamental diagnostic in astrophysics; in general, the more massive the star at birth, the more massive the white dwarf at its death.

However, analysis of the newly discovered white dwarfs revealed their masses were notably larger than expected.

“Our study interprets this kink in the initial-final mass relationship as the signature of the synthesis of carbon made by low-mass stars in the Milky Way,” Marigo says.

In the last phases of their lives, stars twice as massive as the Sun produced new carbon atoms in their hot interiors, transported them to the surface, and finally spread them into the interstellar medium through gentle stellar winds, the researchers say.

Their detailed stellar models indicate that the stripping of the carbon-rich outer mantle occurred slowly enough to allow the central cores of these stars, the future white dwarfs, to grow appreciably in mass.

They conclude that stars bigger than two solar masses contributed to the galactic enrichment of carbon, while stars of less than 1.5 solar masses did not. In other words, 1.5 is the minimum mass for a star to spread carbon-enriched ashes upon its death.

Such a finding places stringent constraints on how and when carbon was produced.

“One of most exciting aspects of this research is that it impacts the age of known white dwarfs, which are essential cosmic probes to understand the formation history of the Milky Way,” says co-author Pier-Emmanuel Tremblay, from the University of Warwick, UK.

“The initial-to-final mass relation is also what sets the lower mass limit for supernovae, the gigantic explosions seen at large distances and that are really important to understand the nature of the Universe.”

Related reading: Elements of surprise in new star study

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