Carbon atoms make about one-fifth of all the atoms in the human body and they are key to all-important life molecules such as DNA and proteins. All carbon atoms were created in stars by fusing three helium atoms together, but astronomers are unsure which is the primary creators of carbon in our galaxy, stars that go supernova or those that eventually turn in white dwarfs?
A new study published in Nature Astronomy offers some new insights into the origin of carbon. The analysis establishes white dwarfs as an active producer of carbon as long as its original star was at least 1.5 times the mass of the Sun. White dwarfs are the exposed cores of stars that are not massive enough to go supernova. When the nuclear fuel of these stars is spent, they expand into a red giant and eventually they lose the outer layers leaving behind just the core. Around 90 percent of all stars will end their lives as white dwarfs.
Researchers believe that there is a straightforward relationship between the mass of the original star and the mass of the eventual white dwarf. This is known as the initial-final mass relation and can be tested by looking at a group of stars that are bound together, such as those in open clusters.
These stars all formed from the same giant molecular cloud roughly at the same time. Researchers can reconstruct the original mass distribution and estimate how massive the progenitors of these white dwarfs should be. However, this is where things got interesting. The initial-final mass relation had an unexpected kink.
“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,” lead author Dr Paola Marigo at the University of Padua in Italy, explained in a statement.
According to the team, the presence of carbon in the stellar interior alters the evolution of the star in one important way. The element is stripped from the stellar mantle over a longer period of time and during this interval, the core of the star, what will become the white dwarf, can continue to gain mass.
They found that stars bigger than two solar masses contributed to the galaxy's carbon, while those less than 1.5 solar masses did not, which puts constraints on the minimum mass a star must be to spread its carbon-rich material when it dies.
“One of the 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,” co-author Dr Pier-Emmanuel Tremblay of the University of Warwick, added. “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.”
These findings have consequences beyond the chemistry of the cosmos. It also tells us something about the ages of these stars and given the role of white dwarfs in cosmological studies, this will have a wide impact.