White dwarfs reveal new insights into the origin of carbon in the
universe

Date:
July 6, 2020
Source:
University of California - Santa Cruz
Summary:
A new analysis of white dwarf stars supports their role as a key
source of carbon in galaxies. Every carbon atom in the universe
was created by stars, but astrophysicists still debate which types
of stars are the primary source of the carbon in our galaxy. Some
studies favor low-mass stars that blew off their envelopes in
stellar winds and became white dwarfs, while others favor massive
stars that eventually exploded as supernovae.



FULL STORY ==========================================================================
A new analysis of white dwarf stars supports their role as a key source
of carbon, an element crucial to all life, in the Milky Way and other
galaxies.


========================================================================== Approximately 90 percent of all stars end their lives as white dwarfs,
very dense stellar remnants that gradually cool and dim over billions
of years. With their final few breaths before they collapse, however,
these stars leave an important legacy, spreading their ashes into the surrounding space through stellar winds enriched with chemical elements, including carbon, newly synthesized in the star's deep interior during
the last stages before its death.

Every carbon atom in the universe was created by stars, through the
fusion of three helium nuclei. But astrophysicists still debate which
types of stars are the primary source of the carbon in our own galaxy,
the Milky Way. Some studies favor low-mass stars that blew off their
envelopes in stellar winds and became white dwarfs, while others favor
massive stars that eventually exploded as supernovae.

In the new study, published July 6 in Nature Astronomy, an international
team of astronomers discovered and analyzed white dwarfs in open star
clusters in the Milky Way, and their findings help shed light on the
origin of the carbon in our galaxy. Open star clusters are groups of
up to a few thousand stars, formed from the same giant molecular cloud
and roughly the same age, and held together by mutual gravitational
attraction. The study was based on astronomical observations conducted in
2018 at the W. M. Keck Observatory in Hawaii and led by coauthor Enrico Ramirez-Ruiz, professor of astronomy and astrophysics at UC Santa Cruz.

"From the analysis of the observed Keck spectra, it was possible to
measure the masses of the white dwarfs. Using the theory of stellar
evolution, we were able to trace back to the progenitor stars and derive
their masses at birth," Ramirez-Ruiz explained.

The relationship between the initial masses of stars and their final
masses as white dwarfs is known as the initial-final mass relation, a fundamental diagnostic in astrophysics that integrates information from
the entire life cycles of stars, linking birth to death. In general, the
more massive the star at birth, the more massive the white dwarf left
at its death, and this trend has been supported on both observational
and theoretical grounds.



==========================================================================
But analysis of the newly discovered white dwarfs in old open clusters
gave a surprising result: the masses of these white dwarfs were notably
larger than expected, putting a "kink" in the initial-final mass relation
for stars with initial masses in a certain range.

"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," said lead author Paola Marigo at the University of Padua
in Italy.

In the last phases of their lives, stars twice as massive as our 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 team's 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.

Analyzing the initial-final mass relation around the kink, the researchers concluded that stars bigger than 2 solar masses also contributed to the galactic enrichment of carbon, while stars of less than 1.5 solar masses
did not. In other words, 1.5 solar masses represents the minimum mass
for a star to spread carbon-enriched ashes upon its death.

These findings place stringent constraints on how and when carbon, the
element essential to life on Earth, was produced by the stars of our
galaxy, eventually ending up trapped in the raw material from which the
Sun and its planetary system were formed 4.6 billion years ago.

"Now we know that the carbon came from stars with a birth mass of not
less than roughly 1.5 solar masses," said Marigo.

Coauthor Pier-Emmanuel Tremblay at University of Warwick said, "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. 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." By combining the theories of
cosmology and stellar evolution, the researchers concluded that bright carbon-rich stars close to their death, quite similar to the progenitors
of the white dwarfs analyzed in this study, are presently contributing
to a vast amount of the light emitted by very distant galaxies.

This light, carrying the signature of newly produced carbon, is
routinely collected by large telescopes to probe the evolution of
cosmic structures. A reliable interpretation of this light depends on understanding the synthesis of carbon in stars.


========================================================================== Story Source: Materials provided by
University_of_California_-_Santa_Cruz. Original written by Tim
Stephens. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Paola Marigo, Jeffrey D. Cummings, Jason Lee Curtis, Jason Kalirai,
Yang
Chen, Pier-Emmanuel Tremblay, Enrico Ramirez-Ruiz, Pierre Bergeron,
Sara Bladh, Alessandro Bressan, Le'o Girardi, Giada Pastorelli,
Michele Trabucchi, Sihao Cheng, Bernhard Aringer, Piero Dal
Tio. Carbon star formation as seen through the non-monotonic
initial-final mass relation.

Nature Astronomy, 2020; DOI: 10.1038/s41550-020-1132-1 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/07/200706140858.htm

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