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R. N. Mohapatra
was just noted. This then points to the fact that there must have been an
r-process in this galaxy. However, if the r-process took place in supernovae in
this galaxy, that would require too many supernovae to produce the amount of
these heavy elements observed and there are not enough stars in them to meet
the requirement. So the r-process most likely took place in a binary neutron
star merger, and one such merger is enough to produce the amount of heavy
element abundance observed. The fact that neutron stars merge is also now
observationally established by the gravitational wave signal from a kilonova
event by LIGO and Virgo (GW170817) observatories in 2017. This makes the
case stronger for neutron star mergers as the source of the r-process.
The figure below illustrates a binary neutron star merger (Fig. 19.4). The
scientists are working on how the details of r-process actually take place in a
binary neutron star merger. To quote the authors Frebel and Beers “Nuclear
physicists are still working to model the r-process, and astrophysicists need
to estimate the frequency of neutron-star mergers to assess whether r-process
heavy-element production solely or at least significantly takes place in the
merger environment.” To be sure, there are also several other sites which
are under active consideration as active r-process sites for heavy nucleus formation [48]—for instance, binary black hole-neutron star mergers. Another
possibility is the so-called magneto-hydrodynamic jet model where magnetic
turbulence in a supernova leads to an ejection of neutron rich material in a
jet. This does not have the complication of excessive neutrino density of the
conventional supernova r-process and can therefore facilitate heavy nucleosynthesis. Other possibilities include carbon-enhanced metal-poor (CEMP) stars
in the Galactic Halo (the environment inside a galaxy) or collapsars (collapsers
are failed supernovae with a black hole at the center). Uncovering the sites of
the r-process is an extremely active area of research and will go on for a while
before we know the final answer to this.
1
Also, some elements are formed by cosmic ray collision with light nuclei
in the stars, although, this is only a small contribution to the abundance of
elements like lithium, beryllium, and boron. The cosmic rays come mostly
from extra-galactic space and are quite energetic. So they tend to break up
nuclei. Astronomy research that goes on in this field requires meticulous
detective work. It involves searching around our galaxy to locate stars with
a high abundance of certain heavy elements and looking at the history of the
star to determine what could have happened to create this environment. It
must be stated though that despite all the active heavy element formation in
various sites in the universe, most of the universe is still only hydrogen and
helium, with only 2% being heavier elements.
1 A parsec is about 3.2 light years or 3 × 10 18 cm—i.e. light takes 3.2 years to travel the distance of 1 parsec.
R. N. Mohapatra
was just noted. This then points to the fact that there must have been an
r-process in this galaxy. However, if the r-process took place in supernovae in
this galaxy, that would require too many supernovae to produce the amount of
these heavy elements observed and there are not enough stars in them to meet
the requirement. So the r-process most likely took place in a binary neutron
star merger, and one such merger is enough to produce the amount of heavy
element abundance observed. The fact that neutron stars merge is also now
observationally established by the gravitational wave signal from a kilonova
event by LIGO and Virgo (GW170817) observatories in 2017. This makes the
case stronger for neutron star mergers as the source of the r-process.
The figure below illustrates a binary neutron star merger (Fig. 19.4). The
scientists are working on how the details of r-process actually take place in a
binary neutron star merger. To quote the authors Frebel and Beers “Nuclear
physicists are still working to model the r-process, and astrophysicists need
to estimate the frequency of neutron-star mergers to assess whether r-process
heavy-element production solely or at least significantly takes place in the
merger environment.” To be sure, there are also several other sites which
are under active consideration as active r-process sites for heavy nucleus formation [48]—for instance, binary black hole-neutron star mergers. Another
possibility is the so-called magneto-hydrodynamic jet model where magnetic
turbulence in a supernova leads to an ejection of neutron rich material in a
jet. This does not have the complication of excessive neutrino density of the
conventional supernova r-process and can therefore facilitate heavy nucleosynthesis. Other possibilities include carbon-enhanced metal-poor (CEMP) stars
in the Galactic Halo (the environment inside a galaxy) or collapsars (collapsers
are failed supernovae with a black hole at the center). Uncovering the sites of
the r-process is an extremely active area of research and will go on for a while
before we know the final answer to this.
1
Also, some elements are formed by cosmic ray collision with light nuclei
in the stars, although, this is only a small contribution to the abundance of
elements like lithium, beryllium, and boron. The cosmic rays come mostly
from extra-galactic space and are quite energetic. So they tend to break up
nuclei. Astronomy research that goes on in this field requires meticulous
detective work. It involves searching around our galaxy to locate stars with
a high abundance of certain heavy elements and looking at the history of the
star to determine what could have happened to create this environment. It
must be stated though that despite all the active heavy element formation in
various sites in the universe, most of the universe is still only hydrogen and
helium, with only 2% being heavier elements.
1 A parsec is about 3.2 light years or 3 × 10 18 cm—i.e. light takes 3.2 years to travel the distance of 1 parsec.