'Ageless' silicon throughout Milky Way may indicate a well-mixed galaxy

April 26, 2017

As galaxies age, some of their basic chemical elements can also show signs of aging. This aging process can be seen as certain atoms "put on a little weight," meaning they change into heavier isotopes—atoms with additional neutrons in their nuclei.

Surprisingly, new surveys of the Milky Way with the National Science Foundation's (NSF) Green Bank Telescope (GBT) in West Virginia, found no such aging trend for the element silicon, a fundamental building block of rocks throughout our solar system. This "ageless" appearance may mean that the Milky Way is more efficient at mixing its contents than previously thought, thereby masking the telltale signs of chemical aging.

When massive first-generation stars in young end their lives as violent supernovas, they fill the cosmos with so-called primary isotopes—elements like oxygen, carbon, and silicon with a balance of neutrons and protons in their nuclei.

"Massive stars are the cauldrons in which heavy elements like silicon are fabricated," said Ed Young, a scientist at the University of California at Los Angeles and author on a study appearing in the Astrophysical Journal. "First-generation stars make silicon 28—an isotope with 14 protons and 14 neutrons in its nucleus. Over billions of years, later generations of stars are able to create the heavier silicon 29 and 30 isotopes. When these later-generation stars explode as supernovas, the heavier isotopes are blasted into the , subtly altering the chemical profile of the galaxy."

Astronomers cannot directly measure these long-term chemical changes. They can, however, do the next best thing: measuring the apparent maturing of isotopes from the outskirts of our galaxy toward its center.

Since there is a greater concentration of the closer you get to the center of the Milky Way, including that end of their lives as supernovas, astronomers expect to find a greater percentage of heavier isotopes among the elements there.

Past radio telescope studies of carbon and oxygen atoms in the Milky Way provided some indication that there is in fact a steady progression from light to heavy isotopes the closer you move toward the .

Intervening interstellar clouds, however, made these observations difficult and the results were inconclusive.

"There were some tantalizing hints in past studies that carbon and oxygen isotope ratios shifted as expected. But it was difficult to account for the material in the interstellar medium, so we were uncertain how reliable these data were," said Young. "Silicon, as detected in molecules of silicon monoxide, has a spectral signature that makes it much easier to account for the dust and gas in our galaxy. We therefore had to make fewer assumptions than were necessary for the surveys done for oxygen and carbon."

Using the 100-meter GBT, the astronomers surveyed vast swaths of the Milky Way, starting from the region near our sun and then moving all the way toward the galactic center. In each region, they probed the radio spectra naturally emitted by silicon monoxide molecules. Differences in the would be seen as subtle changes in the radio spectra.

Counter to their expectations, the researchers found none of the expected gradient in the isotope ratios.

"There was no evidence of a gradient," said Nathaniel Monson, a member of the research team and a graduate student at UCLA. "That was a bit surprising. We may have to reassess what we think we know about our galaxy."

These data may mean that the Milky Way is remarkably efficient at mixing its material, circulating molecules and atoms from the galactic center out into the galaxy's spiral arms and back. It is also possible that type 1a supernovas—which are formed in binary systems when a white dwarf star steals too much material from its companion and detonates—produce an overabundance of Si 28 later in the lifespan of a galaxy.

If subsequent surveys of carbon and oxygen are better able to account for past uncertainties and show the same lack of gradient, it would point to mixing as being the most likely scenario.

"There's a lot about the galaxy we don't understand yet," concluded Young. "It's possible that further studies with the GBT will teach us a bit more about the Milky Way."

Explore further: Examining exploding stars through the atomic nucleus

More information: Nathaniel N. Monson et al. Uniform Silicon Isotope Ratios Across the Milky Way Galaxy, The Astrophysical Journal (2017). DOI: 10.3847/1538-4357/aa67e6

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Tuxford
1 / 5 (5) Apr 26, 2017
Since there is a greater concentration of stars the closer you get to the center of the Milky Way, including massive stars that end of their lives as supernovas, astronomers expect to find a greater percentage of heavier isotopes among the elements there.

Counter to their expectations, the researchers found none of the expected gradient in the isotope ratios.


Ah shucks, merger maniacs are disappointed once again.

Actually, stars are moving away from our central core star, not toward it. Didn't you maniacs pay attention this story years ago?

https://phys.org/...ays.html
Bigbangcon
1 / 5 (3) Apr 27, 2017
This is complete nonsense. In attempts to "prove" their pet theories of "Big Bang" creation, ageing (put on a little weight) etc. ad nauseum, these astrophysicists defy even commonsense to show the "mastery" of their profession and their bankruptcy at the same time!
Anybody with common sense would know that the stability of atoms or isotopes depends on their "binding energy", no matter how they are formed. In any process, the more stable the isotope of an atom is, the higher proportion of that isotope will be formed. For lower atomic number (fewer protons in the nuclei) atoms like silicon, the isotope with 1:1 ratio of protons and neutrons is the most stable, irrespective of whether it was formed 12 billion years ago or formed now. So what "ageing" (putting on more weight or more neutrons!) of the atoms, these "astrophysicists" are talking about? Do they have no shame that they lack elementary knowledge of chemistry and physics or even plain common sense?
434a
not rated yet Apr 27, 2017
This is complete nonsense.


The way I read this is in the young galaxy stars will produce the Si 28 isotope of Silicon.
Later generations of stars will produce heavier isotopes. My interpretation would be later stars would absorb some of the Si 28 created by the earlier generation of now supernovaed stars and those Si 28 isotopes would be able to capture additional neutrons or electrons - I don't know if electron capture can happen in stars but it is a way for atoms to gain neutrons https://en.wikipe..._capture
Consequently, as the galaxy ages, there becomes an ever diverse range and increasing abundance of isotopes, which by their nature, are heavier than those created in early generations of stars. One would therefore expect older galaxies to have a greater proportion of heavier isotopes and those isotopes to be clustered in the star forming regions, this paper says this is not the case and the distribution is more evenly spread than expected.
Bigbangcon
1 / 5 (1) Apr 27, 2017
"The way I read this is in the young galaxy stars will produce the Si 28 isotope of Silicon. Later generations of stars will produce heavier isotopes."

It should be rather the other way around, if at all. Proportion of a particular isotope formed would be proportional to its higher stability. If some proportion of less stable isotopes were formed (statistically) in a previous cycle of production, their proportion will be less in the next cycle, because of their relative instability. Formation of isotopes depends solely on the energy requirements and does not depend on the presence (concentration) of other isotopes in a stoichiometric way like a chemical equilibrium. There is no equilibrium in nuclear processes, these are instant (relatively) and irreversible processes, energy alone determines the outcome! Can you reverse or stop the decay of a radioactive nuclei to its initial state in any possible way?
434a
not rated yet May 02, 2017
It should be rather the other way around, if at all. Proportion of a particular isotope formed would be proportional to its higher stability. If some proportion of less stable isotopes were formed (statistically) in a previous cycle of production, their proportion will be less in the next cycle, because of their relative instability.


Si28 is the most stable isotope and its relative abundance -on earth- is 92.33% the other two stable isotopes are Si29 (4.67%) and Si30 (3.1%)

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