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Perhaps the greatest tool astronomers have is the ability to look backward in time. Since starlight takes time to reach us, astronomers can observe the history of the cosmos by capturing the light of distant galaxies. This is why observatories such as the James Webb Space Telescope (JWST) are so useful. With it, we can study in detail how galaxies formed and evolved. We are now at the point where our observations allow us to confirm long-standing galactic models, as a recent study shows.

This particular model concerns how galaxies become chemically enriched. In the early universe, there was mostly just hydrogen and helium, so the first stars were massive creatures with no planets. They died quickly and spewed heavier elements, from which more complex stars and planets could form. Each generation adds more elements to the mix. But as a galaxy nurtures a menagerie of stars from blue supergiants to red dwarfs, which stars play the greatest role in chemical enrichment?

One model argues that it is the most massive stars. This makes sense because giant stars explode as supernovae when they die. They toss their enriched outer layers deep into space, allowing the material to mix within great molecular clouds from which new stars can form. But about 20 years ago, another model argued that smaller, more sunlike stars played a greater role.

The Cat’s Eye nebula is a remnant of an AGB star. Credit: ESA, NASA, HEIC and the Hubble Heritage Team, STScI/AURA

Stars like the Sun don’t die in powerful explosions. Billions of years from now, the Sun will swell into a red giant star. In a desperate attempt to keep burning, the core of a sun-like star heats up significantly to fuse helium, and its diffuse outer layers swell. On the Hertzsprung-Russell diagram, they are known as asymptotic giant branch (AGB) stars. While each AGB star might toss less material into interstellar space, they are far more common than giant stars. So, the model argues, AGB stars play a greater role in the enrichment of galaxies.

Both models have their strengths, but proving the AGB model over the giant star model would prove difficult. It’s easy to observe supernovae in galaxies billions of light years away. Not so much with AGB stars. Thanks to the JWST, we can now test the AGB model.

Using JWST the study looked at the spectra of three young galaxies. Since the Webb’s NIRSpec camera can capture high-resolution infrared spectra, the team could see not just the presence of certain elements but their relative abundance. They found a strong presence of carbon and oxygen bands, which is common for AGB remnants, but also the presence of more rare elements such as vanadium and zirconium. Taken altogether, this points to a type of AGB star known as thermally pulsing AGBs, or TP-AGBs.

Many red giant stars enter a pulsing phase at the end of their lives. The hot core swells the outer layers, things cool down a bit, and gravity compresses the star a bit, which heats the core, and the whole process starts over. This study indicates that TP-AGBs are particularly efficient at enriching galaxies, thus confirming the 20-year-old model.

Reference: Lu, Shiying, et al. “Strong spectral features from asymptotic giant branch stars in distant quiescent galaxies.” Nature Astronomy (2024): 1-13.

The post Webb Confirms a Longstanding Galaxy Model appeared first on Universe Today.

Neutron stars are extraordinarily dense objects, the densest in the Universe. They pack a lot of matter into a small space and can squeeze several solar masses into a radius of 20 km. When two neutron stars collide, they release an enormous amount of energy as a kilonova.

That energy tears atoms apart into a plasma of detached electrons and atomic nuclei, reminiscent of the early Universe after the Big Bang.

Even though kilonova are extraordinarily energetic, they’re difficult to observe and study because they’re transient and fade quickly. The first conclusive kilonova observation was in 2017, and the event is named AT2017gfo. AT stands for Astronomical Transient, followed by the year it was observed, followed by a sequence of three letters that are assigned to uniquely identify the event.

New research into AT2017gfo has uncovered more details of this energetic event. The research is “Emergence hour-by-hour of r-process features in the kilonova AT2017gfo.” It’s published in the journal Astronomy and Astrophysics, and the lead author is Albert Sneppen from the Cosmic Dawn Center (DAWN) and the Niels Bohr Institute, both in Copenhagen, Denmark.

A kilonova explosion creates a spherical ball of plasma that expands outward, similar to the conditions shortly after the Big Bang. Plasma is made up of ions and electrons, and the intense heat prevents them from combining into atoms.

However, as the plasma cools, atoms form via nucleosynthesis, and scientists are intensely interested in this process. There are three types of nucleosynthesis: slow neutron capture (s-process), proton process (p-process), and rapid neutron capture (r-process). Kilonovae form atoms through the r-process and are known for forming heavier elements, including gold, platinum, and uranium. Some of the atoms they form are radioactive and begin to decay immediately, and this releases the energy that makes a kilonova so luminous.

This study represents the first time astronomers have watched atoms being created in a kilonova.

“For the first time we see the creation of atoms.”

Rasmus Damgaard, co-author, PhD student at Cosmic DAWN Center

Things happen rapidly in a kilonova, and no single telescope on Earth can watch as it plays out because the Earth’s rotation removes it from view.

“This astrophysical explosion develops dramatically hour by hour, so no single telescope can follow its entire story. The viewing angle of the individual telescopes to the event is blocked by the rotation of the Earth,” explained lead author Sneppen.

This research is based on multiple ground telescopes that each took their turn watching the kilonova as Earth rotated. The Hubble also contributed observations from its perch in low-Earth orbit.

“But by combining the existing measurements from Australia, South Africa and The Hubble Space Telescope, we can follow its development in great detail,” Sneppen said. “We show that the whole shows more than the sum of the individual sets of data.”

As the plasma cools, atoms start to form. This is the same thing that happened in the Universe after the Big Bang. As the Universe expanded and cooled and atoms formed, light was able to travel freely because there were no free electrons to stop it. AT2017gfo produced

The research is based on spectra collected from 0.5 to 9.4 days after the merger. The observations focused on optical and near-infrared (NIR) wavelengths because, in the first few days after the merger, the ejecta is opaque to shorter wavelengths like X-rays and UV. Optical and NIR are like open windows into the ejecta. They can observe the rich spectra of newly-formed elements, which are a critical part of kilonovae.

This figure from the research shows how different telescopes contributed to the observations of AT2017gfo. Image Credit: Sneppen et al. 2024.

The P Cygni spectral line is also important in this research. It indicates that a star, or in this case, a kilonova, has an expanding shell of gas around it. It’s both an emission line and an absorption line and has powerful diagnostic capabilities. Together, they reveal velocity, density, temperature, ionization, and direction of flow.

Strontium plays a strong role in this research and in kilonovae. It produces strong emission and absorption features in Optical/NIR wavelengths, which also reveal the presence of other newly formed elements. These spectral lines do more than reveal the presence of different elements. Along with P Cygni, they’re used to determine the velocity of the ejecta, the velocity structures in the ejecta, and the temperature conditions and ionization states.

The spectra from AT2017gfo are complex and anything but straightforward. However, in all that light data, the researchers say they’ve identified elements being synthesized, including Tellurium, Lanthanum, Cesium, and Yttrium.

“We can now see the moment where atomic nuclei and electrons are uniting in the afterglow. For the first time we see the creation of atoms, we can measure the temperature of the matter and see the micro physics in this remote explosion. It is like admiring the cosmic background radiation surrounding us from all sides, but here, we get to see everything from the outside. We see before, during and after the moment of birth of the atoms,” says Rasmus Damgaard, PhD student at Cosmic DAWN Center and co-author of the study.

“The matter expands so fast and gains in size so rapidly, to the extent where it takes hours for the light to travel across the explosion. This is why, just by observing the remote end of the fireball, we can see further back in the history of the explosion,” said Kasper Heintz, co-author and assistant professor at the Niels Bohr Institute.

The kilonova produced about 16,000 Earth masses of heavy elements, including 10 Earth masses of the elements gold and platinum.

Neutron star mergers also create black holes, and AT2017gfo created the smallest one ever observed, though there’s some doubt. The gravitational wave GW170817 is associated with the kilonova and was detected by LIGO in August 2017. It was the first time a GW event was seen in conjunction with its electromagnetic counterpart. Taken together, the GW data and other observations suggest that a black hole was created, but overall, there’s uncertainty. Some researchers think a magnetar may be involved.

This artist’s illustration shows a neutron star collision that, in addition to the radioactive fire cloud, leaves behind a black hole and jets of fast-moving material from its poles. Illustration: O.S. SALAFIA, G. GHIRLANDA, CXC/NASA, GSFC, B. WILLIAMS ET AL

Kilonovae are complex objects. They’re like mini-laboratories where scientists can study extreme nuclear physics. Kilonovae are important contributors of heavy elements in the Universe, and researchers are keen to model and understand how elements are created in these environments.

The post The Aftermath of a Neutron Star Collision Resembles the Conditions in the Early Universe appeared first on Universe Today.

Today’s photos are from California tidepools and were taken by UC Davis math professor Abigail Thompson, a recognized “hero of intellectual freedom.” Abby’s notes and IDs are indented, and you can enlarge the photos by clicking on them.

September-October tidepools (Northern California).

September and October tides are not as extreme as the tides of midsummer, and by mid-October the lowest tides occur after sunset, which altogether makes finding creatures and taking pictures a bit more challenging.  As usual I got help with some of the IDs from people on inaturalist.

Phyllocomus hiltoni: this Doctor-Suessian marine worm washed up on the beach in a clump of eelgrass.    It was tiny; the photo is through a microscope.     I already thought it was amazing, but then (see the next picture) as a bonus it also sprouted tentacles:

Phyllocomus hiltoni with frills!

Porychthis notatus: these tiny fish showed up when I turned over a rock. They were very small, I assume newly-hatched:

Porychthis notatus: close-up:

Anthopleura sola (starburst anemone), one of the more spectacular sea anemones:

Phragmatopoma californica (California sandcastle worm): These worms often live in groups and form large conglomerations of the tubes they live in (the “sandcastles”).    The black shell-like thing on the left is the worm’s operculum, like a lid to close off the top of the tube when the worm withdraws.  The next picture is a close-up of the operculum:

Operculum close-up:

Triopha maculata: nudibranch; this one looks like he’s eating the pink bryozoan, but he may just be passing over it, I’m not sure what this species eats (nudibranchs are very picky eaters):

Epiactis prolifera (brooding anemone: probably): there are a few species of Epiactis sea anemones along the California coast; prolifera is the most common:

Halosydna brevisetosa: Eighteen-scaled worm, found on the underside of a rock.   There are 18 pairs of scales, with a close-up of them in the next picture.

Close-up of scales:

Low tide on this day was about an hour after sunset, which is a lovely time to be out on the beach:

Camera info:  Mostly Olympus TG-7, in microscope mode, pictures taken from above the water.  The first picture was taken with my iphone through the eyepiece of a microscope.