Giant stars undergo dramatic weight loss program

Astronomers at the University of Sydney have found a slimmer type of red giant star for the first time. These stars have undergone dramatic weight loss, possibly due to the presence a greedy neighbour. Published in Nature Astronomy, the discovery is an important step forward to understanding the life of stars in the Milky Way -- our closest stellar neighbours.

There are millions of 'red giant' stars found in our galaxy. These cool and luminous objects are what our Sun will become in four billion years. For some time, astronomers have predicted the existence of slimmer red giants. After finding a smattering of them, the University of Sydney team can finally confirm their existence.

"It's like finding Waldo," said lead author, PhD candidate Mr Yaguang Li from the University of Sydney. "We were extremely lucky to find about 40 slimmer red giants, hidden in a sea of normal ones. The slimmer red giants are either smaller in size or less massive than normal red giants."

How and why did they slim down? Most stars in the sky are in binary systems -- two stars that are gravitationally bound to each other. When the stars in close binaries expand, as stars do as they age, some material can reach the gravitational sphere of their companion and be sucked away. "In the case of relatively tiny red giants, we think a companion could possibly be present," Mr Li said.

An intragalactic treasure hunt

The team analysed archival data from NASA's Kepler space telescope. From 2009 to 2013, the telescope continuously recorded brightness variations on tens of thousands of red giants. Using this incredibly accurate and large dataset, the team conducted a thorough census of this stellar population, providing the groundwork for spotting any outliers.

Two types of unusual stars were revealed: very low-mass red giants, and underluminous (dimmer) red giants.

The very low-mass stars weigh only 0.5 to 0.7 solar mass -- around half the weight of our Sun. If the very low-mass stars had not suddenly lost weight, their masses would indicate they were older than the age of the Universe -- an impossibility.

"So, when we first obtained the masses of these stars, we thought there was something wrong with the measurement," Mr Li said. "But it turns out there wasn't."

The underluminous stars, on the other hand, have normal masses, ranging from 0.8 to 2.0 solar mass. "However, they are much less 'giant' than we expect," said study co-author, Dr Simon Murphy from the University of Southern Queensland. "They've slimmed down somewhat and because they're smaller, they're also fainter, hence 'underluminous' compared to normal red giants."

Only seven such underluminous stars were found, and the authors suspect many more are hiding in the sample. "The problem is that most of them are very good at blending in. It was a real treasure hunt to find them," Dr Murphy said.

These unusual data points could not be explained by simple expectations from stellar evolution. This led the researchers to conclude that another mechanism must be at work, forcing these stars to undergo dramatic weight loss: theft of mass by nearby stars.

Stellar population census

The researchers relied on asteroseismology -- the study of stellar vibrations -- to determine the properties of the red giants.

Traditional methods to study a star are limited to their surface properties, for example, surface temperature and luminosity. By contrast, asteroseismology, which uses sound waves, probes beneath this. "The waves penetrate the stellar interior, giving us rich information on another dimension," Mr Li said.

The researchers could precisely determine stars' evolutionary stages, masses, and sizes with this method. And when they looked at the distributions of these properties, something unusual was immediately noticed: some stars have tiny masses or sizes.

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Nova outbursts are apparently a source for cosmic rays

Light on, light off -- this is how one could describe the behavior of the nova, which goes by the name RS Ophiuchi (RS Oph). Every 15 years or so, a dramatic explosion occurs in the constellation of the Serpent Bearer. Birthplaces of a nova are systems in which two very different stars live in a parasitic relationship: A white dwarf, a small, burned-out and tremendously dense star -- a teaspoon of its matter weighs about 1 ton -- orbits a red giant, an old star that will soon burn up.

The dying giant star feeds the white dwarf with matter shedding its outer hydrogen layer as the gas flows onto the nearby white dwarf. This flow of matter continues, until the white dwarf over(h)eats itself. The temperature and pressure in the newly gained stellar shells become too large and are flung away in a gigantic thermonuclear explosion. The dwarf star remains intact and the cycle begins again -- until the spectacle repeats itself.

Explosion in the high-energy range

It had been speculated that such explosions involve high energies. The two MAGIC telescopes recorded gamma rays with the value of 250 gigaelectronvolts (GeV), among the highest energies ever measured in a nova. By comparison, the radiation is a hundred billion times more energetic than visible light.

MAGIC was able to make its observations following initial alerts from other instruments measuring at different wavelengths. "The spectacular eruption of the RS Ophiuchi shows that the MAGIC telescopes' fast response really pays off: It takes them no more than 30 seconds to move to a new target," said David Green, a scientist at the Max Planck Institute for Physics and one of the authors of the paper.

Accelerated protons as a part of cosmic rays

After the explosion, several shock fronts propagated through the stellar wind from the Red Giant and the interstellar medium surrounding the binary system. These shock waves work like a giant power plant in which particles are accelerated to near the speed of light. The combined measurements suggest that the gamma rays emanate from energetic protons, nuclei of hydrogen atoms.

"This also makes nova outbursts a source of cosmic rays," explains David Green. "However, they tend to play the role of local heroes -- meaning to only contribute to the cosmic rays in the close neighborhood. The big players for cosmic rays are supernova remnants. The shock fronts created from stellar explosions are far more violent compared to novae.

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NASA's TESS tunes into an all-sky 'symphony' of red giant stars

Using observations from NASA's Transiting Exoplanet Survey Satellite (TESS), astronomers have identified an unprecedented collection of pulsating red giant stars all across the sky. These stars, whose rhythms arise from internal sound waves, provide the opening chords of a symphonic exploration of our galactic neighborhood.

TESS primarily hunts for worlds beyond our solar system, also known as exoplanets. But its sensitive measurements of stellar brightness make TESS ideal for studying stellar oscillations, an area of research called asteroseismology.

Hon presented the research during the second TESS Science Conference, an event supported by the Massachusetts Institute of Technology in Cambridge -- held virtually from Aug. 2 to 6 -- where scientists discuss all aspects of the mission. The Astrophysical Journal has accepted a paper describing the findings, led by Hon.

Sound waves traveling through any object -- a guitar string, an organ pipe, or the interiors of Earth and the Sun -- can reflect and interact, reinforcing some waves and canceling out others. This can result in orderly motion called standing waves, which create the tones in musical instruments.

Just below the surfaces of stars like the Sun, hot gas rises, cools, and then sinks, where it heats up again, much like a pan of boiling water on a hot stove. This motion produces waves of changing pressure -- sound waves -- that interact, ultimately driving stable oscillations with periods of a few minutes that produce subtle brightness changes. For the Sun, these variations amount to a few parts per million. Giant stars with masses similar to the Sun's pulsate much more slowly, and the corresponding brightness changes can be hundreds of times greater.

Oscillations in the Sun were first observed in the 1960s. Solar-like oscillations were detected in thousands of stars by the French-led Convection, Rotation and planetary Transits (CoRoT) space telescope, which operated from 2006 to 2013. NASA's Kepler and K2 missions, which surveyed the sky from 2009 to 2018, found tens of thousands of oscillating giants. Now TESS extends this number by another 10 times.

"With a sample this large, giants that might occur only 1% of the time become pretty common," said co-author Jamie Tayar, a Hubble Fellow at the University of Hawaii. "Now we can start thinking about finding even rarer examples."

The physical differences between a cello and a violin produce their distinctive voices. Similarly, the stellar oscillations astronomers observe depend on each star's interior structure, mass, and size. This means asteroseismology can help determine fundamental properties for large numbers of stars with accuracies not achievable in any other way.

"Our initial result, using stellar measurements across TESS's first two years, shows that we can determine the masses and sizes of these oscillating giants with precision that will only improve as TESS goes on," said Marc Hon, a NASA Hubble Fellow at the University of Hawaii in Honolulu. "What's really unparalleled here is that TESS's broad coverage allows us to make these measurements uniformly across almost the entire sky."

When stars similar in mass to the Sun evolve into red giants, the penultimate phase of their stellar lives, their outer layers expand by 10 or more times. These vast gaseous envelopes pulsate with longer periods and larger amplitudes, which means their oscillations can be observed in fainter and more numerous stars.

TESS monitors large swaths of the sky for about a month at a time using its four cameras. During its two-year primary mission, TESS covered about 75% of the sky, each camera capturing a full image measuring 24-by-24 degrees every 30 minutes. In mid-2020, the cameras began collecting these images at an even faster pace, every 10 minutes.

The images were used to develop light curves -- graphs of changing brightness -- for nearly 24 million stars over 27 days, the length of time TESS stares at each swath of the sky. To sift through this immense accumulation of measurements, Hon and his colleagues taught a computer to recognize pulsating giants. The team used machine learning, a form of artificial intelligence that trains computers to make decisions based on general patterns without explicitly programming them.

To train the system, the team used Kepler light curves for more than 150,000 stars, of which some 20,000 were oscillating red giants. When the neural network finished processing all of the TESS data, it had identified a chorus of 158,505 pulsating giants.

Next, the team found distances for each giant using data from ESA's (the European Space Agency's) Gaia mission, and plotted the masses of these stars across the sky. Stars more massive than the Sun evolve faster, becoming giants at younger ages. A fundamental prediction in galactic astronomy is that younger, higher-mass stars should lie closer to the plane of the galaxy, which is marked by the high density of stars that create the glowing band of the Milky Way in the night sky.

"Our map demonstrates for the first time empirically that this is indeed the case across nearly the whole sky," said co-author Daniel Huber, an assistant professor for astronomy at the University of Hawaii. "With the help of Gaia, TESS has now given us tickets to a red giant concert in the sky."

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New evidence of how and when the Milky Way came together

New research provides the best evidence to date into the timing of how our early Milky Way came together, including the merger with a key satellite galaxy.

Using relatively new methods in astronomy, the researchers were able to identify the most precise ages currently possible for a sample of about a hundred red giant stars in the galaxy.

With this and other data, the researchers were able to show what was happening when the Milky Way merged with an orbiting satellite galaxy, known as Gaia-Enceladus, about 10 billion years ago.

Their results were published today (May 17, 2021) in the journal Nature Astronomy.

"Our evidence suggests that when the merger occurred, the Milky Way had already formed a large population of its own stars," said Fiorenzo Vincenzo, co-author of the study and a fellow in The Ohio State University's Center for Cosmology and Astroparticle Physics.

Many of those "homemade" stars ended up in the thick disc in the middle of the galaxy, while most that were captured from Gaia-Enceladus are in the outer halo of the galaxy.

"The merging event with Gaia-Enceladus is thought to be one of the most important in the Milky Way's history, shaping how we observe it today," said Josefina Montalban, with the School of Physics and Astronomy at the University of Birmingham in the U.K., who led the project.

By calculating the age of the stars, the researchers were able to determine, for the first time, that the stars captured from Gaia-Enceladus have similar or slightly younger ages compared to the majority of stars that were born inside the Milky Way.

A violent merger between two galaxies can't help but shake things up, Vincenzo said. Results showed that the merger changed the orbits of the stars already in the galaxy, making them more eccentric.

Vincenzo compared the stars' movements to a dance, where the stars from the former Gaia-Enceladus move differently than those born within the Milky Way. The stars even "dress" differently, Vincenzo said, with stars from outside showing different chemical compositions from those born inside the Milky Way.

The researchers used several different approaches and data sources to conduct their study.

One way the researchers were able to get such precise ages of the stars was through the use of asteroseismology, a relatively new field that probes the internal structure of stars.

Asteroseismologists study oscillations in stars, which are sound waves that ripple through their interiors, said Mathieu Vrard, a postdoctoral research associate in Ohio State's Department of Astronomy.

"That allows us to get very precise ages for the stars, which are important in determining the chronology of when events happened in the early Milky Way," Vrard said.

The study also used a spectroscopic survey, called APOGEE, which provides the chemical composition of stars -- another aid in determining their ages.

"We have shown the great potential of asteroseismology, in combination with spectroscopy, to age-date individual stars," Montalban said.

This study is just the first step, according to the researchers.

"We now intend to apply this approach to larger samples of stars, and to include even more subtle features of the frequency spectra," Vincenzo said.

"This will eventually lead to a much sharper view of the Milky Way's assembly history and evolution, creating a timeline of how our galaxy developed."

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Gigantic, red and full of spots

Among the Sun's most striking features are its sunspots, relatively darker areas compared to the rest of the surface, some of which are visible from Earth even without magnification. Numerous other stars, which like the Sun are in the prime of their lives, are also covered by spots. In red giants, on the other hand, which are in an advanced stage of stellar evolution, such spots were previously considered to be rare. The reason for this difference can be found deep in the interior of stars. In a dynamo process, the interplay of electrically conductive plasma currents and rotation generates a star's magnetic field that is then washed up to its surface. In some places, particularly strong magnetic fields prevent hot plasma from flowing upwards. These regions appear dark and constitute starspots.

"Rotation and convection are both crucial ingredients for the formation of surface magnetic fields and starspots," explains Dr. Federico Spada of MPS, co-author of the new study. "Stars with outer convective layers have the potential to generate surface magnetic fields via dynamo action, but only when the star rotates fast enough the magnetic activity becomes detectable," he adds. Until now, researchers had assumed that almost all red giants rotate rather slowly around their own axis. After all, stars expand dramatically when they develop into red giants towards the end of their lives. As a result their rotation slows down, like a figure skater doing a pirouette with his arms stretched out. The new study led by scientists from MPS and New Mexico State University (USA) now paints a different picture. About eight percent of the observed red giants rotate quickly enough for starspots to form.

The research team scoured the measurement data of about 4500 red giants recorded by NASA's Kepler space telescope from 2009 to 2013 for evidence of spots. Such spots reduce the amount of light that a star emits into space. Since they usually change only slightly over several months, they gradually rotate out of the telescope's field of view -- and then reappear after some time. This produces typical, regularly recurring brightness fluctuations.

In a second step, the scientists investigated the question why the spotted giants rotate so quickly. How do they muster the necessary energy? "To answer this question, we had to determine as many of the stars' properties as possible and then put together an overall picture," says Dr. Patrick Gaulme, lead author of the publication. At the Apache Point Observatory in New Mexico (USA), for example, the researchers studied how the wavelengths of starlight from some of the stars change over time. This allows conclusions about their exact movement. The team also looked at rapid fluctuations in brightness, which are superimposed on the slower ones caused by starspots. The faster fluctuations are the expression of pressure waves propagating through a star's interior to its surface. They contain information on many internal properties such as the star's mass and age.

The analysis revealed that approximately 15 percent of the spotted giants belong to close binary star systems, usually constituted of a red giant with a small and less massive companion. "In such systems, the rotational speeds of both stars synchronize over time until they rotate in unison like a pair of figure skaters," says Gaulme. The slower red giant thus gains momentum and spins faster than it would have without a companion star.

The other red giants with starspots, about 85 percent, are on their own -- and yet they rotate quickly. Those with a mass roughly equal to that of the Sun probably merged with another star or planet in the course of their evolution and thus gained speed. The somewhat heavier ones, whose masses are two to three times that of the Sun, look back on a different development. In the heyday of their lives before they became red giants, their internal structure prevented the creation of a global magnetic field that gradually carries particles away from the star. Unlike their magnetic counterparts, which therefore rotate slower and slower over time, their rotation has probably never slowed down significantly. Even as red giants, they still rotate almost as quickly as they did in their youth.

"In total, behind the common observational feature that some red giants have spots, we find three groups of rapidly rotating stars, each of which has a very different explanation. So it's no wonder that the phenomenon is more widespread than we previously thought," says Gaulme.

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