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Astronomers have detected elusive pulsation patterns in dozens of rapidly spinning young stars thanks to data from NASA’s Exoplanet in Transit Study Satellite (TESS). The discovery will revolutionize scientists’ ability to study details such as the ages, sizes, and compositions of these stars, all members of a class named for the prototype, the bright star Delta Scuti.
“Delta Scuti stars clearly pulse in interesting ways, but the patterns of those pulsations have so far defied understanding,” said Tim Bedding, professor of astronomy at the University of Sydney. “To use a musical analogy, many stars strike single chords, but Delta Scuti stars are complex, with notes that appear to be mixed. TESS has shown us that this is not true for all of them.”
An article describing the findings, edited by Bedding, appears in the May 14 issue of the magazine. Nature and is now available online.
Geologists studying earthquake seismic waves discovered Earth’s internal structure from the way reverberations changed velocity and direction as they traveled through it. Astronomers apply the same principle to study the interior of stars through their pulsations, a field called asteroseismology.
Sound waves travel through the interior of a star at speeds that change with depth, and all combine in pulsation patterns on the star’s surface. Astronomers can detect these patterns as small fluctuations in brightness and use them to determine the star’s age, temperature, composition, internal structure, and other properties.
Delta Scuti stars are between 1.5 and 2.5 times the mass of the Sun. They are named after Delta Scuti, a star visible to the human eye in the southern Scutum constellation that was first identified as a variable in 1900 Since then, astronomers have identified thousands more as Delta Scuti, many with NASA’s Kepler Space Telescope, another hunting mission planet that operated from 2009 to 2018.
But scientists have had trouble interpreting the Delta Scuti pulsations. These stars generally rotate once or twice a day, at least a dozen times faster than the Sun. The rapid rotation flattens the stars at their poles and mixes the pulsation patterns, making them more complicated and difficult to decipher.
To determine if there is an order to the apparently chaotic pulsations of Delta Scuti stars, astronomers needed to observe a large set of stars multiple times with rapid sampling. TESS monitors large swaths of the sky for 27 days at a time, taking a full image every 30 minutes with each of its four cameras. This observation strategy allows TESS to track changes in stellar brightness caused by planets passing in front of their stars, which is their primary mission, but the half-hour exposures are too long to capture the patterns of Delta stars. Scuti that pulse faster. Those changes can happen in minutes.
But TESS also captures snapshots of a few thousand preselected stars, including a few Delta Scuti stars, every two minutes. When Bedding and his colleagues began to classify the measurements, they found a subset of Delta Scuti stars with regular pulsation patterns. Once they knew what to look for, they looked for other examples in Kepler’s data, using a similar observation strategy. They also made follow-up observations with ground-based telescopes, including one at W.M. Keck Observatory in Hawaii and two in the global network of the Las Cumbres Observatory. In total, they identified a batch of 60 Delta Scuti stars with clear patterns.
“This really is a breakthrough. We now have a regular series of pulsations for these stars that we can understand and compare to the models,” said co-author Simon Murphy, a postdoctoral researcher at the University of Sydney. “It will allow us to measure these stars using asteroseismology in a way that we have never been able to do. But it also shows us that this is just a springboard in our understanding of Delta Scuti stars.”
The pulsations in the well-behaved Delta Scuti group fall into two main categories, both caused by the energy stored and released in the star. Some occur when the entire star expands and contracts symmetrically. Others occur as opposite hemispheres alternately expanding and contracting. Bedding’s team deduced the alterations by studying the brightness fluctuations of each star.
The data has already helped resolve a debate about the age of a star, called HD 31901, a member of a recently discovered star stream that orbits within our galaxy. Scientists placed the age of the mainstream at 1 billion years old, based on the age of a red giant they suspected belonged to the same group. A later estimate, based on the rotation periods of other members of the star stream, suggested an age of only about 120 million years. Bedding’s team used TESS’s observations to create an HD 31901 asteroseism model that is compatible with the younger age.
“Delta Scuti stars have been frustrating targets due to their complicated oscillations, so this is a very exciting discovery,” said Sarbani Basu, a professor of astronomy at Yale University in New Haven, Connecticut, who studies asteroseismology but was not involved. in the study. . “Being able to find simple patterns and identify oscillation modes is changing the game. Since this subset of stars allows for normal seismic analyzes, we will finally be able to characterize them properly.”
The team believes that its 60-star set has clear patterns because they are younger than other Delta Scuti stars, and have recently been established to produce all of their energy through nuclear fusion in their cores. Pulsations occur more rapidly in budding stars. As stars age, the frequency of pulsations decreases and mixes with other signals.
Another factor may be the TESS viewing angle. Theoretical calculations predict that the pulsation patterns of a rotating star should be simpler when its pole of rotation faces us instead of its equator. The team’s TESS dataset included around 1,000 Delta Scuti stars, meaning some of them, by chance, must be seen near the pole.
Scientists will continue to develop their models as TESS begins taking full images every 10 minutes instead of every half hour in July. Bedding said the new observation strategy will help capture the pulsations of even more Delta Scuti stars.
“We knew when we designed TESS that in addition to finding many exciting new exoplanets, the satellite would also advance the field of asteroseismology,” said TESS principal investigator George Ricker of the Kavli Institute for Astrophysics and Space Research at the Massachusetts Institute of Technology in Cambridge. . “The mission has already found a new type of pulsating star on one side and has discovered new facts about known stars. As we complete the initial two-year mission and begin the extended mission, we look forward to a wealth of new stellar discoveries that TESS will do. “
