NASA’s TESS, or Transiting Exoplanet Survey Satellite has one main job: finding exoplanets. But it’s also helping astronomers study a strange type of star that has so far defied thorough explanation. Those stars are Delta Scuti stars, named after their prototype.
Delta Scuti stars exhibit strange pulsating patterns, and they rotate rapidly. So far, astronomers haven’t been able to figure out whey they pulsate the way they do. But a new study based on TESS data is revealing some of the detail of these puzzling stars, if not explaining them completely.
The study is titled “Very regular high-frequency pulsation modes in young intermediate-mass stars.” The lead author is Timothy Bedding, a Professor of Astronomy at the University of Sydney. It’s published in the journal Nature.
TESS watches stars and looks for planets that transit in front of them. Those transits create dips in the starlight. To do that, it monitors huge patches of the sky, and all the stars in them, for 27-day periods. Those long observation times were helpful in this new study, even though it’s not focused on exoplanets, but on Delta Scuti stars.
Delta Scuti stars are larger than the Sun, about 1.5 to 2.5 times larger. Their namesake, the star Delta Scuti, was identified as a variable star back in 1900. Now astronomers know of thousands of them, many of which were discovered with NASA’s other planet-hunter, the Kepler spacecraft.
Delta Scutis are puzzling, compared to other variable stars. They rotate much faster than other stars, once or twice a day, which is at least a dozen times faster than our Sun. Because they rotate so fast, the stars are flattened at the poles, and the patterns of pulsation are scrambled. They’re complicated stars, and challenging to understand.
“Delta Scuti stars clearly pulsate in interesting ways, but the patterns of those pulsations have so far defied understanding,” said Tim Bedding in a press release. “To use a musical analogy, many stars pulsate along simple chords, but Delta Scuti stars are complex, with notes that seem to be jumbled. TESS has shown us that’s not true for all of them.”
TESS, and its 27-day observations of many stars at a time, is just what Bedding and his team needed. TESS’s observing power means that astronomers can monitor many stars at once. TESS has four cameras, and in its normal mode, it captures and image with each of the four every thirty minutes. But those 30 minute exposures are too long to capture changes in Delta Scuti stars, which are variable every few minutes.
Fortunately, TESS also captures two-minute exposures of thousands of pre-selected stars. And some of those stars are Delta Scuti stars. Bedding and his team dug through TESS data, and they found a sub-set of Delta Scuti stars that have regular pulsation patterns. With that data in hand, they knew what to look for.
They then combed through Kepler data, and did follow-up observations with ground telescopes like the Keck. In the end, Bedding and the other astronomers found 60 Delta Scuti stars that had regular pulsation patterns. In their paper they write, “We discovered 60 stars with regular frequency spacings, which define a group of ? Scuti stars for which mode identification is possible.”
“This really is a breakthrough. Now we have a regular series of pulsations for these stars that we can understand and compare with models,” said co-author Simon Murphy, a postdoctoral researcher at the University of Sydney. “It’s going to allow us to measure these stars using asteroseismology in a way that we’ve never been able to do. But it’s also shown us that this is just a stepping-stone in our understanding of Delta Scuti stars.”
Asteroseismology is the study of the internal structure of stars, based on a star’s oscillations. The idea is to determine the interior structure of stars by using their oscillations as seismic waves. Asteroseismology is a quickly-evolving field. In a 2012 paper discussing asteroseismology, Gerald Handler of the Copernicus Astronomical Center said, “Nearly all the physical processes that determine the structure and evolution of stars occur in their (deep) interiors. The production of nuclear energy that powers stars takes place in their cores for most of their lifetime.” In short, the interior of stars is where the business takes place, and asteroseismology is one method of investigating it.
As the team writes in their paper, “Asteroseismology probes the internal structures of stars by using their natural pulsation frequencies. It relies on identifying sequences of pulsation modes that can be compared with theoretical models, which has been done successfully for many classes of pulsators, including low-mass solar-type stars, red giants, high-mass stars and white dwarfs.” However, up until now, Delta Scuti stars have resisted efforts to understand their asteroseismology. That’s largely because they rotate so rapidly.
The team of researchers found that their Delta Scuti stars fell into two groups, and both types involve the build-up and then release of energy. In the first group, the entire star expands and contracts symmetrically. In the second group, opposite hemispheres of the star expand and contract alternately.
“Delta Scuti stars have been frustrating targets because of 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 the modes of oscillation is game changing. Since this subset of stars allows normal seismic analyses, we will finally be able to characterize them properly.”
In fact, the team’s data has already helped understand one puzzling star, and the stream of stars it belongs to.
Recently, astronomers discovered a stream of stars orbiting within the Milky Way. Scientists have debated the age of the stream, with some pinning it at about one billion years. That one billion year number was based on a red giant star in the stream. But other, later measurements of the stream showed that the stars are only about 120 million years old, a major discrepancy.
The team behind this study used the models they developed for Delta Scuti stars to examine a single star in the stream named HD 31901. Their new asteroseismological model confirmed HD 31901’s younger age, and showed that the earlier one billion year age is incorrect.
Their work is already paying off.
According to the team, it’s the age of their sub-set of 60 Delta Scuti stars that’s responsible for the regular pulsation patterns. They’re younger than the others, and so they’ve only recently, in stellar terms, settled down to producing their fusion in only their cores. In these younger stars, the pulsations are more rapid.
But as the stars age, things get a little more complicated. The frequency of the pulstations slows, and the signal gets mixed up with signals coming from the star. Because HD 31901 had a clear, rapid pulsation pattern, it was confirmed as being relative young, settling the debate.
But as it turns out, studying Delta Scuit stars is not totally straight-forward. TESS’s viewing angle of the stars can lead to different measurements. When viewed pole-on, these stars may exhibit more regular pulsation patterns than when viewed at the equator, according to theoretical calculations. With over 1,000 Delta Scuti stars in the data, some of them are bound to be seen at their poles.
The team will continue to work on their Delta Scuti models. Soon, TESS will switch from capturing 30 minute images to 10 minute images. That’ll happen in July, when TESS switches from its initial mission to its extended mission, and it should help the team find even more Delta Scutis.
Due to the way that TESS works, the people behind it knew that it would help advance the science of asteroseismology, even though that’s not the spacecraft’s primary mission.
“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 at the Massachusetts Institute of Technology’s Kavli Institute for Astrophysics and Space Research in Cambridge. “The mission has already found a new type of star that pulsates on one side only and has unearthed new facts about well-known stars. As we complete the initial two-year mission and commence the extended mission, we’re looking forward to a wealth of new stellar discoveries TESS will make.”
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