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A glimpse inside δ Scuti stars

Our knowledge of the stars is based almost exclusively on the study of their light. But the light that reaches us originates from their upper layers — we can’t see inside. There is, however, a tool we can use to look into the interior of the stars: asteroseismology. Writing in Nature, Bedding et al.1 report that a subgroup of the enigmatic δ Scuti stars exhibits regular pulsations that will finally enable the stars to be probed using this tool.

Inside a star, gravity and gas pressure compete with each other. If the two are in balance, the star is in equilibrium, but if one increases more than the other, the star contracts or expands. Hot gas spheres such as stars can show characteristic periodic oscillations in which the star pulsates in this way. These characteristic oscillations, called eigenmodes, are standing waves, like the standing sound waves responsible for the sounds of musical instruments such as violins and oboes. The eigenmodes are determined by the physics of the oscillating system.

Asteroseismology is the study of these stellar oscillations2. The idea is similar to that of seismology, in which the interior structure of our planet is inferred from earthquakes. Each star can have different — and often very large numbers of — eigenmodes, depending on its internal structure. Oscillations that have different periods or frequencies are sensitive to physical conditions in different regions inside the stars. The more eigenmodes that can be determined from observations, the more detailed will be our map of the internal structure.

Oscillations produce brighter and darker areas (corresponding to higher and lower pressures and temperatures; Fig. 1) on the star’s surface2. However, we cannot resolve the surfaces of stars, apart from that of our Sun and a few other special cases (see ref. 3 and references therein, for example). Only their total brightness can be measured. The complicated distribution and variation of surface brightness results in an equally complex temporal variation in the total brightness. By measuring the brightness of a star, a photometric time series known as the light curve is obtained.

Figure 1

Figure 1 | Simple modes of stellar oscillations. Pressure oscillations in stars occur in many different combinations of characteristic patterns and frequencies, such as the simple examples shown here. These oscillations change the brightness of stars, and offer clues about the physical conditions inside them. The oscillatory modes of an important family of stars known as δ Scuti stars have been difficult to identify. Bedding et al.1 have identified a group of δ Scuti stars that pulsate at a high rate and with regular patterns of frequency that agree with theoretical predictions. This allowed the oscillation modes to be identified.

For asteroseismology, then, we need the following steps. To obtain the surface-brightness distribution from the measured light curves, the frequency of the brightness variations must be determined. Next, we must work out how these frequencies correspond to the eigenmodes expected from theoretical models, a process called mode identification. If the mode identification is successful, actual asteroseismology can begin by determining key physical parameters such as stellar mass and age. Then the ultimate goal of asteroseismology can follow: obtaining the total seismic inversion, which means the detailed determination of stratifications of the pressure, temperature and chemical composition inside the star.

For decades, researchers have made tremendous efforts to obtain valuable asteroseismic data sets. Extensive observation campaigns were carried out using ground-based telescopes, but inevitable variations in detectors and weather conditions affected the data. Space missions (such as CoRoT, Kepler and TESS) delivered the real breakthrough. Thanks to the missions’ long, homogeneous and evenly sampled light curves, and the precision of the collected data, asteroseismology has now been successfully applied to thousands of stars across several stellar types that have different internal structures46.

But the abundant star type known as δ Scuti7, named after a star in the constellation Scutum, has remained one of the exceptions. Stars of this type have a slightly larger mass than that of our Sun, but their inner structure is very different. They have long been known to have complicated, low-amplitude light curves7. One might think that the large number of detected frequencies would make these stars ideal targets for asteroseismology.

The theoretical models of δ Scuti stars predict many possible excited eigenmodes and corresponding frequencies. In fact, there are many more such frequencies in models than have been observed8, and usually we do not know which of the possible modes are seen. If there were some regular structure to the frequencies (such as frequencies with comb-like regular differences), we would have a better chance of identifying them. But the theoretical models generally do not predict regular frequency structure for these stars.

Bedding et al. have identified a special subgroup of δ Scuti stars that pulsate at higher frequencies than do most such stars. For this subgroup, both theory and observations suggest the existence of regular frequency structures. Other researchers have previously found regular structures in observed data for some δ Scuti stars (see refs 9–14, for example), but did not identify the oscillating modes conclusively, if at all. Bedding et al. provide unambiguous mode identification for a uniform and relatively large sample of these stars.

A key factor in the authors’ success is that many of the stars in the subgroup rotate more slowly than do other δ Scuti stars. (Alternatively, it could be that some of the stars are observed almost pole-on, resulting in apparently small rotation velocities.) Theoretical models predict that the frequency spectra of stars that have low rotation velocities are less complicated than those with higher rotation speeds14, which makes it easier to recognize their regular frequency structures. Bedding et al. not only identified these structures, but also associated the frequencies with the corresponding eigenmodes.

Sky surveys now and in the near future will target many thousands of δ Scuti stars, including many that are similar to those described by Bedding and co-workers. This is not merely an opportunity to understand the physics of a special group of δ Scuti stars better. The authors show that these are young stars, which means that they can be used as tracers to estimate the age of open star clusters or of young stellar associations in our Galaxy. In this way, we might learn more about the evolution of the Milky Way. Bedding and colleagues’ study is therefore not the last word on δ Scuti stars. Rather, it opens up avenues of investigation for this important stellar group.

Nature 581, 141-142 (2020)

doi: https://doi.org/10.1038/d41586-020-01169-z

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