Asteroseismology helped detect ‘thinner’ red giants

(ORDO NEWS) — Scientists have discovered “impossible” red giants with anomalously low masses or luminosities.

The discovery was made while studying the data array obtained by the Kepler space observatory, which, due to its unprecedented accuracy and length, makes it possible to determine the parameters of stars using the asteroseismology method.

Red giants are a stage in the evolution of stars that occurs after the depletion of hydrogen reserves in their core.

The transformation of the Sun into a red giant in 4-5 billion years is called the final feature for life on Earth: the Sun, swollen a hundred times, will swallow Mercury and Venus, and burn the rest of the inner planets to ashes. Many bright stars in the sky are red giants; for example, these are Betelgeuse, Arcturus and Aldebaran.

The lifespan of a star increases as its mass decreases. Massive stars explode millions of years after they were born. The sun will live for 12 billion years, while red dwarfs can live for trillions of years.

Therefore, in the Universe there should not be red giants lighter than about 0.7 solar masses – in the 13.8 billion years that have passed since the Big Bang, not a single light enough star has yet managed to burn hydrogen in the core.

Astronomers from the University of Sydney, led by Yaguang Li, analyzed data from the Kepler space telescope and found exceptions to this rule. To do this, they determined the masses of stars using the asteroseismology method.

Usually the mass of a single star is determined from its spectral type . More accurate estimates are also obtained on the basis of “instantaneously” observable data. First, the temperature of the star is determined from the spectrum, and the distance to it is determined from the parallax.

Luminosity is determined by brightness and distance, and the radius of a star is determined by luminosity and temperature. Further, according to the ratio of the intensity of some spectral lines, the acceleration of free fall in the photosphere is calculated, and together with the known radius, it gives the mass of the star.

This method, in addition to parallax, requires fairly detailed spectral data and cannot be classified as widely available. To obtain a detailed spectrum of a star, special observations are required, and until the recent arrival of data from the Gaia observatory, parallaxes were known only for fairly close stars.

On the contrary, asteroseismology also makes it possible to determine the mass of a star, and does not require high-resolution spectrum imaging. This branch of astronomy studies the pulsations of stars and the propagation of sound waves within them.

Each star can be thought of as a resonator that has a set of natural oscillations. The interaction of plasma flows in the interiors of stars and activity on their surfaces excite these oscillations, just as a bell rings from blows and buzzes from a strong wind.

You can read more about astro- and helioseismology, as well as about the physics of the Sun, here .

For most stars, the amplitude of their own oscillations, as well as the amplitude of the brightness oscillations caused by them, is very small.

But if you capture the light curve with sufficient accuracy, you can determine the frequencies of natural oscillations (their spectrum), and calculate the distribution of density and temperature inside the star, and from them – the mass of the star and the distribution of chemical elements in it.

Astroseismology helped detect thinner red giants 2

This is exactly what was made possible by the Kepler telescope. His main task was to search for transiting exoplanets , and for this, from 2009 to 2013, he continuously recorded the brightness of several hundred thousand stars with an accuracy of hundred-thousandths.

As a result, a unique array of data was obtained. Its unparalleled accuracy, duration, and continuity opens up many previously inaccessible possibilities for studying the stars themselves, such as detailed studies of stellar activity, and, in fact, asteroseismology.

The study analyzed the light curves of 7,000 red giants and found 39 stars whose parameters do not fit into the evolutionary models of single stars.

Some of them have masses from 0.5 to 0.7 solar, and thus, these 32 luminaries could not have time to reach the red giant stage alone. The other seven were abnormally dim. Their masses, lying in the range 0.8-2.0 solar, turned out to be unexpectedly large for the observed luminosities.

In both cases, the observed properties can be explained by unusually rapid and massive mass loss (which is why the researchers dubbed these stars “thinning giants”). The authors of the study suggest that such red giants could appear due to interactions in binary systems.

When a star begins to turn into a red giant in the presence of a nearby component, it begins to pull its matter towards itself, which leads to a redistribution of masses in the star system, and even outbursts of new ones .

If the component is sufficiently low-mass in this case, it can be “lost” against the background of the red giant, and the system will look like a single red giant with anomalous properties through the eyes of Kepler.

Therefore, to elucidate the mechanism of “rapid weight loss” of red giants, additional observations are required – for example, taking spectra, which makes it possible to determine that the star is a binary one.

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