The giant branches

The evolution and the total lifetime of stars depend strongly on the initial mass. Since the processes that govern the evolution of low, intermediate and high mass stars are very different, those fields are distinct specific areas of research inside the broader topic of stellar evolution. The Red Giant Branch (RGB) and the Asymptotic Giant Branch (AGB) are two of the later stages of intermediate mass stars (with masses between 0.8 and 8 times the mass of the Sun) life.

During all of their lives, stars need to produce energy to generate enough pressure to balance the gravity force (that compresses the star). Although, the main energy source changes during stellar evolution, one (or more, sometimes) is always needed, in order to reach an equilibrium situation. A star is said to be born when Hydrogen starts to be burned into Helium in the core, this is called the Main Sequence (MS) phase. The burning of Hydrogen is the most important energy source during the stellar life and intermediate mass stars will stay on the MS for billions of year in average.

When there is no more Hydrogen left in the core, the structure of the star changes, since equilibrium cannot be reached in the MS configuration anymore. Intermediate mass stars will experience two ascents into the, so called, giant branch after leaving the MS. The name giant given to this period is due the very big sizes and very large luminosity that they present.

During the ascent on the RGB, the star gradually increases in size and luminosity. In the core, constituted basically of Helium, no nuclear burning happens. The energy is obtained by burning Hydrogen around the core in a thin layer. The star will only stop increasing in size, and, therefore, leave the RGB, when the temperature in the core is high enough so that Helium burning can take place. Then, another phase of very stable core burning (similar to the MS) starts but in this case, Helium is the fuel. This phase is referred to as the Horizontal Branch and it lasts for around 10% of the time spent on the MS.

A very similar process happens when Helium is exhausted in the center and the star experiences again a gradual increase in both size and luminosity. The AGB starts and it is one of the very last stages of intermediate stars evolution. Stars in this phase are very luminous (~10000 brighter than the Sun) and large (~ 100 larger than the Sun) but have a low temperature (around half the temperature of the Sun). The low temperature of this objects is translates into a red color for the emitted light, and this can be used to identify them. Due to the characteristics of their envelopes, they also present a slow (~ 10 km/s) wind, that corresponds to very high mass loss rates (10-8 to 10-4 solar masses per year), and through which they enrich the Interstellar medium with the ashes of their nuclear burning.

In the expanding wind, molecules and dust grains are formed. The dust absorbs the stellar radiation and becomes warm (~ 1000 K), being detected in the Spectral Energy Distribution of AGB stars due to excess infrared radiation. The molecules produce absorption or emission lines and also provide an important probe of the physical conditions of the wind.

AGB objects are very important contributors for the chemical evolution of galaxies. Therefore, a good understanding of the mass ejection mechanism and element abundance of the wind is essential for understanding chemical evolution in the Universe.

However, we are not yet capable of predicting the evolution of a star in the AGB given its physical parameters, due to the very complex processes that happen during this phase. And a lot of effort is put into trying to connect the observed properties with models for stellar evolution, wind acceleration, envelope chemistry, dust nucleation and growth, radiative transfer and so on.

Besides studying the physical properties of the wind through models of dust and molecular emission, a lot can be learned from studying the pulsation properties of RGB and AGB stars. Parameters usually very difficult to access can be determined in this way. The science that studies the internal structure of pulsating stars by the interpretation of their frequency spectra is called Asteroseismology.

Different oscillation modes penetrate to different depths inside the star. These oscillations provide information about the otherwise unobservable interiors of stars in a manner similar to how seismologists study the interior of Earth and other solid planets through the use of earthquake oscillations.

In red-giant stars ascending the red-giant branch and in the horizontal branch or red clump, so-called solar-like oscillations have been observed. Solar-like oscillations are stochastically excited by the turbulent outer layers in these stars, similar to the oscillations observed in the Sun. From these oscillations it is possible to derive masses, radii and evolutionary states of the stars.
For the analyses data from the French-led CoRoT satellite and NASA Kepler satellite are used.

Active fields of research are:

  • Determining individual frequencies of the oscillation modes through so-called peakbagging to investigate excitation and damping mechanism.
  • Extending the research to more evolved (AGB) stars and investigate possible connections between these oscillations and mass-loss.
  • Extending the research to subgiants to investigate the rapid stellar interior changes in that evolutionary phase.

 

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