Post Main Sequence Evolution: To AGB & Beyond

As low to intermediate mass stars exhaust hydrogen in their core, they evolve off the main sequence. From this point on the evolutionary path is slightly different for low mass stars than it is for intermediate mass stars. We present a very brief overview in this chapter. The details involved in every step of the evolution are slightly marginal to the purpose of this thesis but are included in an appendix for the interested reader (see Appendix A).

In summary, after the core gradually exhausts hydrogen, it will turn into an inactive helium core with a fusing hydrogen shell. The shell fusion will add more helium to the inactive core. This process leads to the expansion of the hydrogen shell and contraction of the helium core. Upon reaching the Hayashi limit, the star becomes fully convective. The luminosity increase at this stage will cause the star to move up the HR diagram almost vertically onto the Red Giant Branch (RGB). It is worthwhile to mention that convection can penetrate the interior layer and bring up nucleosynthesis products to the surface. This is known as the “first dredge- up” which can change the surface abundance of elements such as13_C,14_{N, and} 12_{C [see e.g.} 59, 41, 35, 19, 5, and references therein for further details]

As the star ascends the RGB branch, the inactive helium core continues to contract gravita- tionally which results in temperature and density increase until it reaches temperatures of about

≈ 108 K when helium can ignite. In case of a low mass star the core will become electron-

degenerate and the helium will burn in an explosive manner also known as the “helium flash”. In the case of an intermediate mass star, there will be no degenerate condition, so helium fu- sion will take place in a gentle manner. Helium fusion (in both degenerate and non-degenerate conditions) occurs through a process known as the triple-alpha (3-α) process. The 3-αprocess

converts threeαparticles (4He) into one carbon nucleus (12C). The triple-alpha process is very

sensitive to the temperature: the nuclear reaction rates scale typically with T40_{[66]. Both low} and intermediate mass stars that are fusing helium steadily in their core will settle down on the horizontal branch (HB) on the HR-diagram.

As helium gradually burns in the core, the supply of helium as well as the radiation pressure decreases; once again, this leads to a slight drop in pressure and the core slowly undergoes gravitational contraction to maintain hydrostatic equilibrium. When the core runs out of He, the gradual contraction heats up the surrounding shell and switches on the shell burning process. As before, this results in an increase in luminosity and a decrease in effective temperature through expansion. The star has then reached the asymptotic giant branch(AGB) phase [76], so called because it asymptotically touches the RGB.

The structure of an early-AGB star consists of a hot and dense electron degenerate core of carbon and oxygen, surrounded by two shells of helium and hydrogen which are mostly

Figure 1.3: The theoretical Hertzsprung-Russel diagram including the zero-age main sequence (solid line) for various initial masses and their corresponding evolutionary tracks (Figure taken from Schaller et al. [107]).

responsible for the luminosity in this phase. These two shells are not always actively burning at the same time. However, the nuclear energy released from one shell can reignite the other shell and begin a phenomenon known as “double shell burning”. It is worthwhile to mention a phenomenon that occurs in this stage known as the “second dredge-up”. It only occurs in intermediate mass stars with masses greater than 4 M_⊙ [47, 3] because the convection can penetrate the region in between the hydrogen and helium shells and it can bring up the leftover products of nucleosynthesis in that region. This can lead to changes in surface abundances of a few elements such as16_O,14_N,18_O,22_{Ne [see 3, & Appendix A].}

The He shell becomes thinner as more He is consumed. It eventually undergoes a certain type of thermal instability described in detail by Schwarzschild H´arm [112] and give rise to

“thermal pulses” or “helium shell flash”. Thermal pulses introduce a second stage of AGB evolution known as the thermally pulsing AGB (or TP-AGB). As a result of the helium shell flash, the region between two shells becomes convective. As in the first and second dredge-up, the convective layer will bring nucleosynthesis products to the surface in a series of events col- lectively referred to as “third dredge-up” which changes the surface abundances of particularly 12_{C and}4_{He [43, 47].}

A key ingredient in the evolution of AGB stars is that stellar winds reduce the mass of the envelope from the outside through mass loss (see Appendix A). Unlike most other phases in stellar evolution for which the amount and consumption rate of fuel determines the evolution, the AGB stellar evolution is characterized by rates at which material is removed from the outer layers of the star. At rates of 10−8_–10−3 _M

⊙ per year (with mass loss rates increasing as stars climb the AGB), mass loss sets an AGB lifetime of_≈2_×106_{yr [54]. It is thought that near the} tip of the AGB, AGB stars will experience a so-called superwind phase with mass-loss rates of up to 10−3 _M

⊙/yr, which effectively ejects the entire remaining stellar envelope in a short amount of time (_∼1000 years), and leads to the termination of AGB phase.

After that, in what is known as post-AGB phase, the deeper and hotter layers will grad- ually be exposed as the outer layers are expelled. The steps include the formation of a pre- planetary nebula phase(PPN) which then turns into a short phase of planetary nebula (PN) from which the remnant is a compact electron degenerate core of carbon and oxygen known as white dwarf. Upon formation, the white dwarf is quite stable and it will start its cooling journey on timescales of billions of years.