Zero age main sequence

Red giant stars, both in the field and in globular clusters, present abundance anomalies that can not be explained by standard stellar evolution models. Some of these peculiarities, such as the decline of 12C/13C, and that of Li and 12C surface abundances for stars more luminous than the bump, clearly point towards the existence of extra-mixing processes at play inside the stars, the nature of which remains unclear. Rotation has often been invoked as a possible source for mixing inside Red Giant Branch (RGB) stars ([8], [1], [2]). In this framework, we present the first fully consistent computations of rotating low mass and low metallicity stars from the Zero Age Main Sequence (ZAMS) to the upper RGB. 

The B-N object may be considered to be a zero-age main sequence star that evolves with increasing surface temperature and luminosity at optical wavelengths. The descent from the right-hand upper quarter of the HR diagram, along with what has been called the Hayashi track or birth lines, precedes the entrance onto the... 

Figure 1. Lines of constant mass and varying chemical composition for the computed Wolf-Rayet models of 3, 5, 7, 10, 15, 20, SO, 40, and 60Mq in the HR diagram (solid lines). The pure helium stars are connected through a dashed line, while the extreme helium poor stars are connected through a dotted line. Also the HRD positions after applying a correction for the partly optically thick stellar wind on the effective temperature are shown. Furthermore, the theoretical zero age main sequence (ZAMS) is indicated, together with schematic evolutionary tracks for stars of 15, 30, and IOOMq. The crosses and circles correspond to HRD positions of observed WNE and WC stars, respectively, according to Smith and Willis (198S, Astron. Astrophys. Suppl. 54,229).

Saio, Kato, and Nomoto (1988) recently examined under what conditions a massive star undergoes a blue-red-blue evolution. The evolution of a star of initial mass 20 M0 star in the HR diagram is shown in Figure 1 from the zero-age main-sequence through carbon ignition at the center. The metallicity in the envelope was assumed to be Z = 0.005 and the Schwarzschild criterion was adopted. The star shows the three types of evolutionary path (A, B, C) depending on the mass loss, metallicity, and the change in the helium abundance Y in the envelope. 

The thermal time scale mass transfer is initiated by the shrinkage of the orbit as a consequence of angular momentum conservation, as long as Md > Mwd- the Roche lobe filling main sequence star is squeezed into a continuously shrinking volume. The stellar radius can fall much below the zero-age main sequence mass-radius relation. Consequently, orbital periods can be achieved which are smaller by a factor / than periods corresponding to unperturbed main sequence stars filling their Roche lobes. For conser-... 

In the non-rotating case, the value of F evaluated as described above is 0.78 on the zero age main sequence (log 7 eff = 50 000 K) and rises to 0.96 at t = 2.610syr (log Teff = 25 000 K) due to the iron opacity peak. The core H-burning phase is finished at t = 3.4 106yr and log Teff = 20 000 K with F = 0.84. The evolution of F is reflected in the behaviour of the critical rotational velocity (cf. Eq. 5.31) as shown in Fig. 20 it has a pronounced minimum of vcrjtimin — lOOkrns-1 at t 2.610syr. Since the rotational velocity tends to remain almost constant on the main sequence (Fig. 20) the question arises what happens to a star with an initial rotational velocity above lOOkrns-1 ... 

FIGURE 21. Evolutionary tracks of non-rotating models in the initial mass range 60. .. 300 Mq and metallicities of Z=0.04 (thick continuous lines) and Z=0.02 (thin continuous lines Figer et al. 1998). Dashed lines mark the corresponding zero age main sequences. Filled and open dots on the tracks mark the first occurrence of nuclear processed material at the stellar surface, for Z=0.04 and Z=0.02, respectively. Two possible positions of the Pistol star are indicated (cf. Figer et al. ). 

As contraction continues, the core continues to heat until Tc exceeds the critical value for the ignition of deuterium burning (the second reaction in the p-p I chain). Providing the star is sufficiently massive, this will be followed by the p-p reaction and/or the CNO-cycle and full hydrogen burning will be established. At this point the star has reached the zero-age main sequence (ZAMS). 

Fig. 2. Hertzsprung-Russell diagram for a 1M and a 5Mq star of solar composition. Evolutionary tracks have been plotted from the zero-age main sequence to the asymptotic giant branch. The location of the main sequence is noted by the position of the label. The x-axis is the effective temperature of the model star, shown in units of log10 Teff, whereas the y-axis is the luminosity, in units of log10(L/LQ)...

Fig. 3. Hertzsprung-Russell diagram for a 1M star of solar composition, from the zero-age main sequence to the tip of the asymptotic giant branch. Figure provided by J. C. Lattanzio...

Lammer et al., 2008 [192] discussed the origin and evolution of Venus , Earth s, Mars and Titan s atmospheres from the time when the active young Sun arrived at the Zero-Age-Main-Sequence. Thermal and various nonthermal atmospheric escape processes influenced the evolution and isotope fractionation of the atmospheres and water inventories of the terrestrial planets efficiently. 

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