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Astron. Astrophys. 326, 1001-1012 (1997)

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5. Post accretion tracks

In this section we focus on evolution from the end of the accretion phase to the arrival on the zero age main sequence (ZAMS) at an age of about [FORMULA] yr (for 1.2 [FORMULA]).
Once accretion is finished, deuterium abundance falls immediately to its equilibrium value corresponding to a mass fraction [FORMULA]. At the same time, the nuclear energy production vanishes and restores gravitation as the only energy source able to provide the luminosity. Their further evolution is very slow and the structural differences present at the end of the accretion phase are maintained. During this phase, contraction proceeds, the central temperature increases and 7 Li is depleted. When [FORMULA] reaches [FORMULA] K, the [FORMULA] reaction ignites and its highly temperature sensitive energy production results in the development

of a convective core. The central temperature rise is halted when most of the energy radiated away by the star is supplied by the nuclear sources (the contraction stops). As [FORMULA] is depleted near the center and reaches its equilibrium abundance, the CNO cycle now provides the dominant source of energy and the star joins the main sequence (MS). Fig. 14 summarizes the evolution of key variables from the end of the accretion phase to their arrival on the MS.

[FIGURE] Fig. 14. Post accretion phase. This figure represents a panel of relevant variables from the end of accretion to the arrival on the MS. The solid, dotted and small-dashed curves refer to the accretion laws (R1), (R2) and (R3), respectively. The long dashed line is associated with a standard evolution of a 1.2 [FORMULA] with the same chemical composition. Open circles indicate the time of the exponential decrease for the (R1) and (R2) prescription while empty circles mark the end of accretion for each star.
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© European Southern Observatory (ESO) 1997

Online publication: April 8, 1998