 Study of the Pre-Main Sequence of Low and Intermediate-Mass Stars J. P. Marques Abstract. The pre-main sequence (PMS) phase of stellar evolution spans the period between star formation and the main sequence, when nuclear reaction supply most of the energy of the star. Interest in this phase has been renewed by the realization that intermediate mass stars (1.5Msun < Mstar < 4Msun) experience Scuti-type pulsations, with several PMS delta Scuti stars discovered recently. Stellar models as accurate as possible are necessary to calculate oscillation frequencies to compare with observations. CESAM is one stellar evolution code specifically designed for asteroseismic uses, and we adapted it during this work to better model the PMS phase. We have performed a calibration of the binary EK Cephei. This binary was chosen because it has well determined masses, radii, luminosities, effective temperatures and metallicity; most importantly, its secondary component is a PMS star. We found that a correct treatment of the time step is fundamental to model PMS evolution. Calibrating the radius of the primary component is more difficult; the difficulty can be overcome by the use of a different mixture and/or lower 14N burning reaction rates. Convective overshoot has been considered unimportant during PMS evolution. We show that it can make a difference near the ZAMS for stars with masses high enough to sustain a convective core for the whole duration of the main sequence. Besides prolonging the PMS phase, it causes an extra loop on the evolutionary tracks on the HR diagram near the ZAMS. Stars experiencing this loop can be distinguished from stars in the same location on the HR diagram by the use of asteroseismology. Finally, as the highest mass PMS delta scuti stars are at or near the birthline, the effects of the initial conditions for PMS evolution can not be ignored. We adapted the CESAM stellar evolution code to calculated the evolution of accreting protostars; in this way, protostellar and PMS stellar evolution can be calculated using the same code (with the same numerical methods and input physics) in a self-consistent way. We produced birthlines using a variety of parameters, especially initial deuterium abundance and mass accretion rates. Doutoramento em Astronomia
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