Our study of accreting low-mass stars has revealed that deuterium burning is the main energetic support of the star. To follow its abundance evolution, it was necessary to couple the nucleosynthesis of this element with the equations of the stellar structure. The extreme temperature sensitivity of the deuterium destruction reaction rate produces a thermostatic effect that modifies the global structure. The more energy is provided to the star, either by increasing the accretion rate, the deuterium abundance or with the addition of , the more it is converted to work against contraction. The reaction of the star prevents the temperature and, in the same way, the nuclear energy production to climb drastically. It results from the thermostatic effect of deuterium burning. Consequently the star expands, its effective temperature being imposed by the great dependence of opacity on the temperature in the photosphere (along the Hayashi line), the emergent luminosity increases as well. Finally, once accretion ends, the star relaxes to its standard structure. On the main sequence, the star does not present any difference with respect to a non accreting model within similar properties.
The profile of accreted matter modifies the structural response of the star. When mass deposition is confined to the surface layers, less internal energy is given to the central regions and the star experiences a stronger contraction that yields to a higher central temperature. Moreover, this study has shown the small hydrostatic influence of mass accretion; this effect is much smaller than the effect of deuterium burning.
Comparison with standard PMS evolution indicates that the accretion process accelerates the evolution of the star. The central temperature of an accreting star is always larger than in a non accreting scheme. From its position in the HRD, it appears systematically younger. Especially during the earlier accretion phase, age estimate can be affected by up to a factor 2-3. These results as well as prediction concerning chemical abundances will be confronted to observations in Siess et al. (1997).
This study has revealed that the evolution of an accreting star results from two antagonistic effects, the deuterium burning which tends to force the star to expand and the mass addition which reinforces the gravitational potential of the star and accelerates the contraction. Therefore, it is the combination of both and that determines the further evolution of PMS accreting stars.
We will present, in a forthcoming paper, results of computations performed with high ( ) accretion rates, in order to reproduce the birthline. These calculations will be compared with similar results obtained by Palla & Stahler (1990 and following papers).
© European Southern Observatory (ESO) 1997
Online publication: April 8, 1998