Modeling He-rich subdwarfs through the Hot-Flasher Scenario.

MILLER BERTOLAMI; ALTHAUS; UNGLAUB; WEISS

ASTRONOMY AND ASTROPHYSICS

EDP Sciences

Lugar: Paris; Año: 2008

0004-6361

We present 1D numerical simulations aimed at studying the Hot-Flasherscenario for the formation of He-rich subdwarf stars.  Sequences havebeen calculated for a wide range of metallicities and physicalassumptions, like the stellar mass at the moment of the helium coreflash. This allows us to study the two previously proposed flavors ofthe Hot-Flasher scenario (``deep´´ and ``shallow´´ mixing cases) andto identify a third transition type.  Our sequences are calculated bysimultaneously solving the mixing and burning equations within adiffusive convection picture, in the context of the standard mixinglength theory. This allows us to follow the chemical evolution duringdeep mixing events where hydrogen is violently burnt and to present ahomogeneous set of abundances for different metallicities and allkinds of Hot-Flashers.  We extend the scope of our work by analyzingthe effects of non-standard assumptions such as the effect of chemicalgradients, extra-mixing at convective boundaries, possible reductionsin convective velocities or the interplay between difussion and massloss. Particular emphasis is put on the predicted surface propertiesof the models.We find that the hot-flasher scenario is a viable explanation forthe formation and surface properties of He-sdO stars. Also, ourresults show that, during the early He-core burning stage, elementdiffusion may produce the transformation of (post Hot-flasher) He-richatmospheres into He-deficient ones. If this is so, then we find thatHe-sdO stars would be the progenitors of some of the hottest sdBstars.