On the age of Galactic bulge microlensed dwarf and subgiant stars

Context. Recent results by Bensby and collaborators on the ages of microlensed dwarf and subgiant stars in the Galactic bulge have challenged the picture of an exclusively old stellar population, because ages significantly younger than 9 Gyr have been found. Aims. However, these age estimates have not been independently confirmed with different techniques and theoretical stellar models. One of the aims of this paper is to verify these results by means of a grid-based method. We also quantify the systematic biases that might be induced by some assumptions adopted to compute stellar models. In particular, we explore the impact of increasing the initial helium abundance, neglecting the element microscopic diffusion, and changing the mixing-length calibration in theoretical stellar track computations. Methods. We adopt the SCEPtER pipeline with a newly computed stellar model grid for metallicities [Fe/H] from - 2.00 dex to 0.55 dex, and masses in the range [0.60; 1.60] M<inf>⊙</inf> from the zero-age main sequence to the helium flash at the red giant branch tip. By means of Monte Carlo simulations we show for the considered evolutionary phases that our technique provides unbiased age estimates. Results. Our age results are in good agreement with Bensby and collaborators findings and show 16 stars younger than 5 Gyr and 28 younger than 9 Gyr over a sample of 58. The effect of a helium enhancement as large as ΔY/ ΔZ = 5 is quite modest, resulting in a mean age increase of metal rich stars of 0.6 Gyr. Even simultaneously adopting a high helium content and the upper values of age estimates, there is evidence of 4 stars younger than 5 Gyr and 15 younger than 9 Gyr. For stars younger than 5 Gyr, the use of stellar models computed by neglecting microscopic diffusion or by assuming a super-solar mixing-length value leads to a mean increase in the age estimates of about 0.4 Gyr and 0.5 Gyr respectively. Even considering the upper values for the age estimates, there are four stars estimated younger than 5 Gyr is in both cases. Thus, the assessment of a sizeable fraction of young stars among the microlensed sample in the Galactic bulge appears robust.