The solar neighbourhood HR diagram as a quantitative test for evolutionary time scales

The HIPPARCOS parallaxes have provided us with a precise HR diagram (HRD) of the solar neighbourhood, complete within certain limits. The scope of this work is a study of the stellar population densities in characteristic regions of the HRD to test stellar evolutionary time scales. We examine two HRDs (Mv ≤ 4.0), both based on the single-star entries of the HIPPARCOS catalogue. One is constrained by distance d ≤ 50pc (1337 stars found) and the other by d ≤ 100pc (8984 stars found but more scattered). The stellar distribution on the main sequence (MS) was used to find the best matching present day mass function (PDMF) for 1.15 < M∗/M < 10 in synthetic HRDs. For 1.6 < M∗/M < 4 we derive an initial mass function (IMF) of dNIMF/d logM∗ ∝ M−1.8 ∗ . We then compare 4 synthetic HRDs, each based on a grid of evolutionary tracks with a different description of extended mixing (or ”overshooting”), to the HIPPARCOS HRDs. As a diagnostic tool, we take star counts from characteristic HRD regions: the Hertzsprung gap (HG), the lower giant branch (LGB), the K giant clump (KGC) and the ”cool wind” region (CW). The HG region turns out to be very sensitive to overshooting on the MS for models with ≈ 1.6M . We find that those 2 grids, which include the same overshooting as derived from earlier empirical tests, but with an onset (either gradual or sudden) around 1.7M , reproduce all star counts best. Finally, our computed HRD can quantify a giant mass distribution for any evolved stage. Potential applications include the evolution of late stellar activity, or an estimate of the stellar gas and dust injection rates for the chemical evolution of the Galaxy.