1. Galactic Helium-to-Metals enrichment ratio from the analysis of local main sequence stars observed by HIPPARCOS 52° Congresso SAIt – Teramo 2008 * Università di Pisa, Dipartimento di Fisica “E. Fermi” M. Gennaro*, P.G. Prada Moroni*, S. Degl’Innocenti*

2. Introduction Photometric comparison between low-MS stars and ZAMS models calculated with the most up-to-date input physics; If a linear relation is assumed, Y and Z are completely determined by the two values ΔY/ΔZ and [Fe/H] (assuming also a solar-scaled mixture and a value for the primordial Helium abundance Y P ); We calculated a total of 495 evolutionary tracks for 9 ΔY/ΔZ values, 5 [Fe/H] values and 11 stellar masses (from 0.5 to 1.0 M O, in steps of 0.05 M O ). Y P = 0.248 from Izotov et al., 2007 (ApJ, 662, 15) and Peimbert et al., 2007a,b (ASPC, 374, 81 – ApJ, 666, 636)

3. Outlines of the analysis method Theoretical differences between a reference ZAMS and the other ZAMS, at fixed B-V. This “metric” is slightly dependent on the reference point.

4. Outlines of the analysis method Parallaxes from HIPPARCOS catalogue with an error, σ(π)/π ≤ 5 %. Cutoff at a M V = 6 level to minimize the effect of evolution in the CMD. Recent [Fe/H] estimates: Nordström et al., 2004 (A&A, 418, 989) and Taylor, 2006 (ApJS, 161, 444) Comparison with the data

5. Outlines of the analysis method We wanted to determine the value of ΔY/ΔZ which minimizes the differences between theory and observations; A Montecarlo simulation has been developed to account for the observational errors in B-V, M V and [Fe/H].

6. Possible sources of uncertainty Creating artificial data sets, with known (ΔY/ΔZ) in, we could evaluate separately the effect of different sources of uncertainty on the final result Stars on their ZAMS positionSpread due only to the observational uncertainties

7. Possible sources of uncertainty Intrinsic “numeric” spread due to the interpolation in the tracks grid. It has no effect at all on the recovery procedure.

8. Possible sources of uncertainty There can be a real spread of ΔY/ΔZ around its mean value. Stars on their ZAMS positionTotal spread (observational + intrinsic)

9. Possible sources of uncertainty Even if our simulated data sets include only stars with M V ≥ 6, we concluded that evolutionary effect are not negligible. Stars with ages between 0 and 8 Gyr (uniform distribution) An important bias on the final result is clearly visible

10. Possible sources of uncertainty During their lifes, stars in the low-MS move towards the redder part of the CMD; this fact “mimics” a lower ΔY/ΔZ value and introduces a bias in the result, even with our very low luminosity (and hence mass) cutoff. time

11. Possible sources of uncertainty constant SFR exp SFR τ = 1 Gyr

12. Results using HIPPARCOS data With our analysis we obtained a value of ΔY/ΔZ = 4 ± 1 ; here ± 1 is the uncertainty due only to observational errors, as estimated from the MC simulation. Anyway, taking into account the bias due to evolutionary effects, we can say that the “real” peak is probably shifted by ~1 towards higher ΔY/ΔZ values.

13. The Hyades low-MS Cluster age of ~500 Myr we are sure that for M V ≥ 5 stars are really unevolved and can be used for our recovery method. Data for the Hyades are very good (Madsen et al., 2002; A&A, 381, 446), so the obs. uncert. are even lower than those of our original data set. Little spread in the diagramUnbiased final result: ΔY/ΔZ = 5 ± 1

14. The Hyades low-MS Using ΔY/ΔZ = 5 and [Fe/H] from Perryman et al., 1998 (A&A, 331, 81); we calculated isochrones for the Hyades. Very good agreement, even in the upper MS. (Y, Z) values: (0.3160, 0.0136) left (0.3313, 0.0167) right

15. Conclusions Comparing ZAMS models with low MS stars, we obtained a ΔY/ΔZ = 4 ± 1. Moreover we checked that, even using stars with low luminosities (M V ≥ 6) evolutionary effects can introduce an important bias in the result and are probably not negligible for local disk stars. This value is somewhat higher than values obtained from other autors with similar methods; e.g. Jimenez et al., 2003 (Science, 299, 1552) found ΔY/ΔZ = 2.1 ± 0.4 while Casagrande et al., 2007 (MNRAS, 382, 1516) found ΔY/ΔZ = 2.1 ± 0.9; older studies give results which are closer to our value; e.g. Pagel & Portinari, 1998 (MNRAS, 298, 747) found ΔY/ΔZ = 3 ± 2.

16. Conclusions Using the unevolved sample of stars from the Hyades low MS we obtained an unbiased result of ΔY/ΔZ = 5 ± 1, in good agreement with previous result corrected for evolution. Isochrones calculated with the best-fit ΔY/ΔZ value show a very good agreement with the complete MS of the Hyades.

17. Systematic effects Different α MLT values, same age and chemical composition

18. Systematic effects Different model atmospheres, same age, chemical composition and α MLT values

19. Systematic effects Effect of mod. atm. on the ZAMS position for different values of [Fe/H].