The helium abundance and ΔY/ΔZ in lower main-sequence stars

Abstract

We use nearby K dwarf stars to measure the helium-to-metal enrichment ratio Y/ Z, a diagnostic of the chemical history of the solar neighbourhood. Our sample of K dwarfs has homogeneously determined effective temperatures, bolometric luminosities and metallicities, allowing us to fit each star to the appropriate stellar isochrone and determine its helium content indirectly. We use a newly computed set of Padova isochrones which cover a wide range of helium and metal content.Our theoretical isochrones have been checked against a congruous set of main-sequence binaries with accurately measured masses, to discuss and validate their range of applicability. We find that the stellar masses deduced from the isochrones are usually in excellent agreement with empirical measurements. Good agreement is also found with empirical mass-luminosity relations.Despite fitting the masses of the stars very well, we find that anomalously low helium content (lower than primordial helium) is required to fit the luminosities and temperatures of the metalpoor K dwarfs, while more conventional values of the helium content are derived for the stars around solar metallicity.We have investigated the effect of diffusion in stellar models and the assumption of local thermodynamic equilibrium (LTE) in deriving metallicities. Neither of these is able to resolve the low-helium problem alone and only marginally if the cumulated effects are included, unless we assume a mixing-length which is strongly decreasing with metallicity. Further work in stellar models is urgently needed.The helium-to-metal enrichment ratio is found to be Y/ Z = 2.1 ± 0.9 around and above solar metallicity, consistent with previous studies, whereas open problems still remain at the lowest metallicities. Finally, we determine the helium content for a set of planetary host stars.

Galactic evolution of D, 3He and 4He

“…Helium abundances as high as Y ∼ 0.4 are suggested, which for ω Cen imply a helium-to-metal enrichment ratio ΔY /ΔZ 70 (Piotto et al 2005). Such a value is outstandingly larger than that quoted for the solar neighbourhood around and above solar metallicity from a sample of nearby K-dwarf stars, ΔY /ΔZ = 2.1 ± 0.9 (Casagrande et al 2007). Attemps have been made to explain such extreme 4 He abundances in the framework of two main competing scenarios, the so-called 'asymptotic giant branch (AGB) self-pollution scenario' (P. Ventura, this volume) and the so-called 'fast rotating massive star (FRMS) self-pollution scenario' (T. Decressin, this volume).…”

“…First of all, it is worth stressing that, when dealing with the evolution of 4 He in the Galactic disc, we can rely on only a few data (see also M. Peimbert et al, this volume), namely, the abundance of 4 He in the Sun at the time of its birth, which is by now quite well established (Asplund et al 2009), the abundance of 4 He in M 17, an H II region located in the inner Galaxy, and the helium-to-metal enrichment ratio as derived from nearby K-dwarf stars. The latter quantity, however, is affected by a rather large error and can only be trusted around solar metallicity (see Casagrande et al 2007 and contribution by L. Casagrande, this volume). Fig.…”

“…As far as 4 He is concerned, there has been a general consensus that the relative heliumto-metal enrichment ratio in the solar neighbourhood is ΔY /ΔZ ∼ 2, both from a theoretical (Chiosi & Matteucci 1982, Maeder 1992 and an observational point of view (e.g., Casagrande et al 2007). Yet, hints for very different values of this ratio were reported early on in the literature (Danziger 1970, and references therein).…”

Revisiting the helium abundance in globular clusters with multiple main sequences

“…Around solar metallicities this analysis yielded ΔY /ΔZ ∼ 2 but at lower Z, Casagrande et al (2007) found that the observed MS is narrower than expected from stellar models computed under this standard ΔY /ΔZ (i.e. the stars are cooler than the isochrones), rather implying a much steeper value of ∼ 10.…”

“…More recently, Casagrande et al (2006Casagrande et al ( , 2007 further improved the analysis by compiling a much larger sample of K dwarfs with bolometric magnitudes and effective temperatures derived via InfraRed Flux Method (IRFM). The implementation adopted for the IRFM resorted to homogeneous multi-band photometry to determine in a fully self-consistent, (almost) model independent way T eff and M Bol .…”