Permitted Oxygen Abundances and the Temperature Scale of Metal‐poor Turnoff Stars

Abstract

We use high-quality VLT/UVES published data of the permitted O i triplet and Fe ii lines to determine oxygen and iron abundances in unevolved (dwarfs, turnoff, subgiants) metal-poor halo stars. The calculations have been performed both in LTE and non-LTE (NLTE), employing effective temperatures obtained with the new infrared flux method (IRFM ) temperature scale by Ramírez & Meléndez, and surface gravities from Hipparcos parallaxes and theoretical isochrones. A new list of accurate transition probabilities for Fe ii lines, tied to the absolute scale defined by laboratory measurements, has been used. Interstellar absorption has been carefully taken into account by employing reddening maps, stellar energy distributions and Strömgren photometry. We find a plateau in the oxygento-iron ratio over more than 2 orders of magnitude in iron abundance (À3:2 < ½Fe/ H < À0:7), with a mean [O/ Fe ¼ 0:5 dex ( ¼ 0:1 dex), independent of metallicity, temperature, and surface gravity. The flat [O/ Fe] ratio is mainly due to the use of adequate NLTE corrections and the new IRFM temperature scale, which, for metal-poor F/early G dwarfs is hotter than most T eff scales used in previous studies of the O i triplet. According to the new IRFM T eff scale, the temperatures of turnoff halo stars strongly depend on metallicity, a result that is in excellent qualitative and quantitative agreement with stellar evolution calculations, which predict that the T eff of the turnoff at ½Fe/H ¼ À3 is about 600-700 K higher than that at ½Fe/H ¼ À1. Recent determinations of H temperatures in turnoff stars are in excellent relative agreement with the new IRFM T eff scale in the metallicity range À2:7 < ½Fe/H < À1, with a zero-point difference of only 61 K.

The C/O ratio at low metallicity: constraints on early chemical evolution from observations of Galactic halo stars

“…Here we have included non-LTE corrections for both C and O and considered additional C i lines; taken together, these two aspects of the present work represent significant improvements over earlier published studies, by reducing both systematic and statistical uncertainties in the determinations of the C and O abundances. The sample of stars analysed by Akerman et al (2004) was subsequently used by Meléndez et al (2006) in their study of oxygen at low metallicity. However, while Meléndez and collaborators applied their non-LTE corrections directly to published values, we have re-derived the abundances to complement our sample of newly observed stars in a consistent fashion.…”

Abundances and physical parameters for stars in the open clusters NGC 5822 and IC 4756

“…3, the huge differences between photometric and spectroscopic temperatures imply that a zero-point error exist in the photometry of roughly 0.13 mag for NGC 5822, and of the same order of magnitude for IC 4756 but more difficult to evaluate due to the differential reddening. This can explain the difference between the extinctions obtained in this section and those obtained in the extinction maps of Hakkila et al (1997) as described in Meléndez et al (2006). The aforementioned zeropoint error affects the present evaluation of the cluster parameters by an amount that should be added to random errors (the errors given above include it).…”

The non-LTE line formation of neutral carbon in late-type stars

“…Finally, the T eff scale for halo stars is still under debate (e.g. Meléndez et al 2006). As mentionned, the [C/O] ratio should be relatively unaffected, given the similar temperature dependence of the C and O lines we employ.…”