The sodium abundance in Bootis stars
Department of Astronomy, Odessa National University, Shevchenko Park, 65014 Odessa, Ukraine e-mail: email@example.com
2 Odessa Observatory and Isaac Newton Institute of Chile, Odessa Branch, Ukraine
Accepted: 5 October 2005
Context.Numerous observational studies have shown that the atmospheres of λ Boo type stars are strongly deficient in such elements as Ti, Cr, Fe, while some of the light elements (e.g. carbon, oxygen and sulphur) do not show any remarkable abundance anomalies. Such a specific abundance pattern is regarded to be the direct consequence of the selective accretion of stellar envelope material after its depletion of refractory elements. At the same time, sodium with its rather high condensation temperature shows almost normal content in λ Boo star atmospheres in apparent contradiction to a prediction of the accretion hypothesis.Aims.We aim to show that the normal sodium abundance seen in some λ Boo type stars is connected with the specific ionization regime of the sodium atoms which is formed in the circumstellar envelope.Methods.The analytical expression connecting the fraction of ionized atoms and circumstellar envelope characteristics (electron temperature, optical depth, etc.) is used.Results.Under the special assumptions about the circumstellar envelope characteristics, the sodium remains essentially ionized over the whole envelope volume, while elements such as iron are ionized only in the vicinity of the central star. This favours iron dust grain formation in the envelope, and prevents effective adhesion of the sodium charged particles. Thus, the sodium remains in the gaseous medium which is accreted by the star, while such elements as Ti, Fe and some other refractory elements, which are locked in the dust grains, are effectively swept out of the envelope. This results in a specific abundance pattern which is seen in some λ Boo stars whose atmospheres appear to be deficient in Ti, Fe, Ni, but possess almost normal sodium content.
© ESO, 2006