Microturbulence and abundances in Am stars: Inferences concerning metallicism.
Abstract
In this study high dispersion spectra were obtained for sixteen Am stars in three galactic clusters and for four normal standards. A detailed LTE atmosphere curve of growth analysis was made in order to obtain microturbulent parameters and abundances for each star. Examination of the ionization equilibrium of iron confirms that the spectroscopic gravities of Am stars are dwarf-like - The conclusions of Baschek and Reimers (1969) are confirmed that microturbulence exhibits a common maximum in the late A region for both normal and Am stars. This peak agrees precisely with the intersection of the instability strip and the main sequence in the H-R diagram. There is also evidence that microturbulence may be a unique function of Tv along the main sequence. The abundance of iron is enhanced in Am stars by a variable factor of up to five or more. This enhancement is correlated positively with Tvff and inversely with V sin i. The run of abundances in Am stars suggests that the observed peculiarities do not arise through nucleosynthetic processes. In addition, the abundances of the elements relative to iron behave similarly to those of their neighbors in the periodic table. The scatter is surprisingly small except among the deficient elements. The abundance scatter in these latter elements may be accounted for by their correlation with Tvff and V sin i. A postulated metallicism index is shown to be anti-correlated with apparent rotation. Moreover, the recent K-line photometry of Henry (1969) implies that some anomalies become more moderate as Am stars evolve off the main sequence. It thus appears that metafficism involves stability, and that the mechanism responsible for it is subject to rotational or convective disruption. No evidence for subgrouping in Am properties is found, either by cluster membership or otherwise. However, various anomalies vary with Tvff in various ways and this may account for reported differences between "hot" and "classical" (cool) Am stars. It is suggested that metafficism arises from the operation of a subsurface diffusion process operative between the convective zones due to H I, He I and He fl ionization. The location of the Am domain in the H-R diagram and several other boundary conditions appear to be reasonably explained by such a modeL Key words: metafficism - microturbulence - stability - diffusion process
- Publication:
-
Astronomy and Astrophysics
- Pub Date:
- 1971
- Bibcode:
- 1971A&A....11..325S