Microturbulence in A stars as derived from line profiles.
This study represents an attempt to fix a scale of the "microturbulent velocity" in main-sequence A stars by examining the profiles of weak and saturated lines of six sharp-lined Am stars. Although the study is photographic, so that it is difficult to determine accurate core and wing parameters, studies of the loci of half-intensity widths and core shapes provide useful information. If a total broadening due to rotation and microturbulence is chosen on the basis of agreement between observed and computed intermediate-strength lines, it is found that the strongest lines are narrower and deeper than predicted by an amount that is proportional to the measured metallic-line blocking coefficients. If the strong-line cores were influenced strongly by departures from LTE, this effect would not be expected. After also testing explicitly for non-LTE in strong-line cores, it is concluded that LTE blanketed atmospheres are probably adequate to describe the formation of the spectral lines of the strengths studied. The blanketing effect seriously hampers, however, the uniqueness of the determination of the microturbulent component to the line broadening. From several lines of investigation, for example of the core shape of saturated lines, it appears that the microturbulence peaks to a value of not more than, but probably not much less than, 4 km 1, in mid- to late-A stars. One star shows a total broadening of 4 km i, and its projected rotational velocity is apparently negligible. Our present estimate disagrees with our previously determined peak value of 7 km 1 determined from curve of growth studies. An analysis of the possible error sources indicates that the discrepancy is mainly due to a combination of errors in the Corliss and Warner scale of oscillator strengths and to probably high equivalent-width scale due to the intermediate (8-9 A mm -1) coude' dispersion used. This work is actually a pilot study in that relatively precise photoelectric data are needed to describe the microturbulent velocity scale unambiguously. Even then, the question of LTE departures and the effect of blanketing on the temperature stratification is intimately tied to the scale of "microturbulence." Subject headings: line formation - line profiles - turbulence
The Astrophysical Journal
- Pub Date:
- May 1973