Comprehensive study of the magnetic stars HD 5797 and HD 40711 with large chromium and iron overabundances

We present the results of a comprehensive study of the chemically peculiar stars HD 5797 and HD 40711. The stars have the same effective temperature, Teff = 8900 K, and a similar chemical composition with large iron (+1.5 dex) and chromium (+3 dex) overabundances compared to the Sun. The overabundance of rare-earth elements typically reaches +3 dex. We have measured the magnetic field of HD 5797. The longitudinal field component Be has been found to vary sinusoidally between -100 and +1000 G with a period of 69 days. Our estimate of the evolutionary status of the stars suggests that HD 5797 and HD 40711, old objects with an age t \approx 5 \times 108 yr, are near the end of the core hydrogen burning phase.Comment: 26 pages, 5 Encapsulated Postscript figure

Chemical abundances study of three Am stars HD 155375, HD 159560 and HD 196544

A synthetic spectrum analysis of CCD observations in the spectral region λλ 6400-6500 ÅÅ of the stars HD 155375, HD 159560, and HD 196544 was carried out. All stars show typical Am characteristics

Lithium and metal abundances in long period Am binaries

High signal-to-noise CCD spectra of three bright, long period Am binaries (HD 108651, HD 116657 and HD 138213) have been obtained and the atmospheric abundances of identified chemical elements were studied. We found HD 116657 to be a new candidate as a Li deficient, long period Am binary, in addition to 16 Ori. While HD 108651 and HD 116657 have pronounced Am characteristics, HD 138213 is just a mild Am star.
These stars were further studied in a more general context related to the role of binarity in CP phenomenon. It was found that both Li abundance and the equivalent with ratio Ca $\rm{I}$ 6718/Fe $\rm{I}$ 6679 are more peculiar in systems with more pronounced eccentricities. Such behaviour is the opposite of what was previously found in Ap binaries and their anomalies. It indicates that tidal effects are of crucial importance in driving CP phenomena.

Light element non-LTE abundances of

One of the main characteristics proclaimed for the group of the λ Bootis stars is the apparent solar abundance of the light elements C, N, O and S. The typical abundance pattern is completed by the strong underabundances of the Fe-peak elements. In the first paper of this series, we have shown that carbon is less abundant than oxygen but both elements are still significantly more abundant than Fe-peak elements. The mean abundances, based on a detailed non-LTE investigation, were found -0.37 dex and -0.07 dex, respectively. As a further step, we now present non-LTE abundances of nitrogen and sulphur for thirteen members of the λ Bootis group based on several spectral lines between 8590 Å and 8750 Å. Furthermore, LTE abundances for calcium in the same spectral range were derived and compared with values from the literature. Similar to the mean abundances of carbon and oxygen, nearly solar values were found (-0.30 dex for nitrogen and -0.11 dex for sulphur) for our sample of program stars. Among our sample, one previously undetected binary system (HD 64491) was identified. From a statistical point of view, the abundances of the light elements range from slightly overabundant to moderately underabundant compared to the Sun. However, the individual objects always exhibit a similiar pattern, with the Fe-peak elements being significantly more underabundant than the light elements. No correlation of the derived abundances with astrophysical parameters such as the effective temperature, surface gravity or projected rotational velocity was found. Furthermore, the abundances of the light elements do not allow us to discriminate between any proposed theory

First orbital elements for the

We present high-resolution spectroscopic observations of the two λ Bootis type spectroscopic binary systems HD 84948 and HD 171948. Both systems consist of two true λ Bootis stars, as has already been proven by a detailed abundance analysis taking into account the binary nature. Nevertheless, we have obtained non-LTE abundances for carbon and oxygen in order to investigate those important elements. The abundances fit excellently into the typical pattern for this group, leaving no doubt that all four components are true λ Bootis stars. With the help of the derived orbital elements it is possible to establish the ages of the two systems. For the first time we can estimate the evolutionary status of Galactic field λ Bootis stars. The origin of those nonmagnetic, metal-weak, Population I, late B- to early F-type stars is still controversial. The two widely discussed theories explaining the λ Bootis phenomenon (diffusion together with either accretion of circumstellar matter or mass-loss) predict significantly different evolutionary stages for this group of objects (close to either the Zero-Age Main Sequence or Terminal-Age Main Sequence). It is already known that very young members of the group exist in the Orion OBI association and probably in NGC 2264. Hipparcos data established six further Galactic field λ Bootis stars which are close to the Zero-Age Main Sequence, whereas the evolutionary status for the other objects remained undetermined. The Hipparcos data reveal that the Galactic space motions of both the systems that we discuss here are typical of those of Population I objects. The photometric data from the Hipparcos mission confirm the pulsation previously discovered for HD 84948 with a period of about 110 min and a V-amplitude of about 14 mmag. For HD 84948, we estimate from the mass ratio an age of about 1 Gyr, ruling out a possible Pre-Main-Sequence status. HD 171948 has an age of about 0.01 to 0.1 Gyr which is close to the Zero-Age Main Sequence. We therefore conclude that the λ Bootis phenomenon can be found continuously from very early stages to the Terminal-Age Main Sequence, suggesting that different mechanisms might work at different stages of stellar evolution producing the same abundance pattern