The λ Bootis stars are Population I, late B‐ to early F‐type stars, with moderate to extreme (up to a factor 100) surface underabundances of most Fe‐peak elements and solar abundances of lighter elements (C, N, O and S). To put constraints on the various existing theories that try to explain these peculiar stars, we investigate the observational properties of λ Bootis stars compared with a reference sample of normal stars. Using various photometric systems and Hipparcos data, we analyse the validity of standard photometric calibrations, elemental abundances, and Galactic space motions. There crystallizes a clear picture of a homogeneous group of Population I objects found at all stages of their main‐sequence evolution, with a peak at about 1 Gyr. No correlation of astrophysical parameters such as the projected rotational velocities or elemental abundances with age is found, suggesting that the a priori unknown mechanism, which creates λ Bootis stars, works continuously for late B‐ to early F‐type stars in all stages of main‐sequence evolution. Surprisingly, the sodium abundances seem to indicate an interaction between the stars and their local environment.

Context. Light and spectrum variations of the magnetic chemically peculiar (mCP) stars are explained by the oblique rigid rotator model with a rotation period usually assumed to be stable on a long time scale. A few exceptions, such as CU Vir or 56 Ari, have been reported as displaying an increase in their rotation period. A possible increase in the period of light and spectrum variations has also been suggested from observations of the helium-strong mCP star HD 37776 (V901 Ori). Aims. In this paper we attempt to confirm the possible period change of HD 37776 and discuss a possible origin of this change as a consequence of i) duplicity; ii) precession; iii) evolutionary changes; and iv) continuous/discrete/transient angular momentum loss. Results. We confirm the previously suspected gradual increase in the 1. d 5387 period of HD 37776 and find that it has lengthened by a remarkable 17.7 ± 0.7 s over the past 31 years. We also note that a decrease in the rate of the period change is not excluded by the data. The shapes of light curves in all colours were found to be invariable. Conclusions. After ruling out light-time effects in a binary star, precession of the rotational axis, and evolutionary changes as possible causes for the period change, we interpret this ongoing period increase as a braking of the star's rotation, at least in its surface layers, due to the momentum loss through events or processes in the extended stellar magnetosphere.

Aims. We investigate the magnetic dichotomy between Ap/Bp and other A-type stars by carrying out a deep spectropolarimetric study of Am and HgMn stars. Methods. Using the NARVAL spectropolarimeter at the Télescope Bernard Lyot (Observatoire du Pic du Midi, France), we obtained high-resolution circular polarisation spectroscopy of 12 Am stars and 3 HgMn stars. Results. Using least squares deconvolution (LSD), no magnetic field is detected in any of the 15 observed stars. Uncertaintiies as low as 0.3 G (respectively 1 G) have been reached for surface-averaged longitudinal magnetic field measurements for Am (respectively HgMn) stars. Conclusions. Associated with the results obtained previously for Ap/Bp stars, our study confirms the existence of a magnetic dichotomy among A-type stars. Our data demonstrate that there is at least one order of magnitude difference in field strength between Zeeman detected stars (Ap/Bp stars) and non Zeeman detected stars (Am and HgMn stars). This result confirms that the spectroscopically-defined Ap/Bp stars are the only A-type stars harbouring detectable large-scale surface magnetic fields.

Abstract.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 8590Å and 8750Å. 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.

After establishing the synthetic a photometric system in the first paper of this series, we now present model atmospheres computed with individual abundances for a representative sample of chemically peculiar (CP) stars and either confirm or redetermine their input parameters through comparisons with photometric, spectrophotometric and high-resolution spectroscopic data. The final models obtained from this procedure were used to compute synthetic a indices which were compared with observations. The observed behaviour of a is reproduced for several types of CP stars: models for Am stars show negligible (or marginally positive) values of a few mmag, while for λ Bootis stars -and for metal deficient A stars in general -we obtain negative values (as low as −12 mmag in one case). For the coolest CP2 stars with effective temperatures below about 8500 K, we obtain mild (∼ +10 mmag) to moderately large (∼ +30 mmag) flux depressions in agreement with observations. However, a values for slightly hotter members of the CP2 group (for which still T eff < 10 000 K) are underestimated from these new models. The effect of the microturbulence parameter on the a index is revisited and its different role in various types of CP stars for reproducing the flux depression at 5200 Å is explained. We also provide reasons why models based on enhanced microturbulence and scaled solar abundance could not explain the observed flux depression for all types of CP stars. We discuss potential improvements of the current models including the possibility of still missing line opacities (unidentified and autoionization lines), modifications due to an explicit account of a global stellar magnetic field, and the effect of vertical abundance stratification.