Ultraviolet and visual flux and line variations of one of the least variable Bp stars HD 64740

Abstract

Context. The light variability of hot magnetic chemically peculiar stars is typically caused by the flux redistribution in spots with peculiar abundance. This raises the question why some stars with surface abundance spots show significant rotational light variability, while others do not. Aims. We study the Bp star HD 64740 to investigate how its remarkable inhomogeneities in the surface distribution of helium and silicon, and the corresponding strong variability of many spectral lines, can result in one of the faintest photometric variabilities among the Bp stars. Methods. We used model atmospheres and synthetic spectra calculated for the silicon and helium abundances from surface abundance maps to predict the ultraviolet and visual light and line variability of HD 64740. The predicted fluxes and line profiles were compared with the observed ones derived with the IUE, HST, and Hipparcos satellites and with spectra acquired using the FEROS spectrograph at the 2.2 m MPG/ESO telescope in La Silla. Results. We are able to reproduce the observed visual light curve of HD 64740 assuming an inhomogeneous distribution of iron correlated with silicon distribution. The light variations in the ultraviolet are hardly detectable. We detect the variability of many ultraviolet lines of carbon, silicon, and aluminium and discuss the origin of these lines and the nature of their variations. Conclusions. The maximum abundances of helium and silicon on the surface of HD 64740 are not high enough to cause significant light variations. The detected variability of many ultraviolet lines is most likely of atmospheric origin and reflects the inhomogeneous elemental surface distribution. The variability of the C iv resonance lines of carbon is stronger and it probably results from the dependence of the wind mass-loss rate on the chemical composition and magnetic field orientation. We have not been able to detect a clear signature of the matter trapped in the circumstellar clouds.

Search for stellar spots in field blue horizontal-branch stars

“…It is important to note that from the derived upper limits, it is not possible to derive limits for spot sizes straightforwardly. The spot-induced amplitude for a given wavelength range (or filter) depends not only on the size, contrast, and distribution of the spots, but also on the involved elements (Krtička et al 2013;Prvák et al 2015). Furthermore, the abundance patterns of BHB stars are very much different from CP stars (Kinman et al 2000) and no specialized models exist to synthesize light curves for a given surface distribution of stellar spots in these objects.…”

TheKeplerview of magnetic chemically peculiar stars

“…Flux is redistributed in these spots through bound-free (Peterson 1970;Lanz et al 1996) and bound-bound transitions (Wolff & Wolff 1971;Molnar 1973;cf. also Krtička et al 2013).…”

HD 66051, an eclipsing binary hosting a highly peculiar, HgMn-related star

“…Flux is redistributed in these abundance spots and patches as a result of bound-free and bound-bound atomic transitions 10–12 . As has been described in the oblique rotator model 13 , the magnetic axis is not aligned with the rotational axis.…”