Visual and ultraviolet flux variability of the bright CP starθAurigae

Abstract

Context. Chemically peculiar stars of the upper part of the main sequence show periodical variability in line intensities and continua, modulated by the stellar rotation, which is attributed to the existence of chemical spots on the surface of these stars. The flux variability is caused by the changing redistribution rate of the radiative flux predominantly from the short-wavelength part of the spectra to the long-wavelength part, which is a result of abundance anomalies. Many details of this process are still unknown. Aims. We study the nature of the multi-spectral variability of one of the brightest chemically peculiar stars, θ Aur. Methods. We predict the flux variability of θ Aur from the emerging intensities calculated for individual surface elements of the star taking into account horizontal variation of chemical composition. The surface chemical composition was derived from Doppler abundance maps. Results. The simulated optical variability in the Strömgren photometric system and the ultraviolet flux variability agree well with observations. The IUE flux distribution is reproduced in great detail by our models in the near ultraviolet region. A minor disagreement remains between the observed and predicted fluxes in the far ultraviolet region. The resonance lines of magnesium and possibly also some lines of silicon are relatively weak in the ultraviolet domain, which indicates non-negligible vertical abundance gradients in the atmosphere. We also derive a new period of the star, P = 3.618 664(10) d, from all available photometric and magnetic measurements and show that the observed rotational period is constant over decades. Conclusions. The ultraviolet and visual variability of θ Aur is mostly caused by silicon bound-free absorption and chromium and iron line absorption. Manganese also contributes to the variability, but to a lesser extent. These elements redistribute the flux mainly from the far-ultraviolet region to the near-ultraviolet and optical regions in the surface abundance spots. The light variability is modulated by the stellar rotation. The ultraviolet domain is key for understanding the properties of chemically peculiar stars. It provides a detailed test for surface abundance models and comprises many lines that originate from states with a low excitation potential, which enable detecting vertical abundance gradients.

The roAp starα Circinus as seen by BRITE-Constellation

“…Lüftinger et al (2010b), Shulyak et al (2010), and Krtička et al (2015) find that variations of HD 37776 and θ Aurigae in the u, v, b, and y Strömgren bands are caused by spots dominated by Si, Cr, and/or Fe, and those of HD 37776 are dominated by He. The elements Fe and Cr are primarily responsible for the variability of UMa (Shulyak et al 2010).…”

“…Lüftinger et al 2010a,b;Krtička et al 2009Krtička et al , 2012Krtička et al , 2015Shulyak et al 2010). This is a strong argument to favour bright spot solutions over dark spot solutions, if no other information is available.…”

Doppler imaging of chemical spots on magnetic Ap/Bp stars

“…Since the metal overabundance spots are brighter in the optical than the surrounding atmosphere (e.g. Lüftinger et al 2010a;Krtička et al 2015), an abundance DI based on a single, average stellar model atmosphere may overestimate the contrast of surface abundance distribution and lead to other artefacts. However, the studies by Lehmann et al (2007) and Kochukhov et al (2012) did not reveal any major effect of this problem on DI maps.…”

Rotationally modulated variability and pulsations of the He-rich star CPD −62°2124 with an extraordinarily strong magnetic field

“…the well-studied He-rich star HD 37776; Khokhlova et al 2000). The enhanced redistribution of radiative flux from the ultraviolet to the visible region of hot CP stars causes the presence of large bright photometric spots, usually with overabundant silicon, which is the most efficient, having plenty of bound-free transition continua in the ultraviolet (Krtička et al 2007;Krtička et al 2015). The global magnetic dipole-like field, tilted to the rotation axis by the angle β, frozen in the plasma in the outer layers of the star, is also a long-lived phenomenon firmly bound with the rotating stellar surface.…”

“…Its brightness is very probably the result of a redistribution of radiative energy from the ultraviolet into the visible due to the back-warming process in regions with overabundant elements possessing numerous lines in the ultraviolet. Silicon is the element mainly responsible for flux variability in HD 37776 (Krtička et al 2007) and other hot CP stars (Krtička et al 2015).…”