The exceptional Herbig Ae star HD 101412: The first detection of resolved magnetically split lines and the presence of chemical spots in a Herbig star

Abstract

In our previous search for magnetic fields in Herbig Ae stars, we pointed out that HD 101412 possesses the strongest magnetic field among the Herbig Ae stars and hence is of special interest for follow-up studies of magnetism among young pre-main-sequence stars. We obtained high-resolution, high signal-to-noise UVES and a few lower quality HARPS spectra revealing the presence of resolved magnetically split lines. HD 101412 is the first Herbig Ae star for which the rotational Doppler effect was found to be small in comparison to the magnetic splitting and several spectral lines observed in unpolarized light at high dispersion are resolved into magnetically split components. The measured mean magnetic field modulus varies from 2.5 to 3.5 kG, while the mean quadratic field was found to vary in the range of 3.5 to 4.8 kG. To determine the period of variations, we used radial velocity, equivalent width, line width, and line asymmetry measurements of variable spectral lines of several elements, as well as magnetic field measurements. The period determination was done using the Lomb-Scargle method. The most pronounced variability was detected for spectral lines of He I and the iron peak elements, whereas the spectral lines of CNO elements are only slightly variable. From spectral variations and magnetic field measurements we derived a potential rotation period Prot = 13.86 d, which has to be proven in future studies with a larger number of observations. It is the first time that the presence of element spots is detected on the surface of a Herbig Ae/Be star. Our previous study of Herbig Ae stars revealed a trend towards stronger magnetic fields for younger Herbig Ae stars, confirmed by statistical tests. This is in contrast to a few other (non-statistical) studies claiming that magnetic Herbig Ae stars are progenitors of the magnetic Ap stars. New developments in MHD theory show that the measured magnetic field strengths are compatible with a current-driven instability of toroidal fields generated by differential rotation in the stellar interior. This explanation for magnetic intermediate-mass stars could be an alternative to a frozen-in fossil field.

Abundances in the Herbig Ae star HD 101412

“…The lines were shifted by 0.02 to 0.03 Å with respect to the solar wavelengths of these lines, a rather small shift that could be entirely due to measurement error. Hubrig et al (2010) demonstrate equivalent width changes of the order of 20% (0.08 dex). The variations were correlated with an assumed phase, 13. d 86 days nearly one third the newly determined period of 42. d 0.…”

“…The current paper is based on a subset of the spectra described by Hubrig et al (2010). Most equivalent widths were measured on two HARPS spectra (averaged) obtained in 2009 on 23:47 UT of 4 July, and 00:13 UT of 5 July.…”

“…A number of recent studies have been devoted to this star (cf. Hubrig et al 2009Hubrig et al , 2010, including an estimate of its metallicity (Guimarães et al 2006). The absorption lines are unusually sharp, leading to suggestions that the star may be seen nearly "pole on".…”

“…5). Hubrig et al (2010) noted that the Zeeman patterns of several lines were resolved, or partially resolved. Nevertheless, most atomic lines are sufficiently sharp that numerous relatively unblended lines could be found for abundance work.…”

HARPS spectropolarimetry of three sharp-lined Herbig Ae stars: New insights

“…Wade et al 2005Wade et al , 2007Hubrig et al 2009Hubrig et al , 2010Hubrig et al , 2011b. The fundamental parameters, T eff = 8300 K and log g = 3.8, and the projected rotation velocity, v sin i = 3 ± 2 km s −1 , were determined by Cowley et al (2010).…”