Magnetism, chemical spots, and stratification in the HgMn star ensuremathϕ Phoenicis


Context. Mercury-manganese (HgMn) stars have been considered as non- magnetic and non-variable chemically peculiar (CP) stars for a long time. However, recent discoveries of the variability in spectral line profiles have suggested an inhomogeneous surface distribution of chemical elements in some HgMn stars. From the studies of other CP stars it is known that magnetic field plays a key role in the formation of surface spots. All attempts to find magnetic fields in HgMn stars have yielded negative results.
Aims: In this study, we investigate the possible presence of a magnetic field in ensuremathϕ Phe (HD 11753) and reconstruct surface distribution of chemical elements that show variability in spectral lines. We also test a hypothesis that a magnetic field is concentrated in chemical spots and look into the possibility that some chemical elements are stratified with depth in the stellar atmosphere.
Methods: Our analysis is based on high-quality spectropolarimetric time-series observations, covering a full rotational period of the star. Spectra were obtained with the HARPSpol at the ESO 3.6-m telescope. To increase the sensitivity of the magnetic field search, we employed the least-squares deconvolution (LSD) technique. Using Doppler imaging code INVERS10, we reconstructed surface chemical distributions by utilising information from multiple spectral lines. The vertical stratification of chemical elements was calculated with the DDAFit program.
Results: Combining information from all suitable spectral lines, we set an upper limit of 4 G on the mean longitudinal magnetic field. For chemical spots, an upper limit on the longitudinal field varies between 8 and 15 G. We confirmed the variability of Y, Sr, and Ti and detected variability in Cr lines. Stratification analysis showed that Y and Ti are not concentrated in the uppermost atmospheric layers.
Conclusions: Our spectropolarimetric observations rule out the presence of a strong, globally-organised magnetic field in ensuremathϕ Phe. This implies an alternative mechanism of spot formation, which could be related to a non- equilibrium atomic diffusion. However, the typical time scales of the variation in stratification predicted by the recent time- dependent diffusion models exceed significantly the spot evolution time-scale reported for ensuremathϕ Phe.

Based on observations collected at the European Southern Observatory, Chile (ESO programme 084.D-0338). Figures 9-12 are available in electronic form at <A href=``'‘>