Browsing by Author "Stempels, H.C."

Now showing 1 - 3 of 3
  • Thumbnail Image
    Item
    Magnetic field topology and chemical spot distributions in the extreme Ap star HD 75049
    (EDP Sciences, 2015) Kochukhov, O.; Rusomarov, N.; Valenti, J.A.; Stempels, H.C.; Snik, F.; Rodenhuis, M.; Piskunov, N.; Makaganiuk, V.; Keller, C.U.; Johns-Krull, C.M.
    Context. Intermediate-mass, magnetic chemically peculiar (Ap) stars provide a unique opportunity to study the topology of stellar magnetic fields in detail and to investigate magnetically driven processes of spot formation. Aims. Here we aim to derive the surface magnetic field geometry and chemical abundance distributions for the extraordinary Ap star HD 75049. This object hosts a surface field of ~30 kG, one of the strongest known for any non-degenerate star. Methods. We used time-series of high-resolution HARPS intensity and circular polarisation observations. These data were interpreted with the help of magnetic Doppler imaging and model atmospheres incorporating effects of a non-solar chemical composition and a strong magnetic field. Results. Based on high-precision measurements of the mean magnetic field modulus, we refined the rotational period of HD 75049 to Prot = 4.048267 ± 0.000036 d. We also derived basic stellar parameters, Teff = 10 250 ± 250 K and log g = 4.3 ± 0.1. Magnetic Doppler imaging revealed that the field topology of HD 75049 is poloidal and dominated by a dipolar contribution with a peak surface field strength of 39 kG. At the same time, deviations from the classical axisymmetric oblique dipolar configuration are significant. Chemical surface maps of Si, Cr, Fe, and Nd show abundance contrasts of 0.5–1.4 dex, which is low compared with many other Ap stars. Of the chemical elements, Nd is found to be enhanced close to the magnetic pole, whereas Si and Cr are concentrated predominantly at the magnetic equator. The iron distribution shows low-contrast features both at the magnetic equator and the pole. Conclusions. The morphology of the magnetic field and the properties of chemical spots in HD 75049 are qualitatively similar to those of Ap stars with weaker fields. Consequently, whatever mechanism forms and sustains global magnetic fields in intermediate-mass main-sequence stars, it operates in the same way over the entire observed range of magnetic field strengths.
  • Thumbnail Image
    Item
    Magnetically Controlled Accretion on the Classical T Tauri Stars GQ Lupi and TQ Hydrae
    (The American Astronomical Society, 2013) Johns-Krull, Christopher M.; Chen, Wei; Valenti, Jeff A.; Jeffers, Sandra V.; Piskunov, Nikolai E.; Kochukhov, Oleg; Makaganiuk, V.; Stempels, H.C.; Snik, Frans; Keller, Christoph; Rodenhuis, M.
    We present high spectral resolution (R ≈ 108,000) Stokes V polarimetry of the classical T Tauri stars (CTTSs) GQ Lup and TW Hya obtained with the polarimetric upgrade to the HARPS spectrometer on the ESO 3.6 m telescope. We present data on both photospheric lines and emission lines, concentrating our discussion on the polarization properties of the He i emission lines at 5876 Å and 6678 Å. The He i lines in these CTTSs contain both narrow emission cores, believed to come from near the accretion shock region on these stars, and broad emission components which may come from either a wind or the large-scale magnetospheric accretion flow.We detect strong polarization in the narrow component of the two He i emission lines in both stars. We observe a maximum implied field strength of 6.05 ± 0.24 kG in the 5876 Å line of GQ Lup, making it the star with the highest field strength measured in this line for a CTTS. We find field strengths in the two He i lines that are consistent with each other, in contrast to what has been reported in the literature on at least one star. We do not detect any polarization in the broad component of the He i lines on these stars, strengthening the conclusion that they form over a substantially different volume relative to the formation region of the narrow component of the He i lines.
  • Thumbnail Image
    Item
    Magnetism, chemical spots, and stratification in the HgMn star ϕ Phoenicis
    (EDP Sciences, 2012) Makaganiuk, V.; Kochukhov, O.; Piskunov, N.; Jeffers, S.V.; Johns-Krull, C.M.; Keller, C.U.; Rodenhuis, M.; Snik, F.; Stempels, H.C.; Valenti, J.A.
    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 ϕ 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 ϕ 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 ϕ Phe.