Stokes IQUV Magnetic Doppler Imaging of Ap stars I. ESPaDOnS and NARVAL Observations

Silvester, J., Wade, G.A., Kochukhov, O., Bagnulo, S., Folsom, C.P., Hanes, D.

Figure 9. Two observations of HD 32633 obtained at close phases, one with NARVAL (Phase 0.689, taken 17 Dec 2006) and one with ESPaDOns (Phase 0.700, taken 26 Jan 2008). The signal-to-noise ratio is ≈ 400 and ≈ 700 respectively. Excellent agreement between the signatures can been seen, illustrating the consistency between the two instruments.

Abstract

In this paper we describe and evaluate new spectral line polarisation observations obtained with the goal of mapping the surfaces of magnetic Ap stars in great detail. One hundred complete or partial Stokes IQUV sequences, corresponding to 297 individual polarised spectra, have been obtained for 7 bright Ap stars using the ESPaDOnS and NARVAL high resolution spectropolarimeters. The targets span a range of mass from approximately 1.8 to 3.4 M_☉, a range of rotation period from 2.56 to 6.80 days, and a range of maximum longitudinal magnetic field strength from 0.3 to over 4 kG. For 3 of the 7 stars, we have obtained dense phase coverage sampling the entire rotational cycle. These datasets are suitable for immediate magnetic and chemical abundance surface mapping using Magnetic Doppler Imaging (MDI). For the 4 remaining stars, partial phase coverage has been obtained, and additional observations will be required in order to map the surfaces of these stars. The median signal-to-noise ratio of the reduced observations is over 700 per 1.8 km s^òÈÒ1 pixel. Spectra of all stars show Stokes V Zeeman signatures in essentially all individual lines, and most stars show clear Stokes QU signatures in many individual spectral lines. The observations provide a vastly improved data set compared to previous generations of observations in terms of signal-to-noise ratio, resolving power and measurement uncertainties. Measurement of the longitudinal magnetic field demonstrates that the data are internally consistent within computed uncertainties typically at the 50 to 100δ level. Data are also shown to be in excellent agreement with published observations and in qualitative agreement with the predictions of published surface structure models. In addition to providing the foundation for the next generation of surface maps of Ap stars, this study establishes the performance and stability of the ESPaDOnS and NARVAl high-resolution spectropolarimeters during the period 2006–2010.

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Last Revised: 2012 June 28th