A search for magnetic fields in the variable HgMn star $\alpha$ Andromedae

Abstract

With this work, we have conducted an extensive search for magnetic fields in the photosphere of $\alpha$~And. We have acquired new circular polarisation spectra with the MuSiCoS and ESPaDOnS sp ectropolarimeters. We have also obtained FORS1 circular polarisation spectra fro m the ESO Archive, and considered all previously published magnetic data. This e xtensive dataset has been used to systematically test for the presence of magnet ic fields in the photosphere of $\alpha$~And. We have also examined the high-res olution spectra for line profile variability. The polarimetric and magnetic data provide no convincing evidence for photospheric magnetic fields. The highest-S/N phase- and velocity-resolved Stokes $V$ profiles, obtained with ESPaDOnS, allow us to place a $3\sigma$ upper limit of abou t 100~G on the possible presence of any undetected pure dipolar, quadrupolar or octupolar surface magnet ic fields (and just 50~G for fields with significant obliquity). We also conside r and dismiss the possible existence of more complex fossil and dynamo-generated fields, and discuss the implications of these results for explaining the non-un iform surface distribution of Hg. The very high-quality ESPaDOnS spectra have al lowed us to confidently detect variability of Hg~{\sc ii} $\lambda 6149$, $\lamb da 5425$ and $\lambda 5677$. The profile variability of the Hg~{\sc ii} lines is strong, and similar to that of the Hg~{\sc ii} $\lambda 3984$ line. On the ot her hand, variability of other lines (e.g. Mn, Fe) is much weaker, and appears to be attributable to orbital modulation, continuum normalisation differences and weak, variable fringing.

References in corpus (1)

• Searching for links between magnetic fields and stellar evolution. I. A survey of magnetic fields in open cluster A- and B-type stars with FORS1

Cited by in corpus (3)

• The surprising magnetic topology of tauSco: fossil remnant or dynamo output?
• Weak magnetic fields in Ap/Bp stars: Evidence for a dipole field lower limit and a tentative interpretation of the magnetic dichotomy
• Searching for links between magnetic fields and stellar evolution III. Measurement of magnetic fields in open cluster Ap stars with ESPaDOnS