Searching for links between magnetic fields and stellar evolution

Abstract

Context. The evolution of magnetic fields in Ap stars during the main sequence phase is presently mostly unconstrained by observation because of the difficulty of assigning accurate ages to known field Ap stars. Aims. We are carrying out a large survey of magnetic fields in cluster Ap stars with the goal of obtaining a sample of these stars with well-determined ages. In this paper we analyse the information available from the survey as it currently stands. Methods. We select from the available observational sample the stars that are probably (1) cluster or association members and (2) magnetic Ap stars. For the stars in this subsample we determine the fundamental parameters T eff , L/L , and M/M . With these data and the cluster ages we assign both absolute age and fractional age (the fraction of the main sequence lifetime completed). For this purpose we have derived new bolometric corrections for Ap stars. Results. Magnetic fields are present at the surfaces of Ap stars from the ZAMS to the TAMS. Statistically for the stars with M > 3 M the fields decline with advancing age approximately as expected from flux conservation together with increased stellar radius, or perhaps even faster than this rate, on a time scale of about 3 × 10 7 yr. In contrast, lower mass stars show no compelling evidence for field decrease even on a timescale of several times 10 8 yr. Conclusions. Study of magnetic cluster stars is now a powerful tool for obtaining constraints on evolution of Ap stars through the main sequence. Enlarging the sample of known cluster magnetic stars, and obtaining more precise rms fields, will help to clarify the results obtained so far. Further field observations are in progress.

Magnetic field and atmospheric chemical abundances of the magnetic Ap star HD 318107

“…The programme automatically adjusts v sin i and the radial velocity v R to optimal values at each iteration. For the analysis here, T eff was set to 11 800 K, and log g to 4.2, consistent with the value 4.22 ± 0.13 determined from M = 2.95 ± 0.15 M (Landstreet et al 2007) and the radius found above. Note that because only one ionisation stage is detected for almost all elements, and because the shapes of spectral lines are fit only approximately at best, we do not obtain very strong constraints on the value of T eff from modelling the observed spectrum.…”

“…HD 318107 (=NGC 6405 77), a very peculiar magnetic Ap star, has an effective temperature T eff = 11 800 ± 500 K, luminosity log L/L = 1.92 ± 0.1, and mass M/M = 2.95 ± 0.15 (Landstreet et al 2007). It has a large global magnetic field; A&A 535, A25 (2011) the mean line-of-sight magnetic field component B z is sometimes as large as 5 or 6 kG, and the typical size of the mean field modulus B is about 15 kG.…”

“…HD 318107 is also interesting in that it has an age known with relatively high precision. The large sample of magnetic Ap stars in open clusters for which magnetic fields have been measured and characterised by Bagnulo et al (2006) and Landstreet et al (2007Landstreet et al ( , 2008 have already provided the first clear evidence of evolution of magnetic field strength of the fossil fields of middle main sequence stars with time (or stellar age) during the main sequence phase of evolution. This data set, much of which is based on relatively high-dispersion (polarised) spectra, also offers the possibility of studying empirically the evolution with time and stellar age of the chemical peculiarities of magnetic Ap stars.…”

Confronting uncertainties in stellar physics

“…2 with uncertainties of 0.1 mag in the bolometric correction (see e.g. Landstreet et al 2007) and of 300 K in effective temperature (see e.g. Fossati et al 2009, for a best case scenario).…”

New X-ray observations of IQ Aurigae andα2 Canum Venaticorum

“…Most stars are Si-stars, but some of the hotter ones are also classified as He-weak, moreover all stars have detected magnetic fields of a few hundred G up to several kG (Bychkov et al 2003). The used stellar data are again taken from Kochukhov & Bagnulo (2006), additional values are from Landstreet et al (2007). In Fig.…”