On the formation and evolution of magnetic chemically peculiar stars in the solar neighborhood

Abstract

In order to put strict observational constraints on the evolutionary status of the magnetic chemically peculiar stars (CP2) of the upper main sequence, we have investigated a well established sample of galactic field CP2 objects within a radius of 200 pc from the Sun in the (X, Y) plane. In total, 182 stars with accurate parallax measurements from the Hipparcos satellite were divided into Si, SiCr and SrCrEu subgroups based on classification resolution data from the literature. Primarily, it was investigated if the CP2 phenomenon occurs at very early stages of the stellar evolution, significantly before these stars reach 30% of their life-time on the main sequence. This result is especially important for theories dealing with stellar dynamos, angular momentum loss during the pre-as well as main sequence and stellar evolutionary codes for CP2 stars. For the calibration of the chosen sample, the well-developed framework of the Geneva 7-color and Strömgren uvbyβ photometric system was used. We are able to show that the CP2 phenomenon occurs continuously at the zero age main sequence for masses between 1.5 and 4.5 M . The magnetic field strengths do not vary significantly during the evolution towards the terminal age main sequence. Only the effective temperature and magnetic field strength seem to determine the kind of peculiarity for those stars. We found several effects during the evolution of CP2 stars at the main sequence, i.e. there are two "critical" temperatures where severe changes take place. There is a transition between Si, SiCr and SrCrEu stars at 10 000 K whereas a significant decrease to almost zero of evolved SrCrEu objects with masses below 2.25 M at 8000 K occurs. These conclusions have to be incorporated into models that simulate the stellar formation and evolution of stars between 1.5 and 4.5 M in the presence of strong magnetic fields.

Searching for links between magnetic fields and stellar evolution

“…Note that, in contrast to the results of Pöhnl et al (2005) and Kochukhov & Bagnulo (2006), both of whom found a deficiency of young Ap stars with masses below 2 M , all three of the lowmass stars in our survey are fairly close to the ZAMS; the largest fractional age among these stars is 0.16. This effect probably results from the fact that the main sequence lifetime of such stars (10 9 yr or more) is considerably longer than the ages of most clusters in our sample.…”

“…This result, together with the results of Bagnulo et al (2003), Pöhnl et al (2005), Kochukhov & Bagnulo (2006), and our own survey, lead us to reject the hypothesis that stars with M ≤ 3 M becomes magnetic only after they have spent a significant fraction of their life in the main sequence.…”

“…However, this result was contradicted by the discovery of a field in the young 2.1 M star HD 66318 . Furthermore, both Pöhnl et al (2005) and Kochukhov & Bagnulo (2006) were unable to reproduce the results of Hubrig et al of late emergence of surface fields, but both studies found a remarkable shortage of young magnetic stars with M < ∼ 2 M .…”

“…Sample y-axis x-axis Slope log(R 2 B rms ) log t −0.29 ± 0.07 All masses log R 2 log t 0.02 ± 0.04 log B rms log t −0.31 ± 0.06 log(R 2 B rms ) log t 0.07 ± 0.15 M < 3.0 M log R 2 log t 0.23 ± 0.06 log B rms log t −0.16 ± 0.14 log(R 2 B rms ) log t −0.22 ± 0.14 3.0 M ≤ M ≤ 4.0 M log R 2 log t 0.19 ± 0.06 log B rms log t −0.42 ± 0.14 log(R 2 B rms ) log t −0.27 ± 0.12 Bagnulo et al (2003), Pöhnl et al (2005), and Kochukhov & Bagnulo (2006). Based on the analysis of field stars, Hubrig et al (2000) have proposed that, in stars with M < ∼ 3 M , a magnetic field appears at the surface of an Ap stars only after about 30% of its main sequence liftime has elapsed.…”

Evolution of magnetic fields in stars across the upper main sequence: II. Observed distribution of the magnetic field geometry

“…Knowledge of the evolution of the magnetic field and its geometry, especially of the distribution of the obliquity angle β (orientation of the magnetic axis with respect to the rotation axis) is essential to understand the physical processes taking place in these stars and the origin of their magnetic fields (fossil versus contemporary dynamo). Early studies of the evolution of some characteristics of magnetic Corresponding author: shubrig@eso.org Ap and Bp stars were based on smaller stellar samples (e.g., North 1985;Hubrig, North & Mathys 2000;Hubrig, North & Szeifert 2005), or on stellar samples which included CP stars with not well-defined magnetic field strengths or rotation periods (e.g., Rüdiger & Scholz 1988;Pöhnl, Paunzen & Maitzen 2005;Kochukhov & Bagnulo 2006). …”