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The magnetic fields of peculiar A and B stars in open clusters
John D Landstreet University of Western Ontario London, Upper Canada

22.2.200821 June 2005

Armagh Observatory Workshop

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The Team:
· · · · · · Stefano Bagnulo, Armagh Observatory (N Ireland) Vincenzo Andretta, INAF (Italy) Luca Fossati, Vienna Observatory (Austria) Elena Mason, ESO (Chile) Jessie Silaj, University of Western Ontario (Canada) Gregg Wade, Royal Military College of Canada

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Armagh Observatory Workshop

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Magnetic fields some basic problems
· What is the nature of magnetic fields found in some A and B stars? How are they produced? -- Long-term stability, simple structure, lack of "activity", lack of correlation between B and rotation rate suggest that field is a "fossil" left from (at least) PMS · Why do Ap stars have fields while other A & B's do not? -- Not yet understood · How does the field evolve as the star evolves? -- If it is a fossil, there is ohmic decay plus distortion and amplification due to stellar structure changes

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Observational study of Ap evolution
· At present, we know that at some unknown time in its main sequence life, a magnetic Ap star can have an observed field structure and surface chemistry · To make observed characteristics of A stars into far more powerful probes of (M)HD processes, we want to be able to associate a particular field structure and chemistry with a particular mass and age, not just say that the observed state happen sometime, in a star of unknown age and mass

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Armagh Observatory Workshop

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Observational study of Ap evolution - 2
· We need to determine masses, ages, and fractional ages (= fraction of MS lifetime) for a substantial sample of Ap stars · The obvious method is to determine Teff and log(L/Lo) for field stars from parallaxes and photometry, and then compare results with standard evolution models in HR diagram to determine mass and fractional age · We can then associate a particular observed star with a particular age since the PMS 106, 107, 108 yr, etc · We can also look for statistical trends in field strength, chemical abundances, rotation periods, etc
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Hubrig Theory
· Hubrig et al have placed a sample of nearby (field) magnetic stars with Hipparcos parallaxes onto the HR Diagram · They claim that they find evidence that magnetic fields first emerge in stars of M < 3Mo after 30% of the main sequence lifetime. · Is this reasonable? Consider rotation
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Slow rotation of magnetic Ap stars
· Slow rotation of Ap stars probably due to magnetic coupling with circumstellar material (accretion disk, stellar wind) · Even young Ap stars (in clusters, associations) have slow rotation, so loss of angular momentum probably occurs in PMS phase (North) · Slowest rotators are cooler lower mass Ap stars that spend longer in PMS phase (Stepien) · Hubrig et al result conflicts completely with this picture. If it is right, how do Ap's lose angular momentum without a surface field present to couple to surroundings?
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New observational data
· Hubrig results are quite uncertain, even with Hipparcos parallaxes! · (Realistic) errors of ~500 K for Te and ~0.1 dex for log(L/Lo) lead to large age uncertainties, especially if the bulk composition is not known see figures at right · How can we provide better masses and ages from observation to better constrain physical processes active in magnetic Ap stars?

22.2.200821 June 2005

Armagh Observatory Workshop

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Cluster magnetic Ap stars
· Ap's in clusters would have more accurate ages. · Two recent major advances have made cluster magnetic stars accessible: -- New proper motions from Hipparcos and especially Tycho-2 have greatly improved knowledge of cluster membership down to V~ 10 or fainter, and surveys by Maitzen group have greatly expanded information about probable Ap stars beyond classification by Abt, etc. -- Powerful spectropolarimeters on large telescopes (FORS1 on ESO VLT, Espadons on CFHT) make possible field measurement at V ~ 10 or even 12
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How to use these data?
· We determine membership using parallaxes from Hipparcos, proper motions from Tycho2, and occasional radial velocities. · To find mass of a cluster star, we use Te, determined from Geneva or uvby photometry · We find luminosity using cluster distance and new BC's. · Then compare stars to evolution tracks (Geneva, Padova, etc.) as before, but now only for the cluster's known age

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Armagh Observatory Workshop

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Results so far
· Sample is rich in massive and relatively young stars · Fractional ages of young cluster magnetic Ap stars are much more precise than those of field stars · Fields and fluxes definitely decline with age for all mass ranges between 2 and 5 M0 · This evolution is not in very good agreement with field increase during part of MS lifetime predicted by Brathwaite and collaborators · Contrary to Hubrig theory, plenty of magnetic fields in young 2 3 Mo stars · Hardly any stars in sample below 2 Mo of any age, even though Ap's occur down to 1.6 Mo among field stars!?
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Field strength vs age

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Flux vs age

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Next?
· The next step is to study how chemistry evolves with stellar age in our sample. We then have a powerful tool for using observed stars to inform theory about how both fields and atmosphere chemistry depend on mass and evolve with time · This should be very helpful in using observed stellar characteristics to guide and test ideas about the interaction of various stellar hydrodynamic and magnetohydrodynamic processes, our initial goal

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