June 2005Masaryk University, Brno1 The magnetic fields of peculiar A and B stars in open clusters John D Landstreet University of Western Ontario London, Upper Canada

June 2005Masaryk University, Brno2 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

June 2005Masaryk University, Brno3 Stars in the H-R diagram The H-R diagram contains stars in their initial, H-burning phase, on main sequence Following central H exhaustion stars evolve to giant phases, burning He and heavier elements After all fuel is gone, stars become white dwarfs, neutron stars, or black holes On main sequence we expect to find variations of chemistry only due to chemical evolution (metal enrichment) of the galaxy, thus only in cool stars

June 2005Masaryk University, Brno4 Chemically peculiar A and B stars Lower main sequence stars do show variations in chemistry, due to evolution of interstellar abundances with age of galaxy Upper main sequence stars show little chemical variety, due mainly to (a) extreme youth and (b) strong winds In 1897 Antonia Maury identified some middle main sequence A and B stars with very peculiar spectra We now know that these “Ap” stars do have very peculiar atmospheric chemical compositions They constitute about 10% of all “tepid” main sequence stars

June 2005Masaryk University, Brno5 Types of peculiar A and B stars Peculiarities are identified by prominent spectral features (HgMn stars, SrCrEu stars, He-weak stars)‏ Each peculiarity type is found in a restricted mass (or temperature) range of main sequence Limited range in T eff even on main sequence shows that peculiarities are a surface effect, not a bulk property of stars

June 2005Masaryk University, Brno6 Are Ap stars interesting? Observed variety of chemical peculiarities of Ap stars is due to varying competition among gravitational diffusion, radiative levitation, turbulent & convective mixing, mass loss, etc (Michaud, Vauclairs, Alecian, Charland…)‏ In cooler and hotter main sequence stars, this competition is overwhelmed by a single process (hot stars: mass loss; cool stars: deep convection zone)‏ Thus Ap stars are unique laboratories of internal stellar hydrodynamics Furthermore, some (but not all) chemically peculiar A and B stars have strong, global magnetic fields, so we can study interaction of fields with hydrodynamics

June 2005Masaryk University, Brno7 Magnetic fields in stars Our work has so far focused on magnetic field measurements, so review how this is done Magnetic fields in stars are detected by the Zeeman effect In a magnetic field, a single spectral line splits into pi and sigma components, with separation proportional to B Components are polarized, and both line of sight and transverse field components may be measured by spectropolarimetry Observed line profiles are result of summing over stellar disk

June 2005Masaryk University, Brno8 Fields in Ap stars Fields are detected in some peculiar stars via directly observed line splitting (e.g. HD 94660)‏ Most fields are detected by polarization in spectral lines, as in NGC Field strengths are of order G (0.01 – 3 T)‏ Fields appear to cover whole star, they are not spotty as on surface of Sun

June 2005Masaryk University, Brno9 Features of magnetic Ap stars Field strength, photometric brightness, and spectrum are usually periodically variable Periods range from 0.5d to many years, inversely correlated with v sin i => Period is rotation period of star Specific angular momentum typically 0.1x or less of normal A star, => extra braking (when?)‏

June 2005Masaryk University, Brno10 A model of magnetic Ap structure Observed relation between field modulus |B| and line-of- sight component B z => field structure is “simple” Periodic B z field reversal suggests dipole-like topology with dipole axis inclined to rotation axis: the “oblique dipole rotator” Spectrum and light variations => non-uniform distribution of various elements over stellar surface (“abundance patches”)‏ Such patches are possible because magnetic field suppresses “weather” which would mix atmosphere horizontally

June 2005Masaryk University, Brno11 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

June 2005Masaryk University, Brno12 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

June 2005Masaryk University, Brno13 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 T eff and log(L/L o ) 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 – 10 6, 10 7, 10 8 yr, etc We can also look for statistical trends in field strength, chemical abundances, rotation periods, etc

June 2005Masaryk University, Brno14 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 < 3M o after 30% of the main sequence lifetime. Is this reasonable? Consider rotation

June 2005Masaryk University, Brno15 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?

June 2005Masaryk University, Brno16 New observational data Hubrig results are quite uncertain, even with Hipparcos parallaxes! (Realistic) errors of ~500 K for T e and ~0.1 dex for log(L/L o ) 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?

June 2005Masaryk University, Brno17 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

June 2005Masaryk University, Brno18 Cluster star magnetic measurements Bagnulo, Mason, Wade, Silaj & I have been observing probable cluster Ap’s for fields using FORS1 at VLT, and ESPaDOnS at CFHT Candidates are from Δa or Geneva Z photometry, or spectral classifications We now have about 80 field detections in some 30 clusters and associations (including Orion and Sco-Cen)‏

June 2005Masaryk University, Brno19 How to use these data? We determine membership using parallaxes from Hipparcos, proper motions from Tycho- 2, and occasional radial velocities. To find mass of a cluster star, we use T e, 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

June 2005Masaryk University, Brno20 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 M above 3 M o, beyond about 30 Myr, but hardly change with time for lower mass stars Contrary to Hubrig theory, plenty of magnetic fields in young 2 – 3 M o stars Hardly any stars in sample below 2 M o of any age, even though Ap’s occur down to 1.6 M o among field stars!?

June 2005Masaryk University, Brno21 Some conclusions It is now very practical to study fields of cluster Ap stars If we split samples by mass (say or M o ), we find statistical decrease of fields from young to old for stars above 3 M o, but not below this mass. There is no obvious shortage of young magnetic stars between 2 and 3 M o. The Hubrig et al hypothesis of late field emergence is not confirmed. However, there does seem to be a shortage of magnetic stars (and even Ap stars) with M<2M o -- of all ages -- in open clusters, compared to the field where Ap stars down to 1.6 M o are found.

June 2005Masaryk University, Brno22 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 magneto- hydrodynamic processes, our initial goal