MagIcS
magnetism in intermediate-mass and high-mass stars

	© HST

Field structure and implications for stellar evolution

Intermediate-mass and high-mass stars are distinguished by strong, organised magnetic fields which appear to be primarily fossil in origin - remnants of field accumulated from the interstellar medium during the process of star formation, or possibly produced by dynamo activity at the protostellar phase. Due to the relic nature of their magnetic fields, massive stars provide us with the unique capability to study how fields evolve throughout the various phases of stellar evolution, and to explore how fields are influenced by the large variety of structural changes that occur during the pre-main sequence, main sequence, and post-main sequence evolutionary phases. Ultimately, this allows to explore how fields affect, both directly and indirectly, the process of stellar evolution.

The pre-main sequence

Magnetic fields have now been confidently detected in a small number of intermediate-mass and high-mass pre-main sequence Herbig Ae and Be stars. These fields provide a crucial evolutionary connection between the fields present during stellar formation and those which are well-known to exist on the main sequence. Although recent results provide important new constraints on models of field evolution, significant open questions remain. In particular: Structural properties change rapidly and significantly on the PMS HR diagram. How do field properties correlate (if at all) with HRD position (and therefore stellar structure)? How do field properties change according to star formation environment and stellar age? To what extent do fields influence the circumstellar environment? What are the detailed interactions of the field with mass loss and accretion processes, and how do they yield the remarkable slow rotation of many intermediate-mass MS magnetic stars? What are the conditions and timescale necessary for the development of the signature chemical peculiarities observed in many intermediate-mass stars?

The main sequence

Magnetic fields have been known to exist in some main sequence A and B-type stars for over 50 years, and more recently fields have been discovered in a small number of O-type stars. The three most striking systematic characteristics of the magnetism of upper MS stars are the presence of fields in only a small fraction of stars (and the apparent total absence of such fields in the large majority), the complete disappearance of organised fields at masses below about 1.5 M_sun, and the apparent extreme rarity of fields at higher stellar masses. On the main sequence, the presence of magnetic fields in stars has clear and well-documented effects: e.g. the production of chemical peculiarity and inhomogeneity, magnetic confinement and modulation of stellar winds, and interaction with stellar pulsation. Because the main sequence is the evolutionary phase at which we know the most about fossil magnetism, it is at this stage that ESPaDOnS observations can provide the deepest insight. We aim to address questions such as: What physical or evolutionary processes are responsible for the systematic properties of magnetic fields in A/B/O stars? How do the magnetic properties evolve with time on the main sequence, and to what extent are magnetic properties a function of the environment in which a star is born and evolves? Is the rarity of magnetic fields in massive stars the result of observational selection? What is the contribution of magnetic fields to the variability observed in the winds of many massive stars, and are magnetic fields responsible for non-thermal radio and X-ray emission from O stars? By what mechanisms do magnetic fields influence chemical diffusion? We know much about the global-scale fields of upper-MS stars, but what is the smaller-scale field structure? How do magnetic fields interact with/excite stellar pulsation, and how can such pulsations be used to explore the internal structure of magnetic stars?

The post-main sequence

The post-MS evolution of intermediate-mass and high-mass stars leads ultimately to the formation of white dwarfs and neutron stars, classes of degenerate stars which also host strong, organised magnetic fields. We will explore the proposal that there exists a direct evolutionary link between the magnetic properties observed on the MS and those in stellar remnants, a task for which ESPaDOnS is uniquely well-suited. In particular, we have the following questions: Are the real magnetic properties of MS star progenitors of WDs and NSs compatible with field evolution? Can evidence of evolved fossil magnetic fields be established at intermediate evolutionary stages, for example in intermediate-mass red giants, massive supergiants or in Wolf-Rayet stars?

Related threads and associated Large Program

MiMeS, a Large Program with ESPaDOnS at CFHT, focussing on the most massive stars, was awarded time in 2008a.

Core team and collaborators

Thread coordinators: G Wade, C Neiner, N StLouis, S Owocki

Agency/Country	Core team	Collaborators
F	Alecian, Aurière, Bouret, Brun, Catala, Donati, Hubert, Martins, Martayan, Mathis, Montmerle, Neiner, Stehle, Zahn	Böhm, HuiBonHoa, Lignières
O	Bagnulo, Braithwaite, Fremat, Harries, Henrichs, Howarth, Hubrig, Jardine, Kochukhov, Luftinger, Piskunov	Drew, Kaper, Lanz, North, Oudmaijer, Vink
C	Bohlender, Charbonneau, Folsom, Fullerton, Grunhut, Landstreet, Moffat, V Petit, Power, StLouis, Sylvester, Wade	Drissen, Michaud, Robert
H	Harrington, Kuhn
T	Trung
US	Gagné, Oksala, Owocki, Smith, Townsend, UdDoula	Cohen, Feigelson, Ignace