Investigating MAGnetism of INtErmediate-mass and massive stars – IMAGINE

First, We shall take advantage of an unprecedented set of recent and ongoing spectropolarimetric surveys to gather constraints on the stability of the fossil magnetic fields by studying the dependence of its lower bound as a function of rotation and mass, its occurrence in close binary systems and how it survives to post-mainsequence evolution. The stability of large scale magnetic fields in differentially rotating stars will be investigated numerically as an attempt to quantitatively reproduce the observed minimum field and the magnetic desert. Concerning Vega-like magnetism, a deep spectropolarimetric survey will tell whether this new type of magnetism is ubiquitous among intermediate-mass and massive stars, which would provide to stellar evolution models the first constraint on the field strength of a typical star. Meanwhile, the possibility that it originates from a dynamo driven by differential rotation will be investigated theoretically.

An observing run of weak fossil fields has been performed to demonstrate the dependence of the lower bound of Ap/Bp magnetic fields with rotation.
The data are being analyzed. The stablity of a dipolar field submitted to an initial differential rotation has been studied through axisymmetric and 3D numerical simulations. The results provide insight into the bifurcation between strong stable fields and weak unstable ones that is expected to be at the origin of the observed dichotomy between fossil and Vega-like magnetisms. Observations of Vega-like magnetism enabled to confirm the detection of a polarimetric signal in three other intermediate mass stars, Sirius, Beta Uma, Theta Leo. On Vega, evidence of a structured surface showing bright or dark plages with a very tiny contrast have been discovered through spectroscopic studies. Finally, the presence of Vega-like fields among more massive stars
has been constrained through targeted deep spectropolarimetric observations.

The dependence of the lower bound of Ap/Bp fields with rotation will be established and compared with results of numerical simulations describing the stability of fossil fields submitted to differential rotation. These simulations will become more realistic by introducing stable stratification effects and a forcing mechanism for the differential rotation. The properties of Vega-like magnetism and its incidence among more massive stars will be the aim of further observing campaigns.

thanks to the introduction of stratification effects and a mech

Blazère, A; Neiner, C.; Petit, P., 2016, MNRAS 459, L81

Blazère, A.; Petit, P.; Lignières, F.; Aurière, M.; Ballot, J.; Böhm, T.; Folsom, C. P.; Gaurat, M.; Jouve, L.; Lopez Ariste, A.; Neiner, C ; Wade, G.A., 2016, A&A 586A, 97

Blazère, A. ; Neiner, C. ; Petit, P. ; Lignieres, F. ; 2016, Proceedings of the Annual meeting of the French Society of Astronomy and Astrophysics.

Neiner, C.; Buysschaert, B.; Oksala, M.E.; Blazère, A., 2015, MNRAS 454, L56

Blazère, A.; Neiner, C.; Tkachenko, A.; Bouret, J.-C.; Rivinius, Th., 2015, A&A 582A, 110

Gaurat, M., Jouve, L., Lignières, F., Gastine, T. \2015, 580, 103 A&A

Blazère, A.; Neiner, C.; Bouret, J.-C. & Tkachenko A., 2015, IAUS 307, 367

Neiner, C. ; Mathis, S. ; Alecian, E. ; Emeriau, C.; Grunhut, J., 2015, IAUS 305, 61

Blazère, A., Petit, P., Lignières, F., Aurière, M., Ballot, J., Böhm, T., Folsom C., Lopez-Ariste, A, Wade, G. \ 2015, IAU Symposium 305, 67

Böhm, T., Holschneider, M., Lignières, F.,Petit, P.; Rainer, M.; Paletou, F.; Wade, G.; Alecian, E.; Carfantan, H.; Blazère, A.; Mirouh, G. M. \2015, A&A, 577, A64

Jouve, L., Gastine, T., Lignières, F.\ 2015, A&A, 575, A106

Blazère, A., Neiner, C., Bouret, J.-C., Tkachenko, A.\ 2015, IAU Symposium, 307, 367

Blazère, A., Petit, P., Lignières, F., Aurière, M., Böhm, T., Wade, G. \ 2014, SF2A-2014: Proceedings of the Annual meeting of the French Society of Astronomy and Astrophysics, 463

Neiner, C., Folsom, C.P., Blazere, A.\ 2014, SF2A-2014: Proceedings of the Annual meeting of the French Society of Astronomy and Astrophysics, 163

Wade, G.~A., Folsom, C.~P., Petit, P., Petit, V.; Lignières, F., Aurière, M., Böhm, T.\ 2014, MNRAS, 444, 1993

Lignières, F., Petit, P., Aurière, M., Wade, G.A., Böhm, T.\ 2014, IAU Symposium, 302, 338

Petit, P., Lignières, F., Wade, G.A., Aurière, M., Wade, G.A., Böhm, T. et al. \ 2014, A&A, 568, C2

Neiner, C., Monin, D., Leroy, B., Mathis, S., Bohlender, D.\ 2014, A&A, 562, A59

Understanding the magnetism of massive and intermediate-mass stars is critical to make progress in stellar evolution theory. Magnetic fields are known to play a fundamental role in the transport processes that need to be considered to improve stellar models. Yet, until recently, only a few massive stars and a small (~5%) fraction of intermediate-mass stars were known to be magnetic. The current paradigm, the fossil field theory, describes this magnetism as remnant of an early phase of the star-life, but leaves many basic questions unanswered, such as the small fraction of magnetic stars, and in practice provides no constraint to stellar evolution theory.
In recent years, a new generation of spectropolarimeters has provided new clues to understand this magnetism. First, the same proportion of strong magnetic fields have been detected among intermediate-mass and massive stars, suggesting a common origin to their magnetism. Then, unanticipated observations revealed the lower bound to the magnetic fields of intermediate-mass stars and a two orders of magnitude magnetic desert between this lower bound and a new type of sub-Gauss magnetism, first discovered on the bright star Vega. This prompted a scenario where the strong fossil and weak Vega-like magnetisms originate from the bifurcation between stable and unstable large scale magnetic configurations in differentially rotating stars. Based on these new findings, we propose a combination of modelling and observational efforts that will provide a coherent view of massive and intermediate-mass star magnetism.
We shall take advantage of an unprecedented set of recent and ongoing spectropolarimetric surveys to gather constraints on the stability of the fossil magnetic fields by studying (i) the dependence of its lower bound as a function of rotation and mass, (ii) its occurrence in close binary systems and (iii) how it survives to post-main-sequence evolution. The stability of large scale magnetic fields in differentially rotating stars will be investigated numerically as an attempt to quantitatively reproduce the observed minimum field and the magnetic desert. Concerning Vega-like magnetism, a deep spectropolarimetric survey will tell whether this new type of magnetism is ubiquitous among intermediate-mass and massive stars, which would provide to stellar evolution models the first constraint on the field strength of a typical star. Meanwhile, the possibility that it originates from a dynamo driven by differential rotation will be investigated theoretically. The project will benefit from up-to-date computing facilities.