Research

Surface Mapping of Ap Stars

My previous research work exploited the spectropolarimetric capabilities of instruments like ESPaDOnS (on the CFHT, Hawaii) and NARVAL (on the TBL, France) to obtain high signal-to-noise ratio, high resolution, phase-resolved spectra of a selected sample of magnetic Ap stars in all 4 Stokes parameters (Stokes IQUV).

The Canada France Hawaii Telescope (CFHT) and the Telescope Bernard Lyot (TBL)

The resulting datasets are being used to construct detailed, assumption-free maps of the vector magnetic fields and chemical abundance structures in the photospheres of these stars using the technique known as Magnetic Doppler Imaging (MDI). Such maps represent a crucial step toward understanding the origin and evolution of magnetic fields in upper-main sequence stars and for constraining the influence these magnetic fields have on the chemical transport processes (e.g. diffusion, convection, mass loss).

If one observes a star as it rotates, one would observe that one side of the stellar sphere is rotating towards you and the other side is rotating away from you. This results in a redshift on the receding side and a blueshift on the approaching side which ultimately results in the spectral line being broadened about the line centre (central wavelength). If there was a spot on the surface of the star (either a chemical abundance spot or temperature spot), the spot would exhibit itself in a spectral line profile as a distortion. As the spot traverses the stellar sphere from one limb to the other (due to rotation) the distortion will move from one edge of the profile, through the centre of the profile and then eventually to the other edge.

How a spectral line profile changes as a function of stellar rotation when a chemical spot is present on the surface of a star.

Therefore if we observe the star from all aspect angles, it becomes possible to construct a 2-D chemical of the stellar surface. This technique combined with polarimetric data, allows the reconstruction of the vector magnetic map simultaneously and self-consistently with the distributions of chemical elements. Such a map is shown below in the case of the Ap star α 2 CVn or HD 32633.

Magnetic field map of α 2 CVn (Cor Caroli or the Heart of Charles). Silvester, Kochukhov and Wade (2014, MNRAS, 444, 1442)

Magnetic field map of HD 32633. Silvester, Kochukhov and Wade (2015, MNRAS, 453, 2163)


Refereed Publications

Alvarado-Gómez, J.D., Hussain, G. A. J., Drake J. J., Donati, J-F., Sanz-Forcada, J., Stelzer, B., Cohen O., Amazo-Gómez, E.M., Grunhut, J.H., Garraffo, C., Moschou, S.P., Silvester J., Oksala, M.E., 2017, MNRAS, Accepted
Far beyond the Sun: I. The beating magnetic heart in Horologium

Oksala M. E., Silvester J., Kochukhov O., Neiner C., Wade G. A., the MiMeS Collaboration, 2017, MNRAS, Accepted
Mixed poloidal-toroidal magnetic configuration and surface abundance distributions of the Bp star 36 Lyn

Silvester J., Kochukhov O., Rusomarov N., Wade G. A., 2017, MNRAS, 471, 962
The complex magnetic field topology of the cool Ap star 49 Cam

Kochukhov O., Silvester J., Bailey J. D., Landstreet J. D., Wade, G. A., 2017, A&A, 605, 13
Magnetic field topology and chemical abundance distributions of the young, rapidly rotating, chemically peculiar star HR 5624

Silvester J., Kochukhov O., Wade G.A., 2015, MNRAS, 453, 2163
The magnetic field topology and chemical abundance distributions of the Ap star HD 32633

Oksala M. E., Kochukhov O., Krtička J., Townsend R. H. D., Wade G. A., Prvak M., Mikulasek Z., Silvester J., Owocki, S. P., 2015, MNRAS, 451, 2015,
Revisiting the rigidly rotating magnetosphere model for sigma Ori E - II. Magnetic Doppler imaging, arbitrary field RRM, and light variability

Silvester J., Kochukhov O., Wade G. A., 2014, MNRAS, 444, 1442,
Stokes IQUV magnetic Doppler imaging of Ap stars – III. Next generation abundance mapping of α 2 CVn

Silvester J., Kochukhov O., Wade G. A., 2014, MNRAS, 440, 182,
Stokes IQUV magnetic Doppler imaging of Ap stars – II. Next generation magnetic Doppler imaging of α 2 CVn 

Silvester J., Wade G. A., Kochukhov O., Bagnulo S., Folsom C. P., Hanes D.A., 2012, MNRAS, 426, 1003,
Stokes IQUV magnetic Doppler imaging of Ap stars – I. ESPaDOnS and Narval Observations

Bailey J. D., Landstreet J. D.,Bagnulo S., Fossati L., Kochukhov O., Paladini C., Silvester J., Wade G.A., 2011, A&A, 535, 25,
Magnetic Field and Atmospheric Chemical Abundances of the Magnetic Ap Star HD 318107

Folsom C. P., Kochukhov O., Wade G. A., Silvester J., Bagnulo S., 2010, MNRAS, 407, 2383,
Magnetic field, chemical composition and line profile variability of the peculiar eclipsing binary star AR Aur

Shavrina A. V. ; Glagolevskij Yu. V.,  Silvester J., Chuntonov G.A., Khalack V.R., Pavlenko Ya.V., 2010, MNRAS, 401,1882,
Spots structure and stratification of helium and silicon in the atmosphere of He-weak star HD 21699

Silvester J., Neiner C., Henrichs H. F., Wade G. A., Petit V., Alecian E., Huat A.-L., Martayan C., Power J., Thizy O., 2009, MNRAS, 398, 1505,
On the incidence of magnetic fields in slowly pulsating B, Beta Cephei and B-type emission-line stars

Landstreet J.D., Silaj J., Andretta V., Bagnulo S., Berdyugina S.V., Donati J.-F., Fossati L., Petit P., Silvester  J., Wade G. A., 2008, A&A, 481, 2,
Searching for links between magnetic fields and stellar evolution. III. Measurement of magnetic fields in open cluster Ap stars with ESPaDOnS

Alecian  E. ,Catala C., Wade G. A., Donati J.-F., Petit P., Landstreet J. D., Bohm T., Bouret J.-C., Bagnulo S., Folsom C., Grunhut J., Silvester J., 2008, MNRAS, 385, 391,
Characterization of the magnetic field of the Herbig Be star HD200775

Auriere M., Wade G.A.,Silvester J., et al. 2007, A&A, 475, 1053,
Weak magnetic fields in Ap/Bp stars. Evidence for a dipole field lower limit and a tentative interpretation of the magnetic dichotomy

Catala C., Alecian E., Donati J.-F., Wade G. A., Landstreet J. D., Bohm T., Bouret J.-C., Bagnulo S., Folsom C., Silvester J., 2007, A&A, 462, 293,
The magnetic field of the pre-main sequence Herbig Ae star HD 190073

Wade G. A., Drouin D., Bagnulo S., Landstreet J. D., Mason E., Silvester J., Alecian E., Bohm T., Bouret J.-C., Catala C., Donati J.-F., 2005, A&A, 442, 31,
Discovery of the pre-main sequence progenitors of the magnetic Ap/Bp stars ?

Ryabchikova T., Wade G. A., Auriere M., Bagnulo S., Donati J.-F., Jeffers S. V., Johnson  N., Landstreet J. D., Lignieres F., Lueftinger T., Marsden S.,
Mouillet D., Paletou F., Petit P., Reegen P., Silvester J., Strasser S., Toque N., 2005, A&A, 429L, 55,
Rotational periods of four roAp stars