First Zeeman Doppler imaging of a cool star using all four Stokes parameters

Magnetic fields are ubiquitous in active cool stars but they are in general complex and weak. Current Zeeman Doppler imaging (ZDI) studies of cool star magnetic fields chiefly employ circular polarization observations because linear polarization is difficult to detect and requires a more sophisticated radiative transfer modeling to interpret. But it has been shown in previous theoretical studies, and in the observational analyses of magnetic Ap stars, that including linear polarization in the magnetic inversion process makes it possible to correctly recover many otherwise lost or misinterpreted magnetic features. We have obtained phase-resolved observations in all four Stokes parameters of the RS CVn star II Peg at two separate epochs. Here we present temperature and magnetic field maps reconstructed for this star using all four Stokes parameters. This is the very first such ZDI study of a cool active star. Our magnetic inversions reveal a highly structured magnetic field topology for both epochs. The strength of some surface features is doubled or even quadrupled when linear polarization is taken into account. The total magnetic energy of the reconstructed field map also becomes about 2.1-3.5 times higher. The overall complexity is also increased as the field energy is shifted towards higher harmonic modes when four Stokes parameters are used. As a consequence, the potential field extrapolation of the four Stokes parameter ZDI results indicates that magnetic field becomes weaker at a distance of several stellar radii due to a decrease of the large-scale field component.Comment: 16 pages, 16 figures, 4 tables; Accepted for publication in Ap

Beam me up, Spotty: Toward a new understanding of the physics of massive star photospheres

For 30 years, cyclical wind variability in OB stars has puzzled the astronomical community. Phenomenological models involving co-rotating bright spots provide a potential explanation for the observed variations, but the underlying physics remains unknown. We present recent results from hydrodynamical simulations constraining bright spot properties and compare them to what can be inferred from space-based photometry. We also explore the possibility that these spots are caused by magnetic fields and discuss the detectability of such fields.Comment: 2 pages, 1 figure, IAU Symposium 30

Discovery of electron cyclotron MASER emission from the magnetic Bp star HD 133880 with the Giant Metrewave Radio Telescope

We report the discovery of coherent radio emission from the young, rapidly-rotating magnetic Bp star HD 133880 at a frequency of 610 MHz with the Giant Metrewave Radio Telescope (GMRT). This is only the second magnetic star in which coherent radio emission has been detected. In our observations of HD 133880 covering the full rotational cycle of the star (except for a phase window 0.17-0.24), we witness an abrupt order-of-magnitude flux enhancement along with $\approx100$ percent right circular polarization. We attribute this phenomenon to coherent Electron Cyclotron MASER Emission. We attribute the lack of left circularly polarised emission to the asymmetric topology of the star's magnetic field. The phase of enhancement, $0.73$, differs from the previously reported phase of enhancement, $0.16$, (at 610 MHz Chandra et al. 2015) by one-half cycle. However, no flux enhancement is found at phase $0.16$ in our data, which could be due to an unstable or drifting emission region, or a consequence of the reported changes of the star's rotational period. Either of these factors could have shifted the enhancement to the above-mentioned phase window not sampled by our observations.Comment: Accepted for publication in MNRAS Letter

Stokes IQUVIQUV magnetic Doppler imaging of Ap stars - III. Next generation chemical abundance mapping of Alpha 2 CVn

In a previous paper we presented an updated magnetic field map for the chemically peculiar star Alpha 2 CVn using ESPaDOnS and Narval time-resolved high-resolution Stokes $IQUV$ spectra. In this paper we focus on mapping various chemical element distributions on the surface of Alpha 2 CVn. With the new magnetic field map and new chemical abundance distributions we can investigate the interplay between the chemical abundance structures and the magnetic field topology on the surface of Alpha 2 CVn. Previous attempts at chemical abundance mapping of Alpha 2 CVn relied on lower resolution data. With our high resolution (R=65,000) dataset we present nine chemical abundance maps for the elements O, Si, Cl, Ti, Cr, Fe, Pr, Nd and Eu. We also derive an updated magnetic field map from Fe and Cr lines in Stokes $IQUV$ and O and Cl in Stokes $IV$. These new maps are inferred from line profiles in Stokes $IV$ using the magnetic Doppler imaging code Invers10. We examine these new chemical maps and investigate correlations with the magnetic topology of Alpha 2 CVn. We show that chemical abundance distributions vary between elements, with two distinct groups of elements; one accumulates close to the negative part of the radial field, whilst the other group shows higher abundances located where the radial magnetic field is on the order of 2 kG regardless of the polarity of the radial field component. We compare our results with previous works which have mapped chemical abundance structures of Ap stars. With the exception of Cr and Fe, we find no clear trend between what we reconstruct and other mapping results. We also find a lack of agreement with theoretical predictions. This suggests that there is a gap in our theoretical understanding of the formation of horizontal chemical abundance structures and the connection to the magnetic field in Ap stars.Comment: 14 Pages, 8 Figures, Accepted in MNRA

Understanding the fossil magnetic fields of Ap/Bp stars: Conclusions from a volume-limited survey

Various observational properties of Ap/Bp stars have been well-established such as the often-cited 10% incidence rate of strong, organized magnetic fields amongst all A- and B-type stars. However, these inferences have generally been drawn fromsurveys biased towards the strongest most easily detectable fields. A volume-limited spectropolarimetric survey of all intermediate-mass stars within 100 pc was initiated in 2007 in order to avoid the biases inherent in previous studies. This workyielded the magnetic properties of a large number of Ap/Bp stars in the sample; however, nearly half of the sample remained either unobserved or had relatively poor constraints on their field strengths and geometries. We have recently completed thissurvey using measurements obtained by ESPaDOnS and NARVAL. We discuss here some of the recent findings of this survey.Comment: 6 pages, 6 figures; presented at the conference "Stars with a stable magnetic field: from pre-main sequence to compact remnants"; to be published by Contributions of the Astronomical Observatory Skalnat\'e Ples

Deciphering the Surprising Variability of A-type Stars Revealed by Kepler

A recent analysis of high precision photometry obtained using the Kepler spacecraft has revealed two surprising discoveries: (1) over 860 main sequence A-type stars -- approximately 40% of those identified in the Kepler field -- exhibit periodic variability that may be attributable to rotational modulation by spots and (2) many of their light curves indicate the presence of a mysterious and characteristic power spectral feature. We have been carrying out an ongoing analysis designed to expand upon these discoveries and to provide a possible explanation for the unusual power spectral features. In the following, we will put these recent discoveries into context as well as discuss the preliminary findings yielded by our analysis of the Kepler light curves.Comment: 5 pages, 2 figures, to be published in the proceedings of the 3rd BRITE Science Conference held in Saint-Michel-des-Saints (QC, Canada), 2017 August 7-10 -- Proceedings of the Polish Astronomical Societ

First Empirical Constraints on the Low Hα\alpha Mass-Loss Rates of Magnetic O-Stars

A small subset of Galactic O-stars possess surface magnetic fields that alter the outflowing stellar wind by magnetically confining it. Key to the magnetic confinement is that it induces rotational modulation of spectral lines over the full EM domain; this allows us to infer basic quantities, e.g., mass-loss rate and magnetic geometry. Here, we present an empirical study of the H$\alpha$ line in Galactic magnetic O-stars to constrain the mass fed from the stellar base into the magnetosphere, using realistic multi-dimensional magnetized wind models, and compare with theoretical predictions. Our results suggest that it may be reasonable to use mass-feeding rates from non-magnetic wind theory if the absolute mass-loss rate is scaled down according to the amount of wind material falling back upon the stellar surface. This provides then some empirical support to the proposal that such magnetic O-stars might evolve into heavy stellar-mass black holes (Petit et al. 2017)Comment: 4 pages, 3 figures, IAU Symposium 346 proceeding

Detection of coherent emission from the Bp star HD 142990 at uGMRT frequencies

HD 142990 is a Bp-type star with a nearly dipolar surface magnetic field of kG strength. Recently Lenc et al. (2018) reported the tentative discovery of Electron Cyclotron Maser Emission (ECME) from this star at 200 MHz. This type of emission has been observed from only three other hot magnetic stars. In this paper, we present our observations of HD 142990 with the upgraded Giant Metrewave Radio telescope (uGMRT) at 550-804 MHz and with the legacy GMRT at 1420 MHz near the rotational phases of the nulls of the longitudinal magnetic field. We found strong enhancements in flux density in both circular polarisations suggesting an ECME bandwith of at least 1.2 GHz (200-1420 MHz). In one of the observation sessions, we observed enhancements with opposite circular polarisations from the same magnetic pole. This has not been reported in any other hot magnetic star known to exhibit ECME. In order to explain this unusual finding, we suggest a scenario that involves a transition of the dominant mode of ECME between the magneto-ionic modes.Comment: Accepted for publication in the Ap

Investigating the origin of cyclical spectral variations in hot, massive stars

OB stars are known to exhibit various types of wind variability, as detected in their ultraviolet spectra, amongst which are the ubiquitous discrete absorption components (DACs). These features have been associated with large-scale azimuthal structures extending from the base of the wind to its outer regions: corotating interaction regions (CIRs). There are several competing hypotheses as to which physical processes may perturb the star's surface and generate CIRs, including magnetic fields and non radial pulsations (NRPs), the subjects of this paper with a particular emphasis on the former. Although large-scale magnetic fields are ruled out, magnetic spots deserve further investigation, both on the observational and theoretical fronts.Comment: 4 pages, 1 figure, IAU Symposium 30

Emission from the Centrifugal Magnetospheres of Magnetic B-type Stars

Approximately 10% of B-type stars possess strong magnetic fields, and of these, 25% host centrifugal magnetospheres (CMs) in which the radiative wind, magnetic field, and rotational support interact to form a dense circumstellar plasma visible in a variety of diagnostic lines. In this article we review the basic theory behind CMs, outline current theoretical and observational problems, compare the observational properties of CM host stars to those of classical Be stars, and finally present preliminary results of a population study aimed at clarifying the characteristics of this growing sub-class.Comment: 8 pages, 4 figures, proceedings of 'Bright Emissaries: Be Stars as Messengers of Star Disk Physics', conference held at the University of Western Ontario, Aug 10-13, 201