336 research outputs found
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 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, , differs from the previously
reported phase of enhancement, , (at 610 MHz Chandra et al. 2015) by
one-half cycle. However, no flux enhancement is found at phase 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
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
Stokes 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 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
and O and Cl in Stokes . These new maps are inferred from line
profiles in Stokes 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
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 H 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
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
The impact and evolution of magnetic confinement in hot stars
Magnetic confinement of the winds of hot, massive stars has far-reaching
consequences on timescales ranging from hours to Myr. Understanding the
long-term effects of this interplay has already led to the identification of
two new evolutionary pathways to form `heavy' stellar mass black holes and
pair-instability supernova even at galactic metallicity. We are performing 1D
stellar evolution model calculations that, for the first time, account for the
surface effects and the time evolution of fossil magnetic fields. These models
will be thoroughly confronted with observations and will potentially lead to a
significant revision of the derived parameters of observed magnetic massive
stars.Comment: to appear in proceedings of 'Stars with a stable magnetic field: from
pre-main sequence to compact remnants' conference, held in Brno, Czech
Republic, 28 Aug - 1 Sep, 2017, Contributions of the Astronomical Observatory
Skalnate Pleso, 201
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