863 research outputs found
Discovery of a strong magnetic field on the O star HD 191612: new clues to the future of theta1 Orionis C?
From observations made with the ESPaDOnS spectropolarimeter, recently
installed on the 3.6-m Canada--France--Hawaii Telescope, we report the
discovery of a strong magnetic field in the Of?p spectrum variable HD 191612 --
only the second known magnetic O star (following theta1 Ori C). The stability
of the observed Zeeman signature over four nights of observation, together with
the non-rotational shape of line profiles, argue that the rotation period of HD
191612 is significantly longer than the 9-d value previously proposed. We
suggest that the recently identified 538-d spectral-variability period is the
rotation period, in which case the observed line-of-sight magnetic field of
-220+-38 G implies a large-scale field (assumed dipolar) with a polar strength
of about -1.5 kG. If confirmed, this scenario suggests that HD 191612 is,
essentially, an evolved version of the near-ZAMS magnetic O star theta1 Ori C,
but with an even stronger field (about 15 kG at an age similar to that of
theta1Ori C). We suggest that the rotation rate of HD 191612, which is
exceptionally slow by accepted O-star standards, could be due to
angular-momentum dissipation through a magnetically confined wind.Comment: Accepted by MNRAS Letters, 5 pages, 2 figures, 2 table
Magnetic fields and accretion flows on the classical T Tauri star V2129 Oph
From observations collected with the ESPaDOnS spectropolarimeter, we report
the discovery of magnetic fields at the surface of the mildly accreting
classical T Tauri star V2129 Oph. Zeeman signatures are detected, both in
photospheric lines and in the emission lines formed at the base of the
accretion funnels linking the disc to the protostar, and monitored over the
whole rotation cycle of V2129 Oph. We observe that rotational modulation
dominates the temporal variations of both unpolarized and circularly polarized
line profiles. We reconstruct the large-scale magnetic topology at the surface
of V2129 Oph from both sets of Zeeman signatures simultaneously. We find it to
be rather complex, with a dominant octupolar component and a weak dipole of
strengths 1.2 and 0.35 kG, respectively, both slightly tilted with respect to
the rotation axis. The large-scale field is anchored in a pair of 2-kG unipolar
radial field spots located at high latitudes and coinciding with cool dark
polar spots at photospheric level. This large-scale field geometry is unusually
complex compared to those of non-accreting cool active subgiants with moderate
rotation rates. As an illustration, we provide a first attempt at modelling the
magnetospheric topology and accretion funnels of V2129 Oph using field
extrapolation. We find that the magnetosphere of V2129 Oph must extend to about
7R* to ensure that the footpoints of accretion funnels coincide with the
high-latitude accretion spots on the stellar surface. It suggests that the
stellar magnetic field succeeds in coupling to the accretion disc as far out as
the corotation radius, and could possibly explain the slow rotation of V2129
Oph. The magnetospheric geometry we derive produces X-ray coronal fluxes
typical of those observed in cTTSs.Comment: MNRAS, in press (18 pages, 17 figures
Stellar Coronal and Wind Models: Impact on Exoplanets
Surface magnetism is believed to be the main driver of coronal heating and
stellar wind acceleration. Coronae are believed to be formed by plasma confined
in closed magnetic coronal loops of the stars, with winds mainly originating in
open magnetic field line regions. In this Chapter, we review some basic
properties of stellar coronae and winds and present some existing models. In
the last part of this Chapter, we discuss the effects of coronal winds on
exoplanets.Comment: Chapter published in the "Handbook of Exoplanets", Editors in Chief:
Juan Antonio Belmonte and Hans Deeg, Section Editor: Nuccio Lanza. Springer
Reference Work
Modelling the Corona of HD 189733 in 3D
The braking of main sequence stars originates mainly from their stellar wind. The efficiency of this angular momentum extraction depends on the rotation rate of the star, the acceleration profile of the wind and the coronal magnetic field. The derivation of scaling laws parametrizing the stellar wind torque is important for our understanding of gyro-chronology and the evolution of the rotation rates of stars. In order to understand the impact of complex magnetic topologies on the stellar wind torque, we present three-dimensional, dynamical simulations of the corona of HD 189733. Using the observed complex topology of the magnetic field, we estimate how the torque associated with the wind scales with model parameters and compare those trends to previously published scaling laws.AS thank A. Vidotto for discussions about the modelling of the corona of HD 189733. This work was supported by the ANR 2011
Blanc Toupies and the ERC project STARS2 (207430). The authors acknowledge CNRS INSU/PNST and CNES/Solar Orbiter
fundings. AS acknowledges support from the Canada’s Natural Sciences and Engineering Research Council and from the Canadian
Institute of Theoretical Astrophysics (National fellow). We acknowledge access to supercomputers through GENCI (project 1623),
Prace, and ComputeCanada infrastructures
The large-scale axisymmetric magnetic topology of avery-low-mass fully-convective star
Understanding how cool stars produce magnetic fields within their interiors
is crucial for predicting the impact of such fields, such as the activity cycle
of the Sun. In this respect, studying fully convective stars enables us to
investigate the role of convective zones in magnetic field generation. We
produced a magnetic map of a rapidly rotating, very-low-mass, fully convective
dwarf through tomographic imaging from time series of spectropolarimetric data.
Our results, which demonstrate that fully convective stars are able to trigger
axisymmetric large-scale poloidal fields without differential rotation,
challenge existing theoretical models of field generation in cool stars.Comment: 17 pages, 4 figures, supplementary online material (including 2
figures
Non-stationary dynamo & magnetospheric accretion processes of the classical T Tauri star V2129 Oph
We report here the first results of a multi-wavelength campaign focussing on
magnetospheric accretion processes of the classical TTauri star (cTTS)
V2129Oph. In this paper, we present spectropolarimetric observations collected
in 2009 July with ESPaDOnS at the Canada-France-Hawaii Telescope (CFHT).
Circularly polarised Zeeman signatures are clearly detected, both in
photospheric absorption and accretion-powered emission lines, from time-series
of which we reconstruct new maps of the magnetic field, photospheric brightness
and accretion-powered emission at the surface of V2129Oph using our newest
tomographic imaging tool - to be compared with those derived from our old 2005
June data set, reanalyzed in the exact same way.
We find that in 2009 July, V2129Oph hosts octupolar & dipolar field
components of about 2.1 & 0.9kG respectively, both tilted by about 20deg with
respect to the rotation axis; we conclude that the large-scale magnetic
topology changed significantly since 2005 June (when the octupole and dipole
components were about 1.5 and 3 times weaker respectively), demonstrating that
the field of V2129Oph is generated by a non-stationary dynamo. We also show
that V2129Oph features a dark photospheric spot and a localised area of
accretion-powered emission, both close to the main surface magnetic region
(hosting fields of up to about 4kG in 2009 July). We finally obtain that the
surface shear of V2129Oph is about half as strong as solar.
From the fluxes of accretion-powered emission lines, we estimate that the
observed average logarithmic accretion rate (in Msun/yr) at the surface of
V2129Oph is -9.2+-0.3 at both epochs, peaking at -9.0 at magnetic maximum. It
implies in particular that the radius at which the magnetic field of V2129Oph
truncates the inner accretion disc is 0.93x and 0.50x the corotation radius in
2009 July and 2005 June respectively.Comment: MNRAS in press - 16 pages, 9 figure
Estimating magnetic filling factors from Zeeman-Doppler magnetograms
This is the author accepted manuscript. The final version is available from American Astronomical Society via the DOI in this record.Low-mass stars are known to have magnetic fields that are believed to be of dynamo origin. Two complementary techniques are principally used to characterise them. Zeeman-Doppler imaging (ZDI) can determine the geometry of the large-scale magnetic field while Zeeman broadening can assess the total unsigned flux including that associated with small-scale structures such as spots. In this work, we study a sample of stars that have been previously mapped with ZDI. We show that the average unsigned magnetic flux follows an activity-rotation relation separating into saturated and unsaturated regimes. We also compare the average photospheric magnetic flux recovered by ZDI, hBV i, with that recovered by Zeeman broadening studies, hBI i. In line with previous studies, hBV i ranges from a few % to ∼20% of hBI i. We show that a power law relationship between hBV i and hBI i exists and that ZDI recovers a larger fraction of the magnetic flux in more active stars. Using this relation, we improve on previous attempts to estimate filling factors, i.e. the fraction of the stellar surface covered with magnetic field, for stars mapped only with ZDI. Our estimated filling factors follow the well-known activity-rotation relation which is in agreement with filling factors obtained directly from Zeeman broadening studies. We discuss the possible implications of these results for flux tube expansion above the stellar surface and stellar wind models.European CommissionAustrian Space Application Programm
Signatures of Star-planet interactions
Planets interact with their host stars through gravity, radiation and
magnetic fields, and for those giant planets that orbit their stars within
10 stellar radii (0.1 AU for a sun-like star), star-planet
interactions (SPI) are observable with a wide variety of photometric,
spectroscopic and spectropolarimetric studies. At such close distances, the
planet orbits within the sub-alfv\'enic radius of the star in which the
transfer of energy and angular momentum between the two bodies is particularly
efficient. The magnetic interactions appear as enhanced stellar activity
modulated by the planet as it orbits the star rather than only by stellar
rotation. These SPI effects are informative for the study of the internal
dynamics and atmospheric evolution of exoplanets. The nature of magnetic SPI is
modeled to be strongly affected by both the stellar and planetary magnetic
fields, possibly influencing the magnetic activity of both, as well as
affecting the irradiation and even the migration of the planet and rotational
evolution of the star. As phase-resolved observational techniques are applied
to a large statistical sample of hot Jupiter systems, extensions to other
tightly orbiting stellar systems, such as smaller planets close to M dwarfs
become possible. In these systems, star-planet separations of tens of stellar
radii begin to coincide with the radiative habitable zone where planetary
magnetic fields are likely a necessary condition for surface habitability.Comment: Accepted for publication in the handbook of exoplanet
Non-thermal emission processes in massive binaries
In this paper, I present a general discussion of several astrophysical
processes likely to play a role in the production of non-thermal emission in
massive stars, with emphasis on massive binaries. Even though the discussion
will start in the radio domain where the non-thermal emission was first
detected, the census of physical processes involved in the non-thermal emission
from massive stars shows that many spectral domains are concerned, from the
radio to the very high energies.
First, the theoretical aspects of the non-thermal emission from early-type
stars will be addressed. The main topics that will be discussed are
respectively the physics of individual stellar winds and their interaction in
binary systems, the acceleration of relativistic electrons, the magnetic field
of massive stars, and finally the non-thermal emission processes relevant to
the case of massive stars. Second, this general qualitative discussion will be
followed by a more quantitative one, devoted to the most probable scenario
where non-thermal radio emitters are massive binaries. I will show how several
stellar, wind and orbital parameters can be combined in order to make some
semi-quantitative predictions on the high-energy counterpart to the non-thermal
emission detected in the radio domain.
These theoretical considerations will be followed by a census of results
obtained so far, and related to this topic... (see paper for full abstract)Comment: 47 pages, 5 postscript figures, accepted for publication in Astronomy
and Astrophysics Review. Astronomy and Astrophysics Review, in pres
A new concept for the combination of optical interferometers and high-resolution spectrographs
The combination of high spatial and spectral resolution in optical astronomy
enables new observational approaches to many open problems in stellar and
circumstellar astrophysics. However, constructing a high-resolution
spectrograph for an interferometer is a costly and time-intensive undertaking.
Our aim is to show that, by coupling existing high-resolution spectrographs to
existing interferometers, one could observe in the domain of high spectral and
spatial resolution, and avoid the construction of a new complex and expensive
instrument. We investigate in this article the different challenges which arise
from combining an interferometer with a high-resolution spectrograph. The
requirements for the different sub-systems are determined, with special
attention given to the problems of fringe tracking and dispersion. A concept
study for the combination of the VLTI (Very Large Telescope Interferometer)
with UVES (UV-Visual Echelle Spectrograph) is carried out, and several other
specific instrument pairings are discussed. We show that the proposed
combination of an interferometer with a high-resolution spectrograph is indeed
feasible with current technology, for a fraction of the cost of building a
whole new spectrograph. The impact on the existing instruments and their
ongoing programs would be minimal.Comment: 27 pages, 9 figures, Experimental Astronomy; v2: accepted versio
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