9,286 research outputs found
Magnetic topology and surface differential rotation on the K1 subgiant of the RS CVn system HR 1099
We present here spectropolarimetric observations of the RS CVn system HR 1099
(V711 Tau) secured from 1998 February to 2002 January with the
spectropolarimeter MuSiCoS at the Telescope Bernard Lyot (Observatoire du Pic
du Midi, France). We apply Zeeman-Doppler Imaging and reconstruct brightness
and magnetic surface topologies of the K1 primary subgiant of the system, at
five different epochs. We confirm the presence of large, axisymmetric regions
where the magnetic field is mainly azimuthal, providing further support to the
hypothesis that dynamo processes may be distributed throughout the whole
convective zone in this star. We study the short-term evolution of surface
structures from a comparison of our images with observations secured at
close-by epochs by Donati et al. (2003) at the Anglo-Australian Telescope. We
conclude that the small-scale brightness and magnetic patterns undergo major
changes within a timescale of 4 to 6 weeks, while the largest structures remain
stable over several years. We report the detection of a weak surface
differential rotation (both from brightness and magnetic tracers) indicating
that the equator rotates faster than the pole with a difference in rotation
rate between the pole and the equator about 4 times smaller than that of the
Sun. This result suggests that tidal forces also impact the global dynamic
equilibrium of convective zones in cool active stars.Comment: accepted by MNRA
Magnetic fields and differential rotation on the pre-main sequence I: The early-G star HD 141943 - brightness and magnetic topologies
Spectroscopic and spectropolarimetric observations of the pre-main sequence
early-G star HD 141943 were obtained at four observing epochs (in 2006, 2007,
2009 and 2010). The observations were undertaken at the 3.9-m Anglo-Australian
Telescope using the UCLES echelle spectrograph and the SEMPOL
spectropolarimeter visitor instrument. Brightness and surface magnetic field
topologies were reconstructed for the star using the technique of least-squares
deconvolution to increase the signal-to-noise of the data.
The reconstructed brightness maps show that HD 141943 had a weak polar spot
and a significant amount of low latitude features, with little change in the
latitude distribution of the spots over the 4 years of observations. The
surface magnetic field was reconstructed at three of the epochs from a high
order (l <= 30) spherical harmonic expansion of the spectropolarimetric
observations. The reconstructed magnetic topologies show that in 2007 and 2010
the surface magnetic field was reasonably balanced between poloidal and
toroidal components. However we find tentative evidence of a change in the
poloidal/toroidal ratio in 2009 with the poloidal component becoming more
dominant. At all epochs the radial magnetic field is predominantly
non-axisymmetric while the azimuthal field is predominantly axisymmetric with a
ring of positive azimuthal field around the pole similar to that seen on other
active stars.Comment: 18 pages, 17 figures, accepted by MNRA
Magnetometry of the classical T Tauri star GQ Lup: non-stationary dynamos & spin evolution of young Suns
We report here results of spectropolarimetric observations of the classical T
Tauri star (cTTS) GQ Lup carried out with ESPaDOnS at the Canada-France-Hawaii
Telescope (CFHT) in the framework of the "Magnetic Protostars and Planets"
(MaPP) programme, and obtained at 2 different epochs (2009 July & 2011 June).
From these observations, we first infer that GQ Lup has a photospheric
temperature of 4,300+-50\^A K and a rotation period of 8.4+-0.3 d; it implies
that it is a 1.05+-0.07 Msun star viewed at an inclination of ~30deg, with an
age of 2-5 Myr, a radius of 1.7+-0.2 Rsun, and has just started to develop a
radiative core.
Large Zeeman signatures are clearly detected at all times, both in
photospheric lines & in accretion-powered emission lines, probing longitudinal
fields of up to 6 kG and hence making GQ Lup the cTTS with the strongest
large-scale fields known as of today. Rotational modulation of Zeeman
signatures is clearly different between our 2 runs, demonstrating that
large-scale fields of cTTSs are evolving with time and are likely produced by
non-stationary dynamo processes.
Using tomographic imaging, we reconstruct maps of the large-scale field, of
the photospheric brightness & of the accretion-powered emission of GQ Lup. We
find that the magnetic topology is mostly poloidal & axisymmetric; moreover,
the octupolar component of the large-scale field (of strength 2.4 & 1.6 kG in
2009 & 2011) dominates the dipolar component (of strength ~1 kG) by a factor of
~2, consistent with the fact that GQ Lup is no longer fully-convective.
GQ Lup also features dominantly poleward magnetospheric accretion at both
epochs. The large-scale dipole of GQ Lup is however not strong enough to
disrupt the surrounding accretion disc further than about half-way to the
corotation radius, suggesting that GQ Lup should rapidly spin up like other
similar partly-convective cTTSs (abridged).Comment: MNRAS, in press (17 pages, 10 figures, 1 table
Large-scale magnetic topologies of late M dwarfs
We present here the final results of the first spectropolarimetric survey of
a small sample of active M dwarfs, aimed at providing observational constraints
on dynamo action on both sides of the full-convection threshold (spectral type
M4). Our two previous studies (Donati et al. 2008b; Morin et al. 2008b) were
focused on early and mid M dwarfs. The present paper examines 11 fully
convective late M dwarfs (spectral types M5-M8). Tomographic imaging techniques
were applied to time-series of circularly polarised profiles of 6 stars, in
order to infer their large-scale magnetic topologies. For 3 other stars we
could not produce such magnetic maps, because of low variability of the Stokes
V signatures, but were able to derive some properties of the magnetic fields.
We find 2 distinct categories of magnetic topologies: on the one hand strong
axisymmetric dipolar fields (similar to mid M dwarfs), and on the other hand
weak fields generally featuring a significant non-axisymmetric component, and
sometimes a significant toroidal one. Comparison with unsigned magnetic fluxes
demonstrates that the second category of magnetic fields shows less
organization (less energy in the large scales), similarly to partly convective
early M dwarfs. Stars in both categories have similar stellar parameters, our
data do not evidence a separation between these 2 categories in the
mass-rotation plane. We also report marginal detection of a large-scale
magnetic field on the M8 star VB 10 featuring a significant toroidal
axisymmetric component, whereas no field is detectable on VB 8 (M7).Comment: 26 pages, 16 figures, 9 tables, 11 tables in appendix. Accepted for
publication in MNRA
Global 3D Simulations of Disc Accretion onto the classical T Tauri Star BP Tauri
The magnetic field of the classical T Tauri star BP Tau can be approximated
as a superposition of dipole and octupole moments with respective strengths of
the polar magnetic fields of 1.2 kG and 1.6 kG (Donati et al. 2008). We adopt
the measured properties of BP Tau and model the disc accretion onto the star.
We observed in simulations that the disc is disrupted by the dipole component
and matter flows towards the star in two funnel streams which form two
accretion spots below the dipole magnetic poles. The octupolar component
becomes dynamically important very close to the star and it redirects the
matter flow to higher latitudes. The spots are meridionally elongated and are
located at higher latitudes, compared with the pure dipole case, where
crescent-shaped, latitudinally elongated spots form at lower latitudes. The
position and shape of the spots are in good agreement with observations. The
disk-magnetosphere interaction leads to the inflation of the field lines and to
the formation of magnetic towers above and below the disk. The magnetic field
of BP Tau is close to the potential only near the star, inside the
magnetospheric surface, where magnetic stress dominates over the matter stress.
A series of simulation runs were performed for different accretion rates. They
show that an accretion rate is lower than obtained in many observations, unless
the disc is truncated close to the star. The torque acting on the star is about
an order of magnitude lower than that which is required for the rotational
equilibrium. We suggest that a star could lose most of its angular momentum at
earlier stages of its evolution.Comment: 11 pages, 13 figures, submitted to MNRA
Dynamo Processes in the T Tauri star V410 Tau
We present new brightness and magnetic images of the weak-line T Tauri star
V410 Tau, made using data from the NARVAL spectropolarimeter at Telescope
Bernard Lyot (TBL). The brightness image shows a large polar spot and
significant spot coverage at lower latitudes. The magnetic maps show a field
that is predominantly dipolar and non-axisymmetric with a strong azimuthal
component. The field is 50% poloidal and 50% toroidal, and there is very little
differential rotation apparent from the magnetic images.
A photometric monitoring campaign on this star has previously revealed V-band
variability of up to 0.6 magnitudes but in 2009 the lightcurve is much flatter.
The Doppler image presented here is consistent with this low variability.
Calculating the flux predicted by the mapped spot distribution gives an
peak-to-peak variability of 0.04 magnitudes. The reduction in the amplitude of
the lightcurve, compared with previous observations, appears to be related to a
change in the distribution of the spots, rather than the number or area.
This paper is the first from a Zeeman-Doppler imaging campaign being carried
out on V410 Tau between 2009-2012 at TBL. During this time it is expected that
the lightcurve will return to a high amplitude state, allowing us to ascertain
whether the photometric changes are accompanied by a change in the magnetic
field topology.Comment: 12 pages, 11 figures, accepted by MNRA
A polarity reversal in the large-scale magnetic field of the rapidly rotating Sun HD 190771
Aims. We investigate the long-term evolution of the large-scale photospheric
magnetic field geometry of the solar-type star HD 190771. With fundamental
parameters very close to those of the Sun except for a shorter rotation period
of 8.8 d, HD 190771 provides us with a first insight into the specific impact
of the rotation rate in the dynamo generation of magnetic fields in 1
stars.
Methods. We use circularly polarized, high-resolution spectra obtained with
the NARVAL spectropolarimeter (Observatoire du Pic du Midi, France) and compute
cross-correlation line profiles with high signal-to-noise ratio to detect
polarized Zeeman signatures. From three phase-resolved data sets collected
during the summers of 2007, 2008, and 2009, we model the large-scale
photospheric magnetic field of the star by means of Zeeman-Doppler imaging and
follow its temporal evolution.
Results. The comparison of the magnetic maps shows that a polarity reversal
of the axisymmetric component of the large-scale magnetic field occurred
between 2007 and 2008, this evolution being observed in both the poloidal and
toroidal magnetic components. Between 2008 and 2009, another type of global
evolution occured, characterized by a sharp decrease of the fraction of
magnetic energy stored in the toroidal component. These changes were not
accompanied by significant evolution in the total photospheric magnetic energy.
Using our spectra to perform radial velocity measurements, we also detect a
very low-mass stellar companion to HD 190771.Comment: Accepted by Astronomy and Astrophysics (Letter to the Editor
Magnetic cycles of the planet-hosting star Tau Bootis: II. a second magnetic polarity reversal
In this paper, we present new spectropolarimetric observations of the
planet-hosting star Tau Bootis, using ESPaDOnS and Narval spectropolarimeters
at Canada-France-Hawaii Telescope (CFHT) and Telescope Bernard Lyot (TBL),
respectively. We detected the magnetic field of the star at three epochs in
2008. It is a weak magnetic field of only a few Gauss, oscillating between a
predominant toroidal component in January and a dominant poloidal component in
June and July. A magnetic polarity reversal was observed relative to the
magnetic topology in June 2007. This is the second such reversal observed in
two years on this star, suggesting that Tau Boo has a magnetic cycle of about 2
years. This is the first detection of a magnetic cycle for a star other than
the Sun. The role of the close-in massive planet in the short activity cycle of
the star is questioned.
Tau Boo has strong differential rotation, a common trend for stars with
shallow convective envelope. At latitude 40 deg., the surface layer of the star
rotates in 3.31 d, equal to the orbital period. Synchronization suggests that
the tidal effects induced by the planet may be strong enough to force at least
the thin convective envelope into corotation. Tau Boo shows variability in the
Ca H & K and Halpha throughout the night and on a night to night time scale. We
do not detect enhancement in the activity of the star that may be related to
the conjunction of the planet. Further data is needed to conclude about the
activity enhancement due to the planet.Comment: 9 pages, 5 figures, 3 tables Accepted to MNRA
Analysis of the socio-economic impact of the tobacco CMO reform on italian tobacco sector
The Tobacco CMO (Common Market Organization) is involved in a intense debate between the European tobacco industry and those who are against to a crop whose transformed product is dangerous to the health. European institutions have shown a strong interest in this complex issue introducing two Reforms (1992 and 1998) and one revision in 2004. This paper aims to analyse and investigate the socio-economic impact of the tobacco CMO Reform of 2004 in Italy, across the scenarios proposed by the EC Commission (2004), both on the tobacco production and processing sector. The considered socio-economic indicators are harvested surfaces, farm income and overall employment, while the sample of farms used in this research belong to the FADN–Italy sample.Tobacco CMO, CAP reform, decoupling, Positive Mathematical Programming
The relation between stellar magnetic field geometry and chromospheric activity cycles – II The rapid 120-day magnetic cycle of <i>τ</i> Bootis
One of the aims of the BCool programme is to search for cycles in other stars and to understand how similar they are to the Sun. In this paper, we aim to monitor the evolution of τ Boo’s large-scale magnetic field using high-cadence observations covering its chromospheric activity maximum. For the first time, we detect a polarity switch that is in phase with τ Boo’s 120-day chromospheric activity maximum and its inferred X-ray activity cycle maximum. This means that τ Boo has a very fast magnetic cycle of only 240 days. At activity maximum τ Boo’s large-scale field geometry is very similar to the Sun at activity maximum: it is complex and there is a weak dipolar component. In contrast, we also see the emergence of a strong toroidal component which has not been observed on the Sun, and a potentially overlapping butterfly pattern where the next cycle begins before the previous one has finished
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