662 research outputs found
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
Magnetic field, differential rotation and activity of the hot-Jupiter hosting star HD 179949
HD 179949 is an F8V star, orbited by a giant planet at ~8 R* every 3.092514
days. The system was reported to undergo episodes of stellar activity
enhancement modulated by the orbital period, interpreted as caused by
Star-Planet Interactions (SPIs). One possible cause of SPIs is the large-scale
magnetic field of the host star in which the close-in giant planet orbits.
In this paper we present spectropolarimetric observations of HD 179949 during
two observing campaigns (2009 September and 2007 June). We detect a weak
large-scale magnetic field of a few Gauss at the surface of the star. The field
configuration is mainly poloidal at both observing epochs. The star is found to
rotate differentially, with a surface rotation shear of dOmega=0.216\pm0.061
rad/d, corresponding to equatorial and polar rotation periods of 7.62\pm0.07
and 10.3\pm0.8 d respectively. The coronal field estimated by extrapolating the
surface maps resembles a dipole tilted at ~70 degrees. We also find that the
chromospheric activity of HD 179949 is mainly modulated by the rotation of the
star, with two clear maxima per rotation period as expected from a highly
tilted magnetosphere. In September 2009, we find that the activity of HD 179949
shows hints of low amplitude fluctuations with a period close to the beat
period of the system.Comment: Accepted for publication in Monthly Notices of The Royal Astronomical
Societ
Searching for Star-Planet interactions within the magnetosphere of HD 189733
HD 189733 is a K2 dwarf, orbited by a giant planet at 8.8 stellar radii. In
order to study magnetospheric interactions between the star and the planet, we
explore the large-scale magnetic field and activity of the host star.
We collected spectra using the ESPaDOnS and the NARVAL spectropolarimeters,
installed at the 3.6-m Canada-France-Hawaii telescope and the 2-m Telescope
Bernard Lyot at Pic du Midi, during two monitoring campaigns (June 2007 and
July 2008).
HD 189733 has a mainly toroidal surface magnetic field, having a strength
that reaches up to 40 G. The star is differentially rotating, with latitudinal
angular velocity shear of domega = 0.146 +- 0.049 rad/d, corresponding to
equatorial and polar periods of 11.94 +- 0.16 d and 16.53 +- 2.43 d
respectively. The study of the stellar activity shows that it is modulated
mainly by the stellar rotation (rather than by the orbital period or the beat
period between the stellar rotation and the orbital periods). We report no
clear evidence of magnetospheric interactions between the star and the planet.
We also extrapolated the field in the stellar corona and calculated the
planetary radio emission expected for HD 189733b given the reconstructed field
topology. The radio flux we predict in the framework of this model is time
variable and potentially detectable with LOFAR
Hot Jupiters and stellar magnetic activity
Recent observations suggest that stellar magnetic activity may be influenced
by the presence of a close-by giant planet. Specifically, chromospheric hot
spots rotating in phase with the planet orbital motion have been observed
during some seasons in a few stars harbouring hot Jupiters. The spot leads the
subplanetary point by a typical amount of about 60-70 degrees, with the extreme
case of upsilon And where the angle is about 170 degrees. The interaction
between the star and the planet is described considering the reconnection
between the stellar coronal field and the magnetic field of the planet.
Reconnection events produce energetic particles that moving along magnetic
field lines impact onto the stellar chromosphere giving rise to a localized hot
spot. A simple magnetohydrostatic model is introduced to describe the coronal
magnetic field of the star connecting its surface to the orbiting planet. The
field is assumed to be axisymmetric around the rotation axis of the star and
its configuration is more general than a linear force-free field. With a
suitable choice of the free parameters, the model can explain the phase
differences between the hot spots and the planets observed in HD 179949,
upsilon And, HD 189733, and tau Bootis, as well as their visibility modulation
on the orbital period and seasonal time scales. The possible presence of cool
spots associated with the planets in tau Boo and HD 192263 cannot be explained
by the present model. However, we speculate about the possibility that
reconnection events in the corona may influence subphotospheric dynamo action
in those stars producing localized photospheric (and chromospheric) activity
migrating in phase with their planets.Comment: 9 pages, 5 figures, 2 tables, 2 appendixes, accepted by Astronomy &
Astrophysic
Modulation of Ca II H & K Emission by Short-Period Planets
We have detected modulation of the Ca II H&K reversal structure in four out
of five 51 Peg-type stars whose planets have orbital periods between 3 and 4
days. We observe two effects in the K-core: (1) a broad 3-A variation at ~1%
level and (2) changes on a scale of 0.5 A (~1-3%) in each of the three reversal
features. The nightly variations are coherent in both H and K. From
differential radial velocities measured to better than 20 m/s, up-to-date
phases were extracted. The enhancements in the reversals tend to be greatest at
the sub-planetary points which may imply that there is a magnetic interaction
between the star's outer layers and the magnetosphere of the planet. These
high-S/N (500 per pixel in the continuum) and high-resolution (R = 110,000)
data are too few to confirm orbital synchronization.Comment: 4 pages, 2 figures, in newpasp.sty format. To be published by PASP
(ed. Drake Deming) in the proceedings of the conference entitled "Scientific
Frontiers in Research on Extrasolar Planets," held at the Carnegie
Institution of Washington, DC, June 18-21, 200
MOST detects variability on tau Bootis possibly induced by its planetary companion
(abridged) There is considerable interest in the possible interaction between
parent stars and giant planetary companions in 51 Peg-type systems. We
demonstrate from MOST satellite photometry and Ca II K line emission that there
has been a persistent, variable region on the surface of tau Boo A which
tracked its giant planetary companion for some 440 planetary revolutions and
lies ~68deg (phi=0.8) in advance of the sub-planetary point. The light curves
are folded on a range of periods centered on the planetary orbital period and
phase dependent variability is quantified by Fourier methods and by the mean
absolute deviation (MAD) of the folded data for both the photometry and the Ca
II K line reversals. The region varies in brightness on the time scale of a
rotation by ~1 mmag. In 2004 it resembled a dark spot of variable depth, while
in 2005 it varied between bright and dark. Over the 123 planetary orbits
spanned by the photometry the variable region detected in 2004 and in 2005 are
synchronised to the planetary orbital period within 0.0015 d. The Ca II K line
in 2001, 2002 and 2003 also shows enhanced K-line variability centered on
phi=0.8, extending coverage to some 440 planetary revolutions. The apparently
constant rotation period of the variable region and its rapid variation make an
explanation in terms of conventional star spots unlikely. The lack of
complementary variability at phi=0.3 and the detection of the variable region
so far in advance of the sub-planetary point excludes tidal excitation, but the
combined photometric and Ca II K line reversal results make a good case for an
active region induced magnetically on the surface of tau Boo A by its planetary
companion.Comment: 7 pages, 7 figures; accepted for publication in A&
Magnetic cycles of the planet-hosting star tauBootis
We have obtained new spectropolarimetric observations of the planet-hosting
star tauBootis, using the ESPaDOnS and NARVAL spectropolarimeters at the
Canada-France-Hawaii Telescope and Telescope Bernard-Lyot. With this data set,
we are able to confirm the presence of a magnetic field at the surface of
tauBoo and map its large-scale structure over the whole star. The overall
polarity of the magnetic field has reversed with respect to our previous
observation (obtained a year before), strongly suggesting that tauBoo is
undergoing magnetic cycles similar to those of the Sun. This is the first time
that a global magnetic polarity switch is observed in a star other than the
Sun; we speculate that the magnetic cycle period of tauBoo is much shorter than
that of the Sun.
Our new data also allow us to confirm the presence of differential rotation
from the latitudinal shearing that the magnetic structure is undergoing. The
differential rotation surface shear that tauBoo experiences is found to be 6 to
10 times larger than that of the Sun. We propose that the short magnetic cycle
period is due to the strong level of differential rotation. With a rotation
period of 3.0 and 3.9 d at the equator and pole respectively, tauBoo appears as
the first planet-hosting star whose rotation (at intermediate latitudes) is
synchronised with the orbital motion of its giant planet (period 3.3 d).
Assuming that this synchronisation is not coincidental, it suggests that the
tidal effects induced by the giant planet can be strong enough to force the
thin convective enveloppe (though not the whole star) into corotation and thus
to play a role in the activity cycle of tauBoo.Comment: MNRAS, in pres
Photospheric activity, rotation, and star-planet interaction of the planet-hosting star CoRoT-6
The CoRoT satellite has recently discovered a hot Jupiter that transits
across the disc of a F9V star called CoRoT-6 with a period of 8.886 days. We
model the photospheric activity of the star and use the maps of the active
regions to study stellar differential rotation and the star-planet interaction.
We apply a maximum entropy spot model to fit the optical modulation as observed
by CoRoT during a uninterrupted interval of about 140 days. Photospheric active
regions are assumed to consist of spots and faculae in a fixed proportion with
solar-like contrasts. Individual active regions have lifetimes up to 30-40
days. Most of them form and decay within five active longitudes whose different
migration rates are attributed to the stellar differential rotation for which a
lower limit of \Delta \Omega / \Omega = 0.12 \pm 0.02 is obtained. Several
active regions show a maximum of activity at a longitude lagging the
subplanetary point by about 200 degrees with the probability of a chance
occurrence being smaller than 1 percent. Our spot modelling indicates that the
photospheric activity of CoRoT-6 could be partially modulated by some kind of
star-planet magnetic interaction, while an interaction related to tides is
highly unlikely because of the weakness of the tidal force.Comment: 9 pages, 7 figures, accepted to Astronomy & Astrophysic
ЕКОНОМІКО–МАТЕМАТИЧНЕ МОДЕЛЮВАННЯ ОЦІНКИ ФУНКЦІОНУВАННЯ УКРАЇНСЬКОЇ БАНКІВСЬКОЇ СИСТЕМИ
The investigation presents scientific and methodological approach of evaluation of efficiency of the banking system through a generalized Harrington function (desirable function) that allows to determine the current state of the banking system and to predict the perspectives of its development. In the simulation of assessment of the banking system were used data from 49 banks operating in Ukraine for 2003–2016 and 15 financial indicators, which provide formation of input variables of the model. The method of data processing is implemented in the package Viscovery SOMine. The approach is based on the analysis of the dynamics of patterns of banks and building self–organizing Kohonen maps which enables to determine the trajectory of activity of individual bank. This approach enables forecasting of individual bank crisis and the potential crisis of banking system of a country. Conducted trend forecast of the Harington function suggests potential of improving of Ukrainian banking system over the next 3 years.В исследовании представлены научно–методические основы оценки эффективности функционирования банковской системы с помощью обобщенной функции Харрингтона (функции желательности), что дает возможность определить текущее состояние банковской системы и спрогнозировать перспективы ее развития. При моделировании оценки функционирования банковской системы были использованы данные 49 банков, действующих в Украине в 2003–2016 гг. В модели было использовано 15 финансовых показателей, обеспечивающих формирование входных переменных модели. Предложенный научно–методический подход, основанный на анализе динамики паттернов банков и построении самоорганизующихся карт Кохонена позволяет определить траекторию развития отдельного банка. Также на основе данного подхода возможно прогнозирование кризисного финансового состояния как отдельного банка, так и банковской системы страны в целом.У дослідженні представлено науково–методичні засади оцінювання ефективності функціонування банківської системи України за допомогою узагальненої функції Харрингтона (функції бажаності). Запропонований науково–методичний підхід, що ґрунтується на аналізі динаміки патернів банків та побудові самоорганізуючих карт Кохонена, дозволяє визначити траєкторію розвитку окремого банку та спрогнозувати кризовий фінансовий стан як окремого банку так і банківської системи країни в цілому
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