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J F Donati - One of the best experts on this subject based on the ideXlab platform.

  • observations of non solar type dynamo processes in stars with shallow Convective Zones
    Monthly Notices of the Royal Astronomical Society, 2011
    Co-Authors: S V Jeffers, J F Donati, E Alecian, S C Marsden
    Abstract:

    The magnetic field topology and differential rotation are fundamental signatures of the dynamo processes that generate the magnetic activity observed in the Sun and solar-type stars. To investigate how these dynamo processes evolve in stars with shallow Convective Zones, we present high-resolution spectropolarimetric observations of the young GO dwarf HD171488 over three epochs. Using the Zeeman-Doppler tomographic imaging technique, we have reconstructed surface brightness images that are dominated by polar and high-latitude starspots and a magnetic field topology that shows large-scale radial and azimuthal magnetic field components. Over the time-span of our observations, we do not observe a reversal of the magnetic field polarity as has been observed in other solar-type stars with shallow Convective Zones. The phase coverage of our data was sufficient to determine the differential rotation for two epochs where in conjunction with previous work, we conclude that there is no evidence for the temporal evolution of differential rotation.

  • Magnetic cycles of the planet-hosting star τ Bootis
    Monthly Notices of the Royal Astronomical Society, 2008
    Co-Authors: J F Donati, Ac Cameron, Mm Jardine, C. Moutou, R. Farès, D. Bohlender, C. Catala, M. Deleuil, E. Shkolnik, G. A. H. Walker
    Abstract:

    We have obtained new spectropolarimetric observations of the planet-hosting star τ Bootis, using the ESPaDOnS and NARVAL spectropolarimeters at the Canada-France-Hawaii Telescope (CFHT) and Télescope Bernard Lyot (TBL). With this data set, we are able to confirm the presence of a magnetic field at the surface of τ Boo and map its large-scale structure over the whole star. The large-scale magnetic field is found to be fairly complex, with a strength of up to 10 G; it features a dominant poloidal field and a small toroidal component, the poloidal component being significantly more complex than a dipole. The overall polarity of the magnetic field has reversed with respect to our previous observation (obtained a year before), strongly suggesting that τ Boo 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; given the infrequent occurrence of such events in the Sun, we speculate that the magnetic cycle period of τ Boo is much shorter than that of the Sun. Our new data also allow us to confirm the presence of differential rotation, both from the shape of the line profiles and the latitudinal shearing that the magnetic structure is undergoing. The differential rotation surface shear that τ Boo experiences is found to be 6 to 10 times larger than that of the Sun, in good agreement with recent claims that differential rotation is strongest in stars with shallow Convective Zones. 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, τ Boo appears as the first planet-hosting star whose rotation (at intermediate latitudes) is synchronized with the orbital motion of its giant planet (period 3.3 d). Assuming that this synchronization is not coincidental, it suggests that the tidal effects induced by the giant planet can be strong enough to force the thin Convective envelope (though not the whole star) into corotation. We also detect time-dependent activity fluctuations on τ Boo, but cannot unambiguously determine whether they are intrinsic to the star or induced by the planet; more observations of similar type are needed to determine the role of the close-in giant planet orbiting τ Boo on both the activity enhancements and the magnetic cycle of the host star.

  • Magnetospheric accretion on the T Tauri star BP Tauri
    2008
    Co-Authors: J F Donati, Ac Cameron, Mm Jardine, S. G. Gregory, P. Petit, F. Paletou, J. Bouvier, C. Dougados, F. Menard, T. J. Harries
    Abstract:

    From observations collected with the ESPaDOnS and NARVAL spectropolarimeters, we report the detection of Zeeman signatures on the classical T Tauri star BP Tau. Circular polarisation signatures in photospheric lines and in narrow emission lines tracing magnetospheric accretion are monitored throughout most of the rotation cycle of BP Tau at two different epochs in 2006. We observe that rotational modulation dominates the temporal variations of both unpolarised and circularly polarised spectral proxies tracing the photosphere and the footpoints of accretion funnels. From the complete data sets at each epoch, we reconstruct the large-scale magnetic topology and the location of accretion spots at the surface of BP Tau using tomographic imaging. We find that the field of BP Tau involves a 1.2 kG dipole and 1.6 kG octupole, both slightly tilted with respect to the rotation axis. Accretion spots coincide with the two main magnetic poles at high latitudes and overlap with dark photospheric spots; they cover about 2% of the stellar surface. The strong mainly-axisymmetric poloidal field of BP Tau is very reminiscent of magnetic topologies of fully-Convective dwarfs. It suggests that magnetic fields of fully-Convective cTTSs such as BP Tau are likely not fossil remants, but rather result from vigorous dynamo action operating within the bulk of their Convective Zones. Preliminary modelling suggests that the magnetosphere of BP Tau extends to distances of at least 4 R* to ensure that accretion spots are located at high latitudes, and is not blown open close to the surface by a putative stellar wind. It apparently succeeds in coupling to the accretion disc as far out as the corotation radius, and could possibly explain the slow rotation of BP Tau.

  • Magnetospheric accretion on the T Tauri star BP Tauri
    Monthly Notices of the Royal Astronomical Society, 2008
    Co-Authors: J F Donati, Ac Cameron, Mm Jardine, S. G. Gregory, P. Petit, F. Paletou, J. Bouvier, C. Dougados, F. Menard, T. J. Harries
    Abstract:

    From observations collected with the ESPaDOnS and NARVAL spectropolarimeters, we report the detection of Zeeman signatures on the classical T Tauri star (cTTS) BP Tau. Circular polarization signatures in photospheric lines and in narrow emission lines tracing magnetospheric accretion are monitored throughout most of the rotation cycle of BP Tau at two different epochs in 2006. We observe that rotational modulation dominates the temporal variations of both unpolarized and circularly polarized spectral proxies tracing the photosphere and the footpoints of accretion funnels. From the complete data sets at each epoch, we reconstruct the large-scale magnetic topology and the location of accretion spots at the surface of BP Tau using tomographic imaging. We find that the field of BP Tau involves a 1.2 kG dipole and 1.6 kG octupole, both slightly tilted with respect to the rotation axis. Accretion spots coincide with the two main magnetic poles at high latitudes and overlap with dark photospheric spots; they cover about 2 per cent of the stellar surface. The strong mainly axisymmetric poloidal field of BP Tau is very reminiscent of magnetic topologies of fully Convective dwarfs. It suggests that magnetic fields of fully Convective cTTSs such as BP Tau are likely not fossil remants, but rather result from vigorous dynamo action operating within the bulk of their Convective Zones. Preliminary modelling suggests that the magnetosphere of BP Tau extends to distances of at least 4R* to ensure that accretion spots are located at high latitudes, and is not blown open close to the surface by a putative stellar wind. It apparently succeeds in coupling to the accretion disc as far out as the corotation radius, and could possibly explain the slow rotation of BP Tau. Based on observations obtained at the Canada-France-Hawaii Telescope (CFHT) and at the Télescope Bernard Lyot (TBL). CFHT is operated by the National Research Council of Canada, the Institut National des Sciences de l'Univers of the Centre National de la Recherche Scientifique of France (INSU/CNRS) and the University of Hawaii, while TBL is operated by CNRS/INSU. E-mail: donati@ast.obs-mip.fr (J-FD); mmj@st-andrews.ac.uk (MMJ); sg64@st-andrews.ac.uk (SGG); petit@ast.obs-mip.fr (PP); fpaletou@ast.obs-mip.fr (FP); jerome.bouvier@obs.ujf-grenoble.fr (JB); catherine.dougados@obs.ujf-grenoble.fr (CD); francois.menard@obs.ujf-grenoble.fr (FM); acc4@st-andrews.ac.uk (ACC); th@astro.ex.ac.uk (TJH); ghussain@eso.org (GAJH); y.unruh@imperial.ac.uk (YU); jmorin@ast.obs-mip.fr (JM); scm@aao.gov.au (SCM); manset@cfht.hawaii.edu (NM); auriere@ast.obs-mip.fr (MA); claude.catala@obspm.fr (CC); evelyne.alecian@obspm.fr (EA)

  • The large-scale axisymmetric magnetic topology of avery-low-mass fully-Convective star
    2006
    Co-Authors: J F Donati, T Forveille, Ac Cameron, Jr Barnes, X Delfosse, Mm Jardine, Ja Valenti
    Abstract:

    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.

Mm Jardine - One of the best experts on this subject based on the ideXlab platform.

  • Magnetic cycles of the planet-hosting star τ Bootis
    Monthly Notices of the Royal Astronomical Society, 2008
    Co-Authors: J F Donati, Ac Cameron, Mm Jardine, C. Moutou, R. Farès, D. Bohlender, C. Catala, M. Deleuil, E. Shkolnik, G. A. H. Walker
    Abstract:

    We have obtained new spectropolarimetric observations of the planet-hosting star τ Bootis, using the ESPaDOnS and NARVAL spectropolarimeters at the Canada-France-Hawaii Telescope (CFHT) and Télescope Bernard Lyot (TBL). With this data set, we are able to confirm the presence of a magnetic field at the surface of τ Boo and map its large-scale structure over the whole star. The large-scale magnetic field is found to be fairly complex, with a strength of up to 10 G; it features a dominant poloidal field and a small toroidal component, the poloidal component being significantly more complex than a dipole. The overall polarity of the magnetic field has reversed with respect to our previous observation (obtained a year before), strongly suggesting that τ Boo 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; given the infrequent occurrence of such events in the Sun, we speculate that the magnetic cycle period of τ Boo is much shorter than that of the Sun. Our new data also allow us to confirm the presence of differential rotation, both from the shape of the line profiles and the latitudinal shearing that the magnetic structure is undergoing. The differential rotation surface shear that τ Boo experiences is found to be 6 to 10 times larger than that of the Sun, in good agreement with recent claims that differential rotation is strongest in stars with shallow Convective Zones. 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, τ Boo appears as the first planet-hosting star whose rotation (at intermediate latitudes) is synchronized with the orbital motion of its giant planet (period 3.3 d). Assuming that this synchronization is not coincidental, it suggests that the tidal effects induced by the giant planet can be strong enough to force the thin Convective envelope (though not the whole star) into corotation. We also detect time-dependent activity fluctuations on τ Boo, but cannot unambiguously determine whether they are intrinsic to the star or induced by the planet; more observations of similar type are needed to determine the role of the close-in giant planet orbiting τ Boo on both the activity enhancements and the magnetic cycle of the host star.

  • Magnetospheric accretion on the T Tauri star BP Tauri
    2008
    Co-Authors: J F Donati, Ac Cameron, Mm Jardine, S. G. Gregory, P. Petit, F. Paletou, J. Bouvier, C. Dougados, F. Menard, T. J. Harries
    Abstract:

    From observations collected with the ESPaDOnS and NARVAL spectropolarimeters, we report the detection of Zeeman signatures on the classical T Tauri star BP Tau. Circular polarisation signatures in photospheric lines and in narrow emission lines tracing magnetospheric accretion are monitored throughout most of the rotation cycle of BP Tau at two different epochs in 2006. We observe that rotational modulation dominates the temporal variations of both unpolarised and circularly polarised spectral proxies tracing the photosphere and the footpoints of accretion funnels. From the complete data sets at each epoch, we reconstruct the large-scale magnetic topology and the location of accretion spots at the surface of BP Tau using tomographic imaging. We find that the field of BP Tau involves a 1.2 kG dipole and 1.6 kG octupole, both slightly tilted with respect to the rotation axis. Accretion spots coincide with the two main magnetic poles at high latitudes and overlap with dark photospheric spots; they cover about 2% of the stellar surface. The strong mainly-axisymmetric poloidal field of BP Tau is very reminiscent of magnetic topologies of fully-Convective dwarfs. It suggests that magnetic fields of fully-Convective cTTSs such as BP Tau are likely not fossil remants, but rather result from vigorous dynamo action operating within the bulk of their Convective Zones. Preliminary modelling suggests that the magnetosphere of BP Tau extends to distances of at least 4 R* to ensure that accretion spots are located at high latitudes, and is not blown open close to the surface by a putative stellar wind. It apparently succeeds in coupling to the accretion disc as far out as the corotation radius, and could possibly explain the slow rotation of BP Tau.

  • Magnetospheric accretion on the T Tauri star BP Tauri
    Monthly Notices of the Royal Astronomical Society, 2008
    Co-Authors: J F Donati, Ac Cameron, Mm Jardine, S. G. Gregory, P. Petit, F. Paletou, J. Bouvier, C. Dougados, F. Menard, T. J. Harries
    Abstract:

    From observations collected with the ESPaDOnS and NARVAL spectropolarimeters, we report the detection of Zeeman signatures on the classical T Tauri star (cTTS) BP Tau. Circular polarization signatures in photospheric lines and in narrow emission lines tracing magnetospheric accretion are monitored throughout most of the rotation cycle of BP Tau at two different epochs in 2006. We observe that rotational modulation dominates the temporal variations of both unpolarized and circularly polarized spectral proxies tracing the photosphere and the footpoints of accretion funnels. From the complete data sets at each epoch, we reconstruct the large-scale magnetic topology and the location of accretion spots at the surface of BP Tau using tomographic imaging. We find that the field of BP Tau involves a 1.2 kG dipole and 1.6 kG octupole, both slightly tilted with respect to the rotation axis. Accretion spots coincide with the two main magnetic poles at high latitudes and overlap with dark photospheric spots; they cover about 2 per cent of the stellar surface. The strong mainly axisymmetric poloidal field of BP Tau is very reminiscent of magnetic topologies of fully Convective dwarfs. It suggests that magnetic fields of fully Convective cTTSs such as BP Tau are likely not fossil remants, but rather result from vigorous dynamo action operating within the bulk of their Convective Zones. Preliminary modelling suggests that the magnetosphere of BP Tau extends to distances of at least 4R* to ensure that accretion spots are located at high latitudes, and is not blown open close to the surface by a putative stellar wind. It apparently succeeds in coupling to the accretion disc as far out as the corotation radius, and could possibly explain the slow rotation of BP Tau. Based on observations obtained at the Canada-France-Hawaii Telescope (CFHT) and at the Télescope Bernard Lyot (TBL). CFHT is operated by the National Research Council of Canada, the Institut National des Sciences de l'Univers of the Centre National de la Recherche Scientifique of France (INSU/CNRS) and the University of Hawaii, while TBL is operated by CNRS/INSU. E-mail: donati@ast.obs-mip.fr (J-FD); mmj@st-andrews.ac.uk (MMJ); sg64@st-andrews.ac.uk (SGG); petit@ast.obs-mip.fr (PP); fpaletou@ast.obs-mip.fr (FP); jerome.bouvier@obs.ujf-grenoble.fr (JB); catherine.dougados@obs.ujf-grenoble.fr (CD); francois.menard@obs.ujf-grenoble.fr (FM); acc4@st-andrews.ac.uk (ACC); th@astro.ex.ac.uk (TJH); ghussain@eso.org (GAJH); y.unruh@imperial.ac.uk (YU); jmorin@ast.obs-mip.fr (JM); scm@aao.gov.au (SCM); manset@cfht.hawaii.edu (NM); auriere@ast.obs-mip.fr (MA); claude.catala@obspm.fr (CC); evelyne.alecian@obspm.fr (EA)

  • The large-scale axisymmetric magnetic topology of avery-low-mass fully-Convective star
    2006
    Co-Authors: J F Donati, T Forveille, Ac Cameron, Jr Barnes, X Delfosse, Mm Jardine, Ja Valenti
    Abstract:

    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.

Ac Cameron - One of the best experts on this subject based on the ideXlab platform.

  • Magnetic cycles of the planet-hosting star τ Bootis
    Monthly Notices of the Royal Astronomical Society, 2008
    Co-Authors: J F Donati, Ac Cameron, Mm Jardine, C. Moutou, R. Farès, D. Bohlender, C. Catala, M. Deleuil, E. Shkolnik, G. A. H. Walker
    Abstract:

    We have obtained new spectropolarimetric observations of the planet-hosting star τ Bootis, using the ESPaDOnS and NARVAL spectropolarimeters at the Canada-France-Hawaii Telescope (CFHT) and Télescope Bernard Lyot (TBL). With this data set, we are able to confirm the presence of a magnetic field at the surface of τ Boo and map its large-scale structure over the whole star. The large-scale magnetic field is found to be fairly complex, with a strength of up to 10 G; it features a dominant poloidal field and a small toroidal component, the poloidal component being significantly more complex than a dipole. The overall polarity of the magnetic field has reversed with respect to our previous observation (obtained a year before), strongly suggesting that τ Boo 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; given the infrequent occurrence of such events in the Sun, we speculate that the magnetic cycle period of τ Boo is much shorter than that of the Sun. Our new data also allow us to confirm the presence of differential rotation, both from the shape of the line profiles and the latitudinal shearing that the magnetic structure is undergoing. The differential rotation surface shear that τ Boo experiences is found to be 6 to 10 times larger than that of the Sun, in good agreement with recent claims that differential rotation is strongest in stars with shallow Convective Zones. 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, τ Boo appears as the first planet-hosting star whose rotation (at intermediate latitudes) is synchronized with the orbital motion of its giant planet (period 3.3 d). Assuming that this synchronization is not coincidental, it suggests that the tidal effects induced by the giant planet can be strong enough to force the thin Convective envelope (though not the whole star) into corotation. We also detect time-dependent activity fluctuations on τ Boo, but cannot unambiguously determine whether they are intrinsic to the star or induced by the planet; more observations of similar type are needed to determine the role of the close-in giant planet orbiting τ Boo on both the activity enhancements and the magnetic cycle of the host star.

  • Magnetospheric accretion on the T Tauri star BP Tauri
    2008
    Co-Authors: J F Donati, Ac Cameron, Mm Jardine, S. G. Gregory, P. Petit, F. Paletou, J. Bouvier, C. Dougados, F. Menard, T. J. Harries
    Abstract:

    From observations collected with the ESPaDOnS and NARVAL spectropolarimeters, we report the detection of Zeeman signatures on the classical T Tauri star BP Tau. Circular polarisation signatures in photospheric lines and in narrow emission lines tracing magnetospheric accretion are monitored throughout most of the rotation cycle of BP Tau at two different epochs in 2006. We observe that rotational modulation dominates the temporal variations of both unpolarised and circularly polarised spectral proxies tracing the photosphere and the footpoints of accretion funnels. From the complete data sets at each epoch, we reconstruct the large-scale magnetic topology and the location of accretion spots at the surface of BP Tau using tomographic imaging. We find that the field of BP Tau involves a 1.2 kG dipole and 1.6 kG octupole, both slightly tilted with respect to the rotation axis. Accretion spots coincide with the two main magnetic poles at high latitudes and overlap with dark photospheric spots; they cover about 2% of the stellar surface. The strong mainly-axisymmetric poloidal field of BP Tau is very reminiscent of magnetic topologies of fully-Convective dwarfs. It suggests that magnetic fields of fully-Convective cTTSs such as BP Tau are likely not fossil remants, but rather result from vigorous dynamo action operating within the bulk of their Convective Zones. Preliminary modelling suggests that the magnetosphere of BP Tau extends to distances of at least 4 R* to ensure that accretion spots are located at high latitudes, and is not blown open close to the surface by a putative stellar wind. It apparently succeeds in coupling to the accretion disc as far out as the corotation radius, and could possibly explain the slow rotation of BP Tau.

  • Magnetospheric accretion on the T Tauri star BP Tauri
    Monthly Notices of the Royal Astronomical Society, 2008
    Co-Authors: J F Donati, Ac Cameron, Mm Jardine, S. G. Gregory, P. Petit, F. Paletou, J. Bouvier, C. Dougados, F. Menard, T. J. Harries
    Abstract:

    From observations collected with the ESPaDOnS and NARVAL spectropolarimeters, we report the detection of Zeeman signatures on the classical T Tauri star (cTTS) BP Tau. Circular polarization signatures in photospheric lines and in narrow emission lines tracing magnetospheric accretion are monitored throughout most of the rotation cycle of BP Tau at two different epochs in 2006. We observe that rotational modulation dominates the temporal variations of both unpolarized and circularly polarized spectral proxies tracing the photosphere and the footpoints of accretion funnels. From the complete data sets at each epoch, we reconstruct the large-scale magnetic topology and the location of accretion spots at the surface of BP Tau using tomographic imaging. We find that the field of BP Tau involves a 1.2 kG dipole and 1.6 kG octupole, both slightly tilted with respect to the rotation axis. Accretion spots coincide with the two main magnetic poles at high latitudes and overlap with dark photospheric spots; they cover about 2 per cent of the stellar surface. The strong mainly axisymmetric poloidal field of BP Tau is very reminiscent of magnetic topologies of fully Convective dwarfs. It suggests that magnetic fields of fully Convective cTTSs such as BP Tau are likely not fossil remants, but rather result from vigorous dynamo action operating within the bulk of their Convective Zones. Preliminary modelling suggests that the magnetosphere of BP Tau extends to distances of at least 4R* to ensure that accretion spots are located at high latitudes, and is not blown open close to the surface by a putative stellar wind. It apparently succeeds in coupling to the accretion disc as far out as the corotation radius, and could possibly explain the slow rotation of BP Tau. Based on observations obtained at the Canada-France-Hawaii Telescope (CFHT) and at the Télescope Bernard Lyot (TBL). CFHT is operated by the National Research Council of Canada, the Institut National des Sciences de l'Univers of the Centre National de la Recherche Scientifique of France (INSU/CNRS) and the University of Hawaii, while TBL is operated by CNRS/INSU. E-mail: donati@ast.obs-mip.fr (J-FD); mmj@st-andrews.ac.uk (MMJ); sg64@st-andrews.ac.uk (SGG); petit@ast.obs-mip.fr (PP); fpaletou@ast.obs-mip.fr (FP); jerome.bouvier@obs.ujf-grenoble.fr (JB); catherine.dougados@obs.ujf-grenoble.fr (CD); francois.menard@obs.ujf-grenoble.fr (FM); acc4@st-andrews.ac.uk (ACC); th@astro.ex.ac.uk (TJH); ghussain@eso.org (GAJH); y.unruh@imperial.ac.uk (YU); jmorin@ast.obs-mip.fr (JM); scm@aao.gov.au (SCM); manset@cfht.hawaii.edu (NM); auriere@ast.obs-mip.fr (MA); claude.catala@obspm.fr (CC); evelyne.alecian@obspm.fr (EA)

  • The large-scale axisymmetric magnetic topology of avery-low-mass fully-Convective star
    2006
    Co-Authors: J F Donati, T Forveille, Ac Cameron, Jr Barnes, X Delfosse, Mm Jardine, Ja Valenti
    Abstract:

    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.

T. J. Harries - One of the best experts on this subject based on the ideXlab platform.

  • Magnetospheric accretion on the T Tauri star BP Tauri
    2008
    Co-Authors: J F Donati, Ac Cameron, Mm Jardine, S. G. Gregory, P. Petit, F. Paletou, J. Bouvier, C. Dougados, F. Menard, T. J. Harries
    Abstract:

    From observations collected with the ESPaDOnS and NARVAL spectropolarimeters, we report the detection of Zeeman signatures on the classical T Tauri star BP Tau. Circular polarisation signatures in photospheric lines and in narrow emission lines tracing magnetospheric accretion are monitored throughout most of the rotation cycle of BP Tau at two different epochs in 2006. We observe that rotational modulation dominates the temporal variations of both unpolarised and circularly polarised spectral proxies tracing the photosphere and the footpoints of accretion funnels. From the complete data sets at each epoch, we reconstruct the large-scale magnetic topology and the location of accretion spots at the surface of BP Tau using tomographic imaging. We find that the field of BP Tau involves a 1.2 kG dipole and 1.6 kG octupole, both slightly tilted with respect to the rotation axis. Accretion spots coincide with the two main magnetic poles at high latitudes and overlap with dark photospheric spots; they cover about 2% of the stellar surface. The strong mainly-axisymmetric poloidal field of BP Tau is very reminiscent of magnetic topologies of fully-Convective dwarfs. It suggests that magnetic fields of fully-Convective cTTSs such as BP Tau are likely not fossil remants, but rather result from vigorous dynamo action operating within the bulk of their Convective Zones. Preliminary modelling suggests that the magnetosphere of BP Tau extends to distances of at least 4 R* to ensure that accretion spots are located at high latitudes, and is not blown open close to the surface by a putative stellar wind. It apparently succeeds in coupling to the accretion disc as far out as the corotation radius, and could possibly explain the slow rotation of BP Tau.

  • Magnetospheric accretion on the T Tauri star BP Tauri
    Monthly Notices of the Royal Astronomical Society, 2008
    Co-Authors: J F Donati, Ac Cameron, Mm Jardine, S. G. Gregory, P. Petit, F. Paletou, J. Bouvier, C. Dougados, F. Menard, T. J. Harries
    Abstract:

    From observations collected with the ESPaDOnS and NARVAL spectropolarimeters, we report the detection of Zeeman signatures on the classical T Tauri star (cTTS) BP Tau. Circular polarization signatures in photospheric lines and in narrow emission lines tracing magnetospheric accretion are monitored throughout most of the rotation cycle of BP Tau at two different epochs in 2006. We observe that rotational modulation dominates the temporal variations of both unpolarized and circularly polarized spectral proxies tracing the photosphere and the footpoints of accretion funnels. From the complete data sets at each epoch, we reconstruct the large-scale magnetic topology and the location of accretion spots at the surface of BP Tau using tomographic imaging. We find that the field of BP Tau involves a 1.2 kG dipole and 1.6 kG octupole, both slightly tilted with respect to the rotation axis. Accretion spots coincide with the two main magnetic poles at high latitudes and overlap with dark photospheric spots; they cover about 2 per cent of the stellar surface. The strong mainly axisymmetric poloidal field of BP Tau is very reminiscent of magnetic topologies of fully Convective dwarfs. It suggests that magnetic fields of fully Convective cTTSs such as BP Tau are likely not fossil remants, but rather result from vigorous dynamo action operating within the bulk of their Convective Zones. Preliminary modelling suggests that the magnetosphere of BP Tau extends to distances of at least 4R* to ensure that accretion spots are located at high latitudes, and is not blown open close to the surface by a putative stellar wind. It apparently succeeds in coupling to the accretion disc as far out as the corotation radius, and could possibly explain the slow rotation of BP Tau. Based on observations obtained at the Canada-France-Hawaii Telescope (CFHT) and at the Télescope Bernard Lyot (TBL). CFHT is operated by the National Research Council of Canada, the Institut National des Sciences de l'Univers of the Centre National de la Recherche Scientifique of France (INSU/CNRS) and the University of Hawaii, while TBL is operated by CNRS/INSU. E-mail: donati@ast.obs-mip.fr (J-FD); mmj@st-andrews.ac.uk (MMJ); sg64@st-andrews.ac.uk (SGG); petit@ast.obs-mip.fr (PP); fpaletou@ast.obs-mip.fr (FP); jerome.bouvier@obs.ujf-grenoble.fr (JB); catherine.dougados@obs.ujf-grenoble.fr (CD); francois.menard@obs.ujf-grenoble.fr (FM); acc4@st-andrews.ac.uk (ACC); th@astro.ex.ac.uk (TJH); ghussain@eso.org (GAJH); y.unruh@imperial.ac.uk (YU); jmorin@ast.obs-mip.fr (JM); scm@aao.gov.au (SCM); manset@cfht.hawaii.edu (NM); auriere@ast.obs-mip.fr (MA); claude.catala@obspm.fr (CC); evelyne.alecian@obspm.fr (EA)

Aliakbar Akbarzadeh - One of the best experts on this subject based on the ideXlab platform.

  • heat extraction from non Convective and lower Convective Zones of the solar pond a transient study
    Solar Energy, 2013
    Co-Authors: Abhijit Date, Yusli Yaakob, Ashwin Date, Shankar Krishnapillai, Aliakbar Akbarzadeh
    Abstract:

    Abstract Heat extraction from the Non-Convective Zone (NCZ) or gradient layer and Lower Convective Zone (LCZ) of the solar pond has been investigated through one-dimensional finite difference transient model. Instantaneous efficiency of the solar pond is introduced and defined in this paper. The solar pond considered in the present study is assumed to have an in-pond heat exchanger for heat extraction. The rate of heat transfer is controlled by the mass flux of heat transfer fluid in this model. As in reality mass flux of heat transfer fluid is the simplest and most practical way to control the rate of heat extraction. In this model for an ideal situation it is assumed that the heat transfer fluid is initially at the local daily average ambient temperature and the in-pond heat exchanger has heat transfer effectiveness equal to unity. With these assumptions the model can predict the thermal performance of the solar pond with maximum heat extraction for a desired mass flux of the heat transfer fluid. This paper presents the comparison of the transient thermal performance of solar pond with heat extraction from LCZ alone and that with combined heat extraction from LCZ and NCZ. It is shown how the efficiency of the solar pond increases when heat is extracted from both NCZ and LCZ. The main objective of this study is to offer a simple method to predict transient thermal performance of a solar pond with heat extraction from NCZ and to estimate the mass flux of heat transfer fluid used in an in-pond heat exchanger for heat extraction from different layers of solar pond.