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 $M_\odot$ 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

The rapid rotation and complex magnetic field geometry of Vega

The recent discovery of a weak surface magnetic field on the normal intermediate-mass star Vega raises the question of the origin of this magnetism in a class of stars that was not known to host magnetic fields. We aim to confirm the field detection and provide additional observational constraints about the field characteristics, by modelling the magnetic geometry of the star and by investigating the seasonal variability of the reconstructed field. We analyse a total of 799 circularly-polarized spectra collected with the NARVAL and ESPaDOnS spectropolarimeters during 2008 and 2009. We employ a cross-correlation procedure to compute, from each spectrum, a mean polarized line profile with a signal-to-noise ratio of about 20,000. The technique of Zeeman-Doppler Imaging is then used to determine the rotation period of the star and reconstruct the large-scale magnetic geometry of Vega at two different epochs. We confirm the detection of circularly polarized signatures in the mean line profiles. The amplitude of the signatures is larger when spectral lines of higher magnetic sensitivity are selected for the analysis, as expected for a signal of magnetic origin. The short-term evolution of polarized signatures is consistent with a rotational period of 0.732 \pm 0.008 d. The reconstructed magnetic topology unveils a magnetic region of radial field orientation, closely concentrated around the rotation pole. This polar feature is accompanied by a small number of magnetic patches at lower latitudes. No significant variability in the field structure is observed over a time span of one year. The repeated observation of a weak photospheric magnetic field on Vega suggests that a previously unknown type of magnetic stars exists in the intermediate-mass domain. Vega may well be the first confirmed member of a much larger, as yet unexplored, class of weakly-magnetic stars.Comment: Accepted by Astronomy & Astrophysics. Abstract shortened to respect the arXiv limit of 1920 character

Exploring the magnetic topologies of cool stars

Magnetic fields of cool stars can be directly investigated through the study of the Zeeman effect on photospheric spectral lines using several approaches. With spectroscopic measurement in unpolarised light, the total magnetic flux averaged over the stellar disc can be derived but very little information on the field geometry is available. Spectropolarimetry provides a complementary information on the large-scale magnetic topology. With Zeeman-Doppler Imaging (ZDI), this information can be retrieved to produce a map of the vector magnetic field at the surface of the star, and in particular to assess the relative importance of the poloidal and toroidal components as well as the degree of axisymmetry of the field distribution. The development of high-performance spectropolarimeters associated with multi-lines techniques and ZDI allows us to explore magnetic topologies throughout the Hertzsprung-Russel diagram, on stars spanning a wide range of mass, age and rotation period. These observations bring novel constraints on magnetic field generation by dynamo effect in cool stars. In particular, the study of solar twins brings new insight on the impact of rotation on the solar dynamo, whereas the detection of strong and stable dipolar magnetic fields on fully convective stars questions the precise role of the tachocline in this process.Postprin

Planck early results XI : Calibration of the local galaxy cluster Sunyaev-Zeldovich scaling relations

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Effects de l'hypoxie sur des poissons et crustacés de l'estuaire de la Gironde (France)

International audienceThe Water Framework Directive (WFD) requires the improvement of water quality in the EU and the assessment of the transitional waters ecological status considering the fish component of the ecosystem. In the WFD, the dissolved oxygen (DO) is considered as one of the physico-chemical quality elements that support the biology and have to be monitored. Estuaries are particularly essential to fish as refuge area, habitats for reproduction, nursery grounds and migration routes but they are impacted by multiple anthropogenic disturbances. Hypoxic threshold is largely characterized by a DO content of 2 mg L-1 whereas the median lethal concentration is about 2.45 mg L-1 for aquatic organisms. Since 2005, this parameter is monitored in the Gironde Estuary thanks to the continuous monitoring MAGEST network that has recorded several summer borderline hypoxic situations (DO close to 2 mg L-1) and a 7 days-long hypoxic event (DO < 2.45 mg L-1) in July 2006 with a minimum measured value of 1.22 mg L-1. Biological responses to hypoxia depend on the period, intensity and extent of these events. Shads, Alosa fallax and Alosa alosa, and some shrimp developmental stages, are among the most sensitive species to hypoxia in the Gironde Estuary. A behavioral study was performed on shad juvenile and allowed to establish the DO threshold that could impact their downstream migration occurring from July to early October. Water temperatures in summer 2006 are the highest recorded in the period 2005-2012 with values higher than 28°C. Almost 43% of conditions (DO thresholds at 20°C and 25°C) are critical to shad juveniles during their migration period in 2006. More than 940 hours reached critical values including 460 hours that would result in the death of individuals. Although the fish taxa appear to be more sensitive towards hypoxia, the crustacean taxa show also a strong sensitivity in the early ontogenic stage and on eggs-bearing females. Because shrimp is a major component of the estuarine food web, this taxon appears as a promising indicator of ecosystem dysfunction

Effects de l'hypoxie sur des poissons et crustacés de l'estuaire de la Gironde (France)

International audienceThe Water Framework Directive (WFD) requires the improvement of water quality in the EU and the assessment of the transitional waters ecological status considering the fish component of the ecosystem. In the WFD, the dissolved oxygen (DO) is considered as one of the physico-chemical quality elements that support the biology and have to be monitored. Estuaries are particularly essential to fish as refuge area, habitats for reproduction, nursery grounds and migration routes but they are impacted by multiple anthropogenic disturbances. Hypoxic threshold is largely characterized by a DO content of 2 mg L-1 whereas the median lethal concentration is about 2.45 mg L-1 for aquatic organisms. Since 2005, this parameter is monitored in the Gironde Estuary thanks to the continuous monitoring MAGEST network that has recorded several summer borderline hypoxic situations (DO close to 2 mg L-1) and a 7 days-long hypoxic event (DO < 2.45 mg L-1) in July 2006 with a minimum measured value of 1.22 mg L-1. Biological responses to hypoxia depend on the period, intensity and extent of these events. Shads, Alosa fallax and Alosa alosa, and some shrimp developmental stages, are among the most sensitive species to hypoxia in the Gironde Estuary. A behavioral study was performed on shad juvenile and allowed to establish the DO threshold that could impact their downstream migration occurring from July to early October. Water temperatures in summer 2006 are the highest recorded in the period 2005-2012 with values higher than 28°C. Almost 43% of conditions (DO thresholds at 20°C and 25°C) are critical to shad juveniles during their migration period in 2006. More than 940 hours reached critical values including 460 hours that would result in the death of individuals. Although the fish taxa appear to be more sensitive towards hypoxia, the crustacean taxa show also a strong sensitivity in the early ontogenic stage and on eggs-bearing females. Because shrimp is a major component of the estuarine food web, this taxon appears as a promising indicator of ecosystem dysfunction