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

Peer reviewe

Characterization of an organic anion-transporting polypeptide (OATP-B) in human placenta

Organic anion-transporting polypeptides (OATPs) are a family of multispecific carriers that mediate the sodium-independent transport of steroid hormone and conjugates, drugs, and numerous anionic endogenous substrates. We investigated whether members of the OATP gene family could mediate fetal-maternal transfer of anionic steroid conjugates in the human placenta. OATP-B (gene symbol SLC21A9) was isolated from a placenta cDNA library. An antiserum to OATP-B detected an 85-kDa protein in basal but not apical syncytiotrophoblast membranes. Immunohistochemistry of first-, second-, and third-trimester placenta showed staining in the cytotrophoblast membranes and at the basal surface of the syncytiotrophoblast. Trophoblasts that reacted with an antibody to Ki-67, a proliferation-associated antigen, expressed lower levels of OATP-B. OATP-B mRNA levels were measured in isolated trophoblasts under culture conditions that promoted syncytia formation. Real-time quantitative PCR estimated an 8-fold increase in OATP-B expression on differentiation to syncytia. The uptake of [(3)H]estrone-3-sulfate, a substrate for OATP-B, was measured in basal syncytiotrophoblast membrane vesicles. Transport was saturable and partially inhibited by pregnenolone sulfate, a progesterone precursor. Pregnenolone sulfate also partially inhibited OATP-B-mediated transport of estrone-3-sulfate in an oocyte expression system. These findings suggest a physiological role for OATP-B in the placental uptake of fetal-derived sulfated steroids

Long-term magnetic field monitoring of the sun-like star ξ Bootis A

peer reviewedPhase-resolved observations of the solar-type star ξ Bootis A were obtained using the Narval spectropolarimeter at Telescope Bernard Lyot (Pic du Midi, France) during years 2007, 2008, 2009 and 2010. The data sets enable us to study both the rotational and the long-term evolution of various activity tracers. Here, we focus on the large-scale photospheric magnetic field (reconstructed by Zeeman-Doppler Imaging), the Zeeman broadening of the FeI 846.84 nm magnetic line, and the chromospheric CaII H and Hα emission