116 research outputs found
Remo Ruffini Festschrift
No full textA conference celebrating Remo Ruffini's 80th birthday was held in Nice, France, from 16 to 18 May 2022, with the participation of over 90 scientists. Among the contributions presented orally there were those by Rashid Sunyaev, Peter Predehl, Demetrios Christodoulou, Thibault Damour, Nathalie Deruelle, Roy Kerr, Tsvi Piran, Claus Laemmerzahl, Asghar Qadir, Chen Pisin, and Marco Tavani, in the presence of Agnès Rampal, the representative of the Mayor of Nice. An extraordinary moment of the meeting in Nice was the delivery of the Marcel Grossmann Award to Rashid Sunyaev and Peter Predehl for the Spectr–Roentgen–Gamma (SRG) mission. This volume contains 10 papers by some of the meeting participants and collaborators of Remo Ruffini
Modelling the hidden magnetic field of low-mass stars
No full textPL acknowledges support from a Science and Technology Facilities Council studentship. JM, AAV and RF acknowledge support from fellowships of the Alexander von Humboldt foundation, the Royal Astronomical Society and Science and Technology Facilities Council, respectively.Zeeman-Doppler imaging is a spectropolarimetric technique that is used to map the large-scale surface magnetic fields of stars. These maps in turn are used to study the structure of the stars' coronae and winds. This method, however, misses any small-scale magnetic flux whose polarization signatures cancel out. Measurements of Zeeman broadening show that a large percentage of the surface magnetic flux may be neglected in this way. In this paper we assess the impact of this 'missing flux' on the predicted coronal structure and the possible rates of spin-down due to the stellar wind. To do this we create a model for the small-scale field and add this to the Zeeman-Doppler maps of the magnetic fields of a sample of 12 M dwarfs. We extrapolate this combined field and determine the structure of a hydrostatic, isothermal corona. The addition of small-scale surface field produces a carpet of low-lying magnetic loops that covers most of the surface, including the stellar equivalent of solar 'coronal holes' where the large-scale field is opened up by the stellar wind and hence would be X-ray dark. We show that the trend of the X-ray emission measure with rotation rate (the so-called 'activity-rotation relation') is unaffected by the addition of small-scale field, when scaled with respect to the large-scale field of each star. The addition of small-scale field increases the surface flux; however, the large-scale open flux that governs the loss of mass and angular momentum in the wind remains unaffected. We conclude that spin-down times and mass-loss rates calculated from surface magnetograms are unlikely to be significantly influenced by the neglect of small-scale field.Peer reviewe
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