Poster Talk abstract details
Direct detection of magnetic field at the surface of slowly rotating giant stars: are all giants magnetically active?
Abstract
Using the spectropolarimeters NARVAL (at TBL, Pic du Midi Observatory) and ESPaDOnS (CFHT) , and the LSD technic, we undergo a sensitive program of detection and measurement of magnetic field at the surface of slowly rotating single giants for which an X-ray emission or variations of CaII H and K lines have been already detected.
Up to now Stokes V signal is significantly detected on 7 slow-rotating single giants of our sample and the infered longitudinal magnetic field is measured to be of the order of a few G. At the upper part of our measurements ladder is the outstanding giant EK Eri for which a very variable longitudinal magnetic field reaching about 100 G is discovered: this supports the star being the descendant from a very magnetic Ap star. At the bottom of the ladder is the weakly active giant Pollux, for which a very weak longitudinal magnetic field (about half a G) is detected.
Since solar-type dynamo is unlikely to work in such slow rotators, two hypothesis are discussed for the origin of magnetic field in these stars: "fossil" magnetic fields for the descendants of Ap stars, and another dynamo regime, for instance "turbulent dynamo" , otherwise. While descendants of Ap stars would concern about 5% of the giant stars in the corresponding mass range, a dynamo process at work in slow rotators might concern a great percentage of red giants of intermediate mass.
Up to now Stokes V signal is significantly detected on 7 slow-rotating single giants of our sample and the infered longitudinal magnetic field is measured to be of the order of a few G. At the upper part of our measurements ladder is the outstanding giant EK Eri for which a very variable longitudinal magnetic field reaching about 100 G is discovered: this supports the star being the descendant from a very magnetic Ap star. At the bottom of the ladder is the weakly active giant Pollux, for which a very weak longitudinal magnetic field (about half a G) is detected.
Since solar-type dynamo is unlikely to work in such slow rotators, two hypothesis are discussed for the origin of magnetic field in these stars: "fossil" magnetic fields for the descendants of Ap stars, and another dynamo regime, for instance "turbulent dynamo" , otherwise. While descendants of Ap stars would concern about 5% of the giant stars in the corresponding mass range, a dynamo process at work in slow rotators might concern a great percentage of red giants of intermediate mass.