A BCool magnetic snapshot survey of solar-type stars

Abstract

Stellar magnetic field measurements obtained from spectropolarimetry offer\nkey data for activity and dynamo studies, and we present the results of a major\nhigh-resolution spectropolarimetric Bcool project magnetic snapshot survey of\n170 solar-type stars from observations with the Telescope Bernard Lyot and the\nCanada-France-Hawaii Telescope. For each target star a high signal-to-noise\ncircularly polarised Stokes V profile has been obtained using Least-Squares\nDeconvolution, and used to detect surface magnetic fields and measure the\ncorresponding mean surface longitudinal magnetic field ($B_{l}$). Chromospheric\nactivity indicators were also measured.\n Surface magnetic fields were detected for 67 stars, with 21 of these stars\nclassified as mature solar-type stars, a result that increases by a factor of\nfour the number of mature solar-type stars on which magnetic fields have been\nobserved. In addition, a magnetic field was detected for 3 out of 18 of the\nsubgiant stars surveyed. For the population of K-dwarfs the mean value of\n$B_{l}$ ($|B_{l}|_{mean}$) was also found to be higher (5.7 G) than\n$|B_{l}|_{mean}$ measured for the G-dwarfs (3.2 G) and the F-dwarfs (3.3 G).\nFor the sample as a whole $|B_{l}|_{mean}$ increases with rotation rate and\ndecreases with age, and the upper envelope for $|B_{l}|$ correlates well with\nthe observed chromospheric emission. Stars with a chromospheric S-index greater\nthan about 0.2 show a high magnetic field detection rate and so offer optimal\ntargets for future studies.\n This survey constitutes the most extensive spectropolarimetric survey of cool\nstars undertaken to date, and suggests that it is feasible to pursue magnetic\nmapping of a wide range of moderately active solar-type stars to improve\nunderstanding of their surface fields and dynamos.\n