Doppler imaging of stellar surface structure

Abstract

Abstract. We present the first Doppler image of a solar-type G dwarf in the Pleiades open cluster. As such, the star represents the Sun at an age of approximately 100 Myr and could be an important target for further progress in magnetic-braking and angular-momentum-transport theories. Our image reconstructions were done from a full spectrum inversion with a total of 38 spectral lines but we also present single-line inversions using the prominent Li i 6707-Å line and the nearby Ca i 6717-Å line. The maps reveal cool spots at or near the pole and within the equatorial regions, in contradiction to our predictions from flux-tube modelling that only medium-latitude spots should be seen. The maps also show several warm spots near the equatorial regions but their reality needs to be confirmed. A polar spot is recovered but likely consists of several smaller spots at very high latitudes touching the visible pole instead of a big cap-like spot as seen on some RS CVn binaries.Key words. stars: activity -starspots -stars: imaging -stars: individual: HII 314 -stars: late-type Surface imaging of solar-type stars in open clustersMagnetic braking of stellar rotation due to a stellar wind along predominantly equatorial magnetic field lines, as worked for our Sun, seems unable in every case to be effective in slowing down stars from their initial angular momentum gained during the contraction of the prestellar cloud. The ultra-fast rotators (UFRs) in the young open clusters α Per (age ≈50 Myr) and Pleiades (age ≈100 Myr) as well as the young field stars AB Dor, LQ Hya, EK Dra are the most cited examples. Their simple existence is something of a problem since magnetic braking of a solar-mass star should have had, by the age of these stars, enough time during pre-main-sequence evolution to halt the rapid rotation. Accretion disks in the early pre-main sequence stages certainly play an important and maybe dominant role in explaining the angular momentum loss of very young objects but cannot easily be incorporated into an explanation of the existence of several hundreds of cluster-ZAMS stars that are not in a close binary but still rapidly rotating (Stauffer et al. 1984;Soderblom et al. 1993b;O'Dell et al. 1995;Barnes & Sofia 1996; and others).What process then might allow these stars to maintain their angular momentum? The currently favored hypothesis is the onset of a dynamo saturation, i.e. a saturation of the atmospheric (coronal) volume with magnetic fields (Vilhu 1984;Keppens et al. 1995) so that there is no torque arm for magnetic braking via a stellar wind. For further reading we refer to Mestel (1999). Solanki et al. (1997) and Buzasi (1997) -and later Strassmeier et al. (1998 for post-main-sequence evolution -have suggested an alternative explanation to dynamo saturation in that magnetic fields concentrated in polar starspots could be the reason for the lack of angular momentum loss. Solanki et al. (1997) presented numerical simulations that show that the effect of polar magnetic fields in the evolution of stellar rotatio...

Doppler imaging of stellar surface structure

“…No water-vapor lines above the nominal noise level are obvious in the wavelength regions of interest and no attempts were made to correct for it. The reduction procedure was nearly identical to that in our paper on HII 314 (Rice & Strassmeier 2001) obtained with the same instrumental set up and we refer the reader to that and previous papers in this series. A radialvelocity standard was observed at the beginning (β CVn) and at the end of each night (γ Ser) and provided the absolute zero point of our radial-velocity calibration.…”

Doppler imaging of stellar surface structure

“…See also the brief discussion in Strassmeier & Bartus (2000). We treated the three mapping lines separately, since the spectral coverage our code can handle is currently limited to 2 nm, which is too small for including all the mapping lines at the given v sin i of UZ Lib (see Rice & Strassmeier 2001) for an application to a different wavelength region).…”

Doppler imaging of Speedy Mic using the VLT

“…can severely deteriorate the reconstruction quality. An extensive series of such simulations can be found in Rice & Strassmeier (2001). We have studied the behaviour of CLDI based on synthetic input data (Wolter 2004;2005, in preparation).…”