Magnetic fields on young, moderately rotating Sun-like stars - I: HD~35296 and HD~29615

Abstract

Observations of the magnetic fields of young solar-type stars provide a way to investigate the signatures of their magnetic activity and dynamos. Spectropolarimetry enables the study of these stellar magnetic fields and was thus employed at the Télescope Bernard Lyot and the Anglo-Australian Telescope to investigate two moderately rotating young Sun-like stars, namely HD 35296 (V119 Tau, HIP 25278) and HD 29615 (HIP 21632). The results indicate that both stars display rotational variation in chromospheric indices consistent with their spot activity, with variations indicating a probable long-term cyclic period for HD 35296. Additionally, both stars have complex, and evolving, large-scale surface magnetic fields with a significant toroidal component. High levels of surface differential rotation were measured for both stars. For the F8V star HD 35296 a rotational shear of $\Delta \Omega$ = 0.22${}_{-0.02}^{+0.04}$ rad/d was derived from the observed magnetic profiles. For the G3V star HD 29615 the magnetic features indicate a rotational shear of $\Delta \Omega$ = 0.48${}_{-0.12}^{+0.11}$ rad/d, while the spot features, with a distinctive polar spot, provide a much lower value of $\Delta \Omega$ of 0.07${}_{-0.03}^{+0.10}$ rad/d. Such a significant discrepancy in shear values between spot and magnetic features for HD 29615 is an extreme example of the variation observed for other lower-mass stars. From the extensive and persistent azimuthal field observed for both targets it is concluded that a distributed dynamo operates in these moderately rotating Sun-like stars, in marked contrast to the Sun's interface-layer dynamo.

18 Pages, 13 figures, 5 tables, accepted for publication in MNRAS

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