Doppler imaging and differential rotation of ${\sigma }^2$ Coronae Borealis using SONG

Abstract

We present new Doppler images of both components of the double-lined binary ${\sigma }^2$ CrB, based on the high-resolution spectroscopic data collected during 11 nights in 2015 March--April. The observed spectra form two independent data sets with sufficient phase coverage. We apply the least-squares deconvolution to all observed spectra to obtain high signal-to-noise mean profiles, from which we derive the Doppler images of both components of ${\sigma }^2$ CrB simultaneously. The surfaces of both F9 and G0 components are dominated by pronounced polar spots. The F9 component exhibits a weak spot at latitude 30${}^{\circ }$ and its mid-to-low latitudes are relatively featureless. The G0 star shows an extended spot structure at latitude 30${}^{\circ }$, and its surface spot coverage is larger than that of the F9 star, which suggests a higher level of magnetic activity. With the cross-correlation method, we derive a solar-like surface differential rotation on the G0 star of ${\sigma }^2$ CrB for the first time, and the surface shear rate is $\Delta \Omega =0.180\pm 0.004$ rad d${}^{-1}$ and $\alpha =\Delta \Omega /{\Omega }_{eq}=0.032\pm 0.001$. We do not obtain a clear surface shear law for the F9 star due to the lack of mid-to-low latitude features, but detect a systematic longitude shift of high-latitude spots, which indicates a slower rotation with respect to the co-rotating frame.

arXiv comment9 figures, 2 tables, accepted for publication in ApJ

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