Long‐Term Spectroscopic Monitoring of Arcturus

Abstract

We evaluate observed spectroscopic activity of Arcturus (HR 5340, K2 III) from 1984 to 2007 using high-resolution spectroscopy and Ca II H+K emission. Line-depth ratios of the V I λ6251.83 Å to Fe I λ6252.56 Å lines and line bisectors from Fe I λ6252.56 Å are used to evaluate spectroscopic activity. Chromospheric emission within H+K lines is used as a magnetic activity indicator. We observe a significant portion of what appears to be a magnetic cycle with a period of ≥14 yr. Line-depth ratios show comparable variation, but with a time lag of 2.0 ± 0.5 yr with respect to H+K variations. This time lag is qualitatively similar to previous observations of G and K dwarfs. No corresponding variation is observed in line bisectors. H+K and line bisectors also show variability within seasons. A Fourier analysis reveals periods of 253 and 207 days during the 1984-1989 H+K seasons. Line bisectors show excess power over the range of ≈115-200 days from 1992 to 2007. We attribute this intraseason variability to inferred magnetic activity along active longitudes. The range of periods observed in H+K and line bisectors (≈115-253 days) may result from migration of active regions within longitude bands, implying differential rotation. We compare these observations with the Sun and other solar-type stars. A 1/f noise component to the Fourier spectrum of the line bisector variations is also observed.