A&A 529, A100 (2011) Astronomy DOI: 10.1051/0004-6361/201015445 & c ESO 2011 Astrophysics First detection of a weak magnetic field on the giant Arcturus: remnants of a solar dynamo? C. Sennhauser1,2 and S. V. Berdyugina2 1 Institute for Astronomy, ETH Zurich, 8093 Zurich, Switzerland e-mail: [email protected] 2 Kiepenheuer Institut für Sonnenphysik, 79104 Freiburg, Germany Received 21 July 2010 / Accepted 28 February 2011 ABSTRACT Context. Arcturus is the second closest K giant and among the brightest stars in the sky. It has not been found to have a magnetic field, even though Ca ii H&K lines as activity indicators imply that Arcturus is magnetically active. Aims. We measure the mean longitudinal magnetic field strengths and interpret them in terms of an intraseasonal activity modulation. Methods. We apply our new Zeeman component decomposition (ZCD) technique to three single sets of Stokes I and V spectra to measure the longitudinal component of the magnetic field responsible for tiny Zeeman signatures detected in spectral line profiles. Results. For two of the spectra, we report the detection of the Zeeman signature of a weak longitudinal magnetic field of 0.65 ± 0.26 G and 0.43 ± 0.16 G. The third measurement is less significant, but all the measurements closely reproduce a rotationally modulated activity cycle with four active longitudes. Conclusions. For the first time, a magnetic field on Arcturus is directly detected. This field can be attributed to a diminishing solar- type αΩ-dynamo acting in the deepening convection zone of Arcturus. We demonstrate that our new method ZCD lowers the detection limit of very weak magnetic fields from spectropolarimetric measurements. Key words. stars: individual: Arcturus – stars: magnetic field – stars: late-type 1. Introduction be attributed to four active longitudes. Observed periods in H&K line bisectors can be explained by differential rotation and Arcturus (α Boo) is a K1.5 III star ascending the red giant the latitudinal migration of active regions. branch. It is the second brightest star in the northern hemi- The direct detection (via Zeeman effect) of a magnetic field sphere, thus has been the subject of many studies. Measurements on Arcturus has therefore been long awaited. The first mag- of the Doppler velocity have found long-period variability on netic field measurement of 2.9 ± 1.8 G reported by Hubrig timescales of a few hundred days (Gray & Brown 2006, and ref- et al. (1994) was taken right at the activity maximum, but er- erences therein), while its short-term variability has been quan- ror bars are too high for an unambiguous detection. We devel- titatively characterized by the identification of individual modes oped a new multi-line analyzing technique for polarized spec- of oscillation (Tarrant et al. 2007). Its granulation properties tra, called Zeeman component decomposition (ZCD). We have were measured by the same group, with an estimated timescale ∼ been encouraged by its performance for simulated Stokes I and of 0.5 d. Being a single star, its mass is relatively poorly known V spectra and its ability to recover very weak longitudinal field but comparable to that of the Sun (0.8 ± 0.3 M by Bonnel ff ffi strengths whose Zeeman signatures are completely embedded & Bell 1993). The debate on the e ective temperature (Gri n in noise (Sennhauser & Berdyugina 2010), thus applied ZCD 1996) has waned, and it is nowadays generally accepted to be to three datasets taken at the Canada-France-Hawaii telescope 4300 K. (CFHT). We report here the detection of a supposedly varying, Ayres et al. (2003) concluded that Arcturus may sustain a very weak magnetic field of about half a Gauss at the photo- modest level of magnetic activity responsible for the heating of sphere of Arcturus. Section 2 describes our observation and the the coronal structures. The long-term study of Ca ii H&K lines principles of ZCD used for the data analysis. In Sect. 3,we as classical activity indicators by Brown et al. (2008) reveals a present our results about the seasonal variability caused by four range of variability periods between the years 1984 and 2007, active longitudes. We discuss possible origins for the magnetic exhibiting an apparent magnetic cycle with an estimated dura- field on Arcturus in Sect. 4, and investigate potential sources of tion of ∼<14 yr. Owing to the known correlation of magnetic systematic errors responsible for spurious signatures in circular activity with surface temperature observed for the Sun (Gray polarization. In Sect. 5, we summarize our conclusions. & Livingston 1997) and other late-type MS stars (Gray et al. 1996b,a, and references therein), periodic temperature changes ≈ of 20 K inferred from line-depth ratio variations support this 2. Observation and analysis theory. Assuming a rotational period of 730 d (Peterson et al. 1993), or about 2 yr (Gray & Brown 2006), with v sin i = The three observations of Arcturus were obtained at the CFHT 1.5 ± 0.3kms−1, the seasonal variability (200−250 days) can using ESPaDOnS (Donati et al. 2006) on August 2 2006, Article published by EDP Sciences A100, page 1 of 6 A&A 529, A100 (2011) Fig. 1. An interval of Stokes I, V spectra of Arcturus, taken at the CFHT/ESPaDOnS on Aug. 2, 2006 (solid lines), and best fits from ZCD (dash-dotted line). ZCD automatically omits spectral features/lines that cannot be fitted well. Lower panel: the recovered Stokes V signal is fully embedded in noise of the original measurement. August 23 2008, and December 6 2008. Data reduction was Table 1. Comparison of recovered BLOS from LSD and ZCD for two performed using the package Libre-ESpRIT installed at CFHT different late-type giants. (Donati et al. 1997). We used ESPaDOnS in circular polarization mode, recording four sub-exposures to obtain the continuum- Name Spec. type (BLOS ± σ)LSD [G] (BLOS ± σ)ZCD [G] normalized Stokes I (intensity) and Stokes V (circular polariza- 32 Cyg K3Ib+ 1.16 ± 0.49 0.53 ± 0.16 tion) parameters. The total integration times were 16 s in 2006 λ Vel K4.5Ib-II 1.72 ± 0.33 0.90 ± 0.13 and 8 s in 2008, resulting in a peak signal-to-noise ratio (SNR) well above 1000 in the wavelength region 650-850 nm. However, the SNR gradually decreases toward shorter wavelengths, where spectrum as the most reliable way of error assessment. We fol- most of the spectral lines can be found, dropping below a value ∼ lowed the Monte Carlo simulation of synthetic data sets de- of 300 (2006) and 450 (2008) at λ 430 nm. scribed in Press (1992), except that we did not rely on uncertain- To recover the mean longitudinal magnetic field, we ap- ties for individual pixels of the spectrum. We compute instead plied the Zeeman component decomposition analysis (hereafter standard deviations for equidistant wavelength intervals of the ZCD), described in Sennhauser & Berdyugina (2010), which measured Stokes V spectrum and simulated a series of hypo- is an inversion technique based on Milne-Eddington assump- thetical data sets (usually 200) with the SNR distribution of the tions. Treating the strength of each line as a free parameter, ZCD original spectrum. We then applied the ZCD to each of these does not rely on pre-calculated line masks, which depend on a data sets to obtain 200 values of BLOS. The standard deviation given set of stellar parameters. Only the central wavelengths and of this distribution gives us an error estimate for the longitudinal atomic configurations of the possible lines typically observed for magnetic field. a given spectral class need to be known. During the inversion, The Stokes I spectrum, which we require to retrieve BLOS us- ZCD extracts a single line-to-continuum opacity profile common ing our ZCD method, remains unchanged. We assume that vari- to both Stokes I and V from thousands of lines, and infers a mean ations in the net longitudinal magnetic field as small as ∼1Gdo ff longitudinal magnetic field strength at the e ective heights of the not influence the intensity spectrum. formation of these lines. In addition, ZCD fits the line strengths to the observations simultaneously for Stokes I and V, reject- ing lines that cannot be fitted well enough, or whose strengths 2.2. Analysis of additional spectra are below a certain threshold value (usually 1% residual depth, depending on the type of spectra). We previously proved the functionality of ZCD with multiple numerical tests (Sennhauser & Berdyugina 2010). As a further In Fig. 1, we show a fragment of the observations (solid test, we present results of applying the ZCD to two late-type lines) for Stokes I (top panel) and Stokes V (bottom panel). supergiants 32 Cyg (HD 192909) and λ Vel (HD 78647). These The fitted spectra obtained from ZCD are superimposed as dash- stars were previously analyzed by Grunhut et al. (2010), who dotted lines. We note that the scaling for V/Ic is smaller than the employed the LSD technique (Donati et al. 1997) and inferred noise level of the observations, to make the fit from ZCD visible. first-order moments of the line-of-sight (LOS) component of the magnetic field. Table 1 compares the results from Grunhut et al. 2.1. Error estimation (2010) with the values recovered by our ZCD. First, we note that whenever there is a magnetic field de- To estimate the error bars for the recovered magnetic field tection from LSD, there is also one from ZCD (true not only strength, a Monte Carlo simulation was used for each individual for these two stars). However, while tests have shown that A100, page 2 of 6 C.